Chapter VIII Planning Highway Systems: Selection of Road Types
A road is a strip of land set apart or appropriated for travel, public or private. When a road has been dedicated to the public or has been used so long that the public has a legal right of easement therein, it becomes a highway.173
The object of a road is to provide a way for transportation. It goes without saying, therefore, that its situation should be such that it can perform this function most efficiently, and a system of highways should perform the same function for the public in the same manner. Efficiency here includes the ideas of economy and satisfaction combined.
In order to make a layout of a system of highways they should first be classified as to use, for the proper treatment will depend upon the use to which the roads are to be put. Anyone attempting a layout will make his own classification suitable to the inherent conditions pertaining to the district covered. The classification of Mr. T. H. MacDonald, Director of the Bureau of Public Roads, U. S. Department of Agriculture, made for another purpose may be adopted:174
1. Those used chiefly related to agriculture.
2. Those which are recreational in character.
3. Those which are commercial.
4. Those which are military.
Agricultural roads comprise those leading from farm to town and are used chiefly for marketing, and for social, educational, and religious activities.
Recreational roads are either local, upon which driving is done for pleasure, or through, those followed by tourists in traveling over the country. Either of which may lead to places of interest within or without the state. The national park roads and forest highways can be classified under the head of recreational.
Commercial highways comprise those exclusive of agricultural, upon which the haul is chiefly of a business nature such as freight and express and bus traffic.
The War Department of the United States during the war refused to designate any roads as special Military Highways, saying a road which would adequately serve the agricultural; recreational, and commercial interests would serve the military. However, it might be well to keep in mind this possible use of the highways. A classification of roads into National, State, County and Town has frequently been suggested. Since national roads do not exist as such in the United States the most densely traveled routes and those used largely for through traffic are usually designated state roads, and all others local roads.
Keeping the cost and use of the roads in view the problem before the road planner is:
(1) To secure the most economical routes in construction and maintenance and future haulage, taking into account topography, alignment, grades, and amount of traffic.
(2) To accommodate the greatest number of people commensurate with the money available for expenditure.
(3) To utilize existing roads as far as practicable.
The system will ordinarily consist of one or more trunk lines to be laid down first and several branch lines connecting with the trunk lines. Some of the essentials to be considered are:
(1) Ruling points. These ordinarily will be (a) the main community centers—the large cities and villages, for to and from these will naturally flow the greatest traffic; (b) Natural configurations such as mountain passes, low points in hills, ridges and valleys, rivers and bridge sites; (c) Parks, scenery, and recreational and amusement centers; (d) Articulation with the highways of adjoining jurisdictions, so that they may eventually be united into one continuous passage.
(2) Branch lines and detours. The question will frequently arise whether to run a branch line to some locality or detour the trunk. Much pressure will be brought by the inhabitants along the way and the deciding power will needs have firmness and fairness in the highest degree.
(3) Alternate routes. If two roads of equal or almost equal importance are available a choice must be made between them.
(4) Existing highways and principal streets of cities should be utilized as much as practicable. Although sometimes betterments so great may come through changes that old routes should be discontinued, in which case the
(5) Vested rights of citizens living along discontinued routes and damages caused by any improvements made must be considered.
(6) The widening of existing roads and streets and costs of the same.
(7) The location of railways, trolley lines, and street car tracks may influence the layout for grade crossings should be eliminated, or at least placed where there is clear vision each way for a considerable distance.
(8) Bridges, culverts, and railroad crossings should, preferably, be directly along the highway and at right angles to the obstruction.
(9) Ruling grades, dependent upon the class of road will also affect the layout.
(10) Esthetics. Perhaps the people of the United States have been too materialistic and have paid too little attention to the beauty and interest elements in highway location. Especially for pleasure riding and recreation should the esthetic side be considered.
Motor Transport Efficiency Outline—At this point it would be well for the road planner to consult an excellent paper by M. C. Horine and his efficiency chart which is substantially as follows:175
Economic Efficiency
Highway Efficiency
Adaptability of road to carrier
Road capacity
Width of road
Pressure capacity
Impact capacity
Seasonal limitations
Tractive resistance
Grades
Route
Curves and corners
Adaptability of carrier to road
Dimensions
Capacity and weight
Speed
Climbing ability
Accelerating ability
Stopping ability
Turning radius
Tractive effort
Transport Efficiency
Adaptability of carrier to volume and character of load
Total load units to be carried
Range of load units to be hauled
Average load units to be hauled
Density of load
Length of haul
Route
Number and probable duration of stops
Comparative adaptability of motor transport
Horse transport
Highway efficiency
Transport efficiency
Vehicle efficiency
Public health
Railway transportation including trolley express
Transport efficiency
Economy
Haulage cost
Packing cost
Adaptability of carrier to traffic
Legal restrictions on equipment and operation
Possible average running speed
Bridges and ferries
Vehicle Efficiency
Operation
Moving factor
Loading delays
Unloading delays
Waiting for loads
Clerical delays
Loafing
Traffic delays
Load factor
Body capacity
Special deliveries
Return loads
Outside hauling (custom work)
Pickups
Deliveries
Trailers
Maintenance
Active factor
Disability layups
Chassis repairs
Body repairs
Accessory repairs
Tire repairs and replacements
Overhaul and painting
Driver’s disability
Requirement layups
Seasonal fluctuations
Off-peak period
Shut downs
Labor troubles
Economy
Earning factor
Unit miles
Packing cost
Loading cost
Unloading cost
Time in transit
Marketability
Insurance
Interest on value
Shrinkage and breakage
Perishability
Tracing and follow up
Advertising value
Goodwill of trade
Increased radius of trade
Increased business turnover
Cost factor
Operating cost
Fixed charges
Maintenance charges
Running charges
Overhead
Loading devices
Shipping room devices
Office and clerical expenses
Telephone
Labor
Loaders
Watchmen
Clerks
Supervisors
Accountants
Traffic department
Miscellaneous
The Highway System Unit—Ordinarily the units will be the same as the political divisions, that is, national, state, or local. A national system would include the whole United States and will comprise trunk lines paralleling each other across the country east and west and north and south, considering, of course, ruling points, with a few branch lines of importance. The total number of miles of such highways should probably never exceed 2 per cent of the total mileage in the United States, or about 50,000 miles. A national highway committee estimated that this mileage of roads would serve 87 per cent of the people. The arguments made in favor of a national system of highways may be briefly summarized as follows:
(1) Political lines would be obliterated, thus welding together and unifying all the people. Commercial and social intercourse would wipe out sectional differences, hatreds, and enmities. The people of one section would learn that the people of another section are human beings as they themselves are, and are actuated by the same instincts and inspirations.
(2) Economic, commercial, and pleasure routes cut across state lines and it would be much better to have the roads continuous and administered by one central control. They would then be kept in a more uniform condition of usefulness. The roads radiating from New York City are mentioned as an example. They pass out of the state of New York and into the states of Connecticut, Rhode Island, Massachusetts, New Jersey, Pennsylvania, Delaware, Maryland, and the District of Columbia. On any one of these roads it would be difficult to say which state you are in by observing the license tags on the trucks and automobiles. A Connecticut traffic census on a road carrying approximately 3000 passenger cars and 500 trucks per day showed that 47 per cent of the cars in the analysis came from without the state.
(3) Military. All roads would become military during war. It is frequently stated that the good roads of France saved Paris from invasion and probably saved the war to the allies. The German General Kluck admits that while he was advancing upon the British and French south of the Marne, an army of which he had no suspicion had struck outward from Paris and put his flank in deadly peril.176 This army was one that Joffre had concentrated in Paris under General Maunouri, which had been brought over two or three good French roads from Paris by motor trucks and lorries to a point within striking distance of the German right flank, where he attacked and began the First Battle of the Marne. In the Second Battle of the Marne, motor cars performed a very important part in the concentration of troops which were to make a surprise attack. In addition the motorized armed cars known as tanks played an important part.
On this side of the ocean the trunk line roads during the war supplemented the railways, which were badly congested at the eastern terminals, by hauling large quantities of men, materials and munitions. It is said that 16,000 trucks were engaged in this work.
(4) The benefit of example. The federal government not hampered by local prejudices may construct roads of the highest type along best possible locations which when completed will serve as an example for state and local officers to pattern after. The people having seen such roads will more willingly vote bonds and taxes for road betterment.
State System—The benefits to be derived from and the arguments for a national system of roads apply with equal force to a state system. In fact it may be better to avoid the two extremes of one consolidated central control for the whole country and a very highly dispersed decentralized local control. The state seems to be a sufficiently large unit to do good work and sufficiently small to be approachable. Competition with other states may bring out developments which under a single central control would never have been thought of. Notwithstanding the excellent research work that is being carried on by the Bureau of Public Roads there is no doubt but that that which is being done by the several states more nearly solves those special problems arising on account of climate, topography, soil, and so on of the particular state.
The state system should cover a greater per cent of the roads than a national system can hope to do. The effort seems to be to take over about 10 per cent of the established roads as state highways. Such roads, if carefully selected and located, can accommodate from 90 to 95 per cent of the inhabitants of the state.
The remaining roads would continue under local—county and town—authorities. A county system might be laid out and money expended upon its roads about in proportion to their use.
Again there is a feeling on the part of many that the first expense of improving a road (that would include grading, bridging, and paving, even if the latter should be done some time subsequent to the former) should be borne by the state and the abutting property, that the maintenance should be under the direction of the local authorities, and paid for by local taxation and by a portion of the state automobile license and gasoline taxes to be returned to the county for this purpose.
The Procedure of Laying Out a Road System—To fix the mind definitely suppose it to be a state road system that is to be laid out. It is generally conceded that the planning should be done by an unprejudiced commission headed by an engineer of wide experience, one who is fair but will show no favoritism. The first question, after deciding to make a layout and who shall make it, is what shall be the determining factor of the lay-out. Shall it be primarily a farm to market system, primarily a commercial system, or primarily a recreational system? A wise commission will, no doubt, attempt to embody all these features in one system, as they should be, for, nowadays, more than half the population of the country reside in the cities and villages. They pay taxes and are as much entitled to road facilities for their particular industries and pleasures as are the “farm bloc,” and every fair-minded person will admit this.
Good maps must of course be obtained. Government contour maps when available will assist materially in selecting roads that will come within a ruling grade. On these maps will first be noted the trunk line terminals and other ruling points. The trunk lines should be as direct as practicable from one ruling point to the next. An endeavor should be made to have the roads with the greatest travel upon them the straightest, so that the total future haul may be a minimum. The roads having the greatest travel will usually be those connecting the largest cities of the state or articulating with roads leading to large cities of other states. Then will be drawn in branch lines and detours so that when the plan is complete every county seat, every village of more than 1000 inhabitants, and every manufacturing, scenic, and pleasure resort of importance will have been reached, as well as connections with the main roads of adjacent states.
This will form a tentative system. Observation upon the roads themselves will now be necessary. A reconnaissance survey may be made quite rapidly from an automobile. Two men are necessary. The engineer will ordinarily drive while the topographer will be supplied with a clip board and ruled sheets of paper. The straight line up the center of the sheet represents the roadway. Small squares on the sheets represent distances and areas. The scale should be made according to the work. On a road with few houses and few topographical features to be recorded 1 inch to the mile might be sufficient. With more houses, and other features, 1 inch to the quarter mile might be none too much. As speedometers (odometers) usually read to a tenth of a mile cross-section paper with ten smaller divisions equal to one larger is convenient. The engineer will observe features and the recorder will record them as the machine is driven along. The miles may be marked off on the sheets ahead then very quickly by a series of notations, such as a for a farm house, a dot with a roof over it for a school house, a steeple or cross on top makes a church. A crossing wagon road, railroad track, a trolley line, stream, and so on. Written explanations can be made along the margin where necessary.
The reconnaissance survey should cover all routes on the tentative map together with alternative or competitive routes and others when found to be worthy of consideration. The record will show all farm houses within a prescribed limiting distance of the roadway, factories, stone quarries, sand and gravel pits, schoolhouses, and churches. All intersecting roads, railroads, trolleys, mail routes, creeks, canals, rivers, drainage and irrigation ditches, culverts and bridges, together with the approximate angle of crossing the highway. Note should be made whether grade crossings may be eliminated by underground or overhead crossings, or by change of route. The character of the soil whether clay, loam, gumbo, or sand, when it differs from the general run of soil should be recorded, also hills, swamps, bad condition generally with brief notes as to how they may be bettered. Turns in the road itself may be noted by an angle thus,,,. Where no turn is shown, straight away is understood. Other information and natural or artificial features that may prove interesting or helpful will suggest themselves for record as the survey proceeds. Of course the mileage of each route as measured by the odometer or speedometer from fixed and known points on the map will be taken. By tabulating and mapping the information recorded it will be possible to get at the comparative merits of alternative routes.
A few simple surveying instruments will be useful in the work of reconnoitering. A steel 100-foot tape, a hand level and inclinometer, a pedometer, a pocket compass, a small aneroid barometer in mountainous countries, a pioneer ax for blazing, and a small spade may be mentioned.
After the reconnaissance, hearings should be held, usually at county seats, notice of such hearings having been given ahead of time. At these hearings the maps are shown and a statement made relative to the procedure. After which an invitation for suggestions and constructive criticism and even complaints is given. From these people who are locally interested in the roads many valuable suggestions will be received, and if they cannot be followed the reasons therefor may be stated. The people will thus know the investigation and the location of the road have been fairly made and that any suggestions that cannot be settled offhand will be duly considered before final location.
The final location will usually be arrived at or at least influenced by the following considerations: Alignment and distance, population served, grades, amount and character of haulage, other kinds of transportation available, character of soil (sand, clay, gumbo, loam), structures, bridges, railroad grade crossings and their possible avoidance, discovery of entirely new routes, topography, geological formation, and other natural features and numerous local conditions, including availability and freight charges of road materials. These are not intended to be in the order of importance, for no two roads may have the same determining factors. The character of the road surface to be used in construction may greatly affect the location. For example it does not pay to use steep grades with hard smooth pavements. But steeper grades may be used with earth and gravel roads without material loss in efficiency.
It will seldom be necessary to resort to preliminary or complete survey to lay out the plan. Sometimes further viewing of alternative roads may be desirable and many times compromises will have to be made. A traffic census on the several routes would be extremely valuable for it would determine to which class, agricultural, commercial, or recreational, the road belongs, and also the character of the traffic and what type of construction is best suited. Especially where there are alternative roads, as is usually the case in midwestern states where the roads were established along the section lines of the U. S. land survey, it is very difficult to determine which is the important highway without a traffic census. It must be remembered, however, in this connection that the improvement of a road will often draw to it much traffic from an equally short competing line. It is quite likely that if 10 per cent of all roads, provided they are properly selected, should be well improved they would carry 90 per cent of all traffic.
Financial Considerations—While little has been said of financial considerations they are, of course, of prime importance. The amount of money as a whole that may be expended is usually limited by the taxes voted or the bonds issued. The planner must cut the garment according to the cloth. He must know the approximate unit costs of the roads that will be constructed as well as the mileage of the several types. To know this he must estimate the amount of traffic that will pass over the road, he should know also about what part of this is local and what part through, in order that a just division of the cost may be made between the local and larger units. If all the money of construction were to be furnished by the national government, say, and only through traffic were considered, directness and grades between controlling points would be the determining factors. But since local traffic cannot be excluded from the use of such roads it is but just that part of the cost of building be paid locally, and if so, that modifications in the lay out be made to accommodate the local conditions.
While the first cost of the road or the road system is of very great importance and will probably be the greatest influencing factor for any particular improvement or layout, the continued cost or cost covering a series of years approximating the life of the road surface should also have consideration.
Traffic Census—An important consideration in the selection of a road alignment, of its grades, its width, and of the type of its foundations and surfacing, is the amount and the character of the traffic which will pass over the roadway after it is built. The best way to get a reasonable estimate of this is to take a traffic census. This is to be used as a basis and to it should be added an estimate of the increase in traffic which the improvement itself will produce by drawing from parallel and contributing lines, and by the probable change that the improvement will work in the character of the farming, the industries and the settlements along its way.
The alignment will be affected by the quantity of traffic, for the cost of haulage depends, though not proportionally, upon the length of haul. No less will the alignment be affected by the class of traffic. With horse-drawn vehicles curves of 40-foot radius were perfectly acceptable, but with the automobile a 200-foot radius is none too great. The new road systems now being adopted by states quite generally endeavor to make all curves to have radii greater than 200 feet except in mountainous regions, with a preference of 500 to 800 feet.
The longer radii allow the turns to be made without slowing up the traffic, providing there is proper superelevation of the outer edge. The longer the radius, that is, the flatter the curve, the less superelevation is required, and the less the tipping sensation experienced by slow-moving vehicles on the turn. Moreover, on short curves a considerable widening of the pavement is required in order that the inner and outer edges, and therefore all traffic lanes, may have the same degree of curvature. Also, clear vision for the longer distances necessary for fast-moving traffic is easier to obtain on flat than on sharp turns.
The minimum grade of a roadway is usually a question of drainage, but the character and quantity of traffic is a determining factor in the establishment of steeper grades. Passenger cars can more easily negotiate grades than can commercial trucks. The average passenger car shifts to second gear at about a 7 per cent grade and there is very little shifting necessary on a long 6 per cent grade, hence for such cars 6 per cent may be considered a maximum for the high-speed gears. This same car will have to drop into low at about 10 per cent. Hence from the standpoint of the convenience of operating a passenger car there is no justification in going to great expense to cut a 10, or a 9, or an 8 per cent grade to a 7 per cent grade. For the average 5-ton truck 4 per cent and 8 per cent are the maximum grades for convenient running in high and intermediate. There may and possibly are many other reasons for cutting grades wherever possible. Where time is an element economy is effected by the possible speeds on grades.
The width of the roadway will likewise be influenced by the quantity and character of traffic. With slow-going wagons a width of 8 feet was sufficient for one lane of traffic, but with the automobile safety demands 10, and the Good Roads Conference of 1922 voted that no road should be less than 22.
The best type of foundation and surface is a factor of quantity and class of traffic, and while as yet all engineers do not agree, the numerous experiments now being made may lead to standardization. Just as an example may be mentioned the change that has taken place in the effect of vehicles on waterbound macadam. Under horse-drawn, iron-wheeled wagons and carriages this was considered an ideal pavement. The horses’ shoes and the iron tires wore off of the stones a sufficient amount of dust to keep the road crust well cemented. The rubber tires of the automobile do not do that; furthermore, what dust is on the road is picked up and scattered to the winds. The force of the drive wheels also is sufficient to loosen the stones and roll them from their bed, causing the roadway to ravel and disintegrate rapidly.
These arguments might be multiplied indefinitely, but enough has been given to demonstrate the value to the road planner and the road designer of a traffic census.
Methods of Taking Traffic Censuses—It is well known that the traffic on any road is not constant. A count, then, made on a single day could not be applied for an entire year. Counting every day for the year would, of course, give a correct total, but that is impracticable. It is customary, therefore, to take the count on a limited number of days and consider their average to be the average for the year. There seems to be a seasonal variation in traffic and a weekly variation. The season variation in crops affects all sorts of commercial enterprises, of which the road traffic is one. Sunday traffic is largely pleasure traffic, and is more dense as a rule than week-day traffic, which is partially pleasure and partially business. In France, where more regular and scientific censuses have been made than in any other country, the counting days are divided uniformly between the seven days of the week and the four seasons of the year, in order to eliminate, as far as possible, the periodic variation. The number of counting days per year is therefore, 28, and these are so arranged that each of the days of the week figures once in each quarter. The interval between counting days is, consequently, 13. For 1923, then, the count might be made as follows:
| Days of the Week | 1st Quarter | 2d Quarter | 3d Quarter | 4th Quarter | ||||
|---|---|---|---|---|---|---|---|---|
| Monday | Jan. | 1 | Apr. | 2 | July | 3 | Oct. | 2 |
| Sunday | 14 | 15 | 16 | 15 | ||||
| Saturday | 27 | 28 | 29 | 28 | ||||
| Friday | Feb. | 9 | May | 11 | Aug. | 11 | Nov. | 10 |
| Thursday | 22 | 24 | 24 | 23 | ||||
| Wednesday | Mar. | 7 | June | 6 | Sept. | 6 | Dec. | 6 |
| Tuesday | 20 | 19 | 19 | 19 | ||||
This method distributes the counting days uniformly over the entire year, but evidently the expense would be considerable unless it can be done by the regular patrolman. For the purpose of laying out a system of highways quicker results might be desired. In Massachusetts, Connecticut and other states two counts were made about two or three months apart. If one were taken in July, say, and another in October and the results averaged it is thought they might fairly represent the year. Each period could cover an entire week, thus giving an average of 14 days during those seasons of the year when traffic might most nearly represent the normal. Blanchard’s method177 contemplates taking the census in four periods of three days each, one in April, May, or June, one in July, one in August, and one in September or October, as local conditions may dictate. The days selected should include Sunday in order to include the heavy automobile traffic on that day. If further information is desired additional three-day periods could be taken during the winter season.
Most of the authorities seem to think that the daylight hours from 5 A.M. to 9 P.M. are sufficient. But in the light of the tests being conducted by the Illinois Highway Commission which show alternate convex and concave warping of pavements from day to night it might be well to take account of night traffic.
Some of the latest censuses, namely Iowa and Connecticut, placed scales on the highways and actually weighed the vehicles. In Connecticut road scales were used which weighed the individual wheel loads.
The observers are supplied with cards on which is printed the classified list of vehicles and animals likely to pass with columns for tallying them during the separate hours. The in-and-out-of-town vehicles are recorded separately, and, if actual weights are not taken, whether loaded or unloaded. Information relative to the weather and condition of the roads is also noted, and there are blank spaces for the station, the date, and the signature of the observer.
The station, or stations, should be so placed that the road or district will be fairly represented, since it will not be practicable to get exact data on every portion of a highway, for every turn-out, branch line, or tributary will alter results. Each station should be established in some place where a good view of the road for some little distance may be had, and where the observers may be reasonably comfortable. The number of observers will depend upon the amount of traffic and the detailed information desired. With considerable traffic it may be necessary to divide up the work, giving one set of observers the in-traffic and another the out-traffic, one man to observe passenger automobiles another trucks and delivery wagons, or one man to jack up and weigh front wheels and another rear. System will result in more accurate results, and in less loss of time for the drivers, and less congestion of traffic. It is customary to take the census over the entire system on the same days although that is not absolutely necessary.
Classification of Traffic—The object of a classification is to obtain: (a) the maximum loads and average number of heavy loads per day. (b) The lighter loads, whether horse drawn or motor driven, iron, solid rubber or pneumatic tire, trailers, traction engines, animals, harnessed and unharnessed, and any other load which might affect the wear of the roadway. (c) Any other interesting data regarding the traffic, such as, local or through, cars belonging within or without the state or county, camping parties, and so on.
Maximum wheel loads are required, primarily, to see if state regulations regarding them are being complied with. In the Connecticut census it was found that a majority of trucks were loaded beyond their rated capacity and many of them beyond the legal maximum wheel loads.
Again heavy loads stress a pavement near to its elastic limit every time they pass over it. It is well established that any structural material when stressed near, to, or beyond the elastic limit will become fatigued with repeated stresses. The higher the stress the quicker they will fail by fatigue. It is not considered good practice to stress a material, especially a brittle material, repeatedly to a point half-way to its elastic limit. (The elastic limit is that stress that may be given to a body without producing a permanent set; below which it will return to its original shape upon the removal of the stress.) A walnut may not crack at the first blow but with a sufficient number of no harder repeated blows will crack. The higher the stresses the sooner the crack occurs. So it is important to know how many loads daily are stressing a pavement near to its elastic limit. From all the loads it has been the endeavor of road engineers to work out “importance factors” that will measure the relative damage done by the several classes. The Road Board of Great Britain has adopted the British ton as a unit and calculations are based on the traffic in tons per yard of width per year or per mile.178
The British Road Board Unit Weights are as follows:
| Classification of Vehicles | Assumed Average Weight in Tons. |
|---|---|
| Ordinary cycles | 0.09 |
| Motor cycles | .13 |
| Motor cars (including motor cabs and any other motor vehicles) | 1.6 |
| Motor vans (covered) | 2.5 |
| Motor omnibuses | 6.0 |
| Motor lorries (rubber tires) | 6.0 |
| Trailers to rubber tired lorries | 5.0 |
| Motor lorries (steel tires) | 10.0 |
| Trailers to steel tired lorries | 5.0 |
| Light tractors | 5.0 |
| Trailers to light tractors | 5.0 |
| Traction engines | 12.0 |
| Trailers to traction engines | 8.0 |
| Light vehicles (one horse) | 0.4 |
| Light vehicles (two or more horses) | 0.6 |
| Heavy vehicles (one horse) | 1.25 |
| Heavy vehicles (two or more horses) | 2.5 |
| Omnibuses (two or more horses) | 3.0 |
| Tram cars (electric, steam or horse, as the case may be) | .... |
| Horses (led or ridden) | 0.5 |
| Cattle | 0.3 |
| Sheep and pigs | 0.1 |
| Hand carts and barrows | .... |
| Horses drawing vehicles (to be calculated from number of vehicles) | 0.5 |
The French unit of traffic is technically known as the “collar,” a draft animal harnessed to a wagon being counted as 1.0. The metric ton, 1000 kg., is also sometimes used. The French, feeling that the dead weight of a vehicle or animal did not truly measure its effect as to wear on a road surface, classified the traffic and assigned importance factors to the several classes. From 1882 to 1903 the classification consisted of: 1st, Trucks and farm wagons, loaded; 2d, Public vehicles designed for transporting passengers and their baggage; 3d, Light vehicles, such as private vehicles, and empty farm wagons; 4th, Larger animals, such as horses, mounted or not, mules, and large cattle; 5th, Small beasts, such as sheep, goats, and pigs. In 1903 motor vehicles were separately listed; they were divided into five classes: 1st, Metallic-tired automobiles, “which in general are heavily loaded, have a slow movement and produce the effect of wearing away the road surface”; 2d, Elastic tired automobiles licensed to make a speed of not more than 30 km. per hour; 3d, Automobiles whose speed was less than 30 km. per hour; 4th, Bicycles or velocipedes propelled by the feet of the rider; and 5th, Motor cycles, whether having two, three, or four wheels. The report of the second international road congress further states that “it is necessary to attribute to each element of the traffic an importance which belongs to it from the viewpoint of the destructive effect exercised on the road crust.” Consequently the numbers of vehicles or animals in the several classes were modified by multiplying them by importance factors arbitrarily assumed.
| Importance | Classification factors | |
|---|---|---|
| An animal harnessed to a loaded vehicle | 1 | collar |
| Loaded trucks and farm wagons | 1 | |
| Public vehicles for transporting passengers | 1 | |
| Light vehicles, and empty farm wagons | 1⁄2 | |
| Harnessed horses to light vehicle or empty wagon | 1⁄4 | |
| Mounted horses or with load on back | 1⁄4 | |
| Unharnessed horses | 1⁄5 | |
| Cattle | 1⁄5 | |
| Small beasts (sheep, pigs, goats) | 1⁄30 | |
| Automobiles with metallic tires, weight in metric tons times 11⁄4 = No. of collars | ||
| Automobiles with elastic tires | ||
| Motor cycles | 3⁄10 | |
| Vehicles licensed to make a speed over 30 km. per hour | 3 | |
| Vehicles licensed to make a speed of less than 30 km. per hour | 1 | |
From this it was possible to reduce all traffic to the unit “collar,” which was used as a comparative measure of the use of the several roadways. The tonnage was calculated by multiplying the numbers by average weights obtained by weighing a sufficiently large number of the units in each class.
Consideration was also made of the weight of the useful load as separate from the weight of the vehicle itself. Animals not harnessed were considered as a part of the useful load.
In Italy traffic censuses followed practically the same classification and methods as in France.
In the United States some of the states have used coefficients of reduction, or importance factors, while many others have contented themselves with a count of vehicles only.
In 1910 Maryland used the following:
| 1. | Ridden horse and one-horse vehicle | 2 |
| 2. | Two-horse vehicles | 4 |
| 3. | Three-horse vehicles | 6 |
| 4. | Four-horse vehicles | 8 |
| 5. | More than four-horse vehicles | 12 |
| 6. | Motor cycles | 2 |
| 7. | Motor runabouts | 10 |
| 8. | Four- or five-seat touring cars | 20 |
| 9. | Six- or seven-seat touring cars | 40 |
| 10. | Motor trucks or drays | 20 |
The New York State Highway Department took a census in 1909 in which the following classification and reduction coefficients were used:
| Class of Traffic | Relative Weight |
|---|---|
| Horse-drawn traffic | |
| Horses with vehicles | 1 |
| One-horse vehicle, light | 2 |
| One-horse vehicle, heavy | 3 |
| Two-horse vehicle, light | 3 |
| Two-horse vehicle, heavy | 4 |
| Three-horse vehicle, heavy | 5 |
| Four-horse vehicle, heavy | 6 |
| Motor vehicles | |
| Motor cycles | 1 |
| Two-passenger cars | 2 |
| Three-passenger cars | 3 |
| Four-passenger cars | 4 |
| Five-passenger cars | 6 |
| Seven-passenger cars | 7 |
| Trucks, omnibuses, etc. | 10 |
| Miscellaneous | |
| Traction engines | 15 |
| Two traction engines | 30 |
| Miscellaneous heavy traffic | 5 upward |
The Massachusetts Highway Commission, 1912 Report, say, “After all it is not numbers which tell the story, it is weight, and it is not weight alone, but the vehicle by which it is transported, whether by horses or by motor.... All these considerations are probably not so important on many road surfaces as the actual weight imposed upon the road per inch width of tire resting upon the road.” There was used in this census the following weights:
| Motors | Tons |
|---|---|
| Runabouts | 1.43 |
| Touring cars | 2.23 |
| Trucks | 6.25 |
| Horse-drawn vehicles | |
| One-horse, light | .36 |
| One-horse, heavy | 1.12 |
| Two or more horses, light | .54 |
| Two or more horses, heavy | 2.46 |
James and Reeves, with the United States Bureau of Public Roads, recommend the ton-mile basis and give the following weights:
| Tons | |
|---|---|
| One-horse wagon, loaded, 0.88; unloaded | 0.28 |
| Two-horse wagon, loaded, 1.57; unloaded | 0.47 |
| Four-horse wagon, loaded; 3.88; unloaded | 0.54 |
| Pleasure vehicles, one-horse, 0.28; two-horse | 0.47 |
| Rubber-tired pleasure vehicle | 0.28 |
| Saddle horse | 0.50 |
| Motor cycle | 0.20 |
| Excessively heavy vehicle | 3.94 |
| Motor, runabout, 1.68, touring car | 2.00 |
| Motor dray, loaded, 2.43; unloaded | 1.23 |
| Draught horses | 0.50 |
In a traffic census taken by the Borough of Brooklyn, New York, the weights were reduced to traffic units per minute per foot width of roadway which was called density. By this rule, “the number of vehicles passing a given point in eight hours times the traffic unit divided by 8 times 60 times the width of the roadway equals the density.” The weights and traffic units used were:
| Rubber-tired vehicles | Weight in tons | Traffic value | ||
|---|---|---|---|---|
| Large automobile trucks, loaded | 9 | 5 | ||
| Large automobile trucks, empty | 4 | 4 | ||
| Small automobile trucks, loaded | 3 | 3 | ||
| Small automobile trucks, empty | 1 | 1⁄2 | 2 | |
| Pleasure automobiles | 1 | 3⁄4 | 1 | |
| Carriages | 2 | 2 | ||
| Steel-tired vehicles ranged in weight from 1 to 71⁄2 tons and in traffic value from 2 to 10. | ||||
A suggested form for a traffic census sheet presented by a committee appointed to study the question of traffic censuses to the New Jersey State Association of Roads is shown on p. 245. This sheet also bears, for the use of the office, blanks for the tabulation of the traffic by classes:
| Kind of Vehicle | No. | Weight | Vehicle-Tons | |
|---|---|---|---|---|
| Motor cycle | ..... | 0.25 | .......... | |
| Light-horse, empty | ..... | 1.25 | .......... | |
| Light-horse, loaded | ..... | 2.00 | .......... | |
| Heavy two-horse, empty | ..... | 3.20 | .......... | |
| Heavy two-horse, loaded | ..... | 6.00 | .......... | |
| Light pleasure motor car | ..... | 1.50 | .......... | |
| Heavy pleasure motor car | ..... | 2.50 | .......... | |
| Light motor truck, empty | ..... | 1.00 | .......... | |
| Light motor truck, loaded | ..... | 2.50 | .......... | |
| Heavy motor truck, empty | ..... | 5.00 | .......... | |
| Heavy motor truck, loaded | ..... | 10.50 | .......... | |
| Specials: | 10 tons | ..... | ..... | .......... |
| 15 tons | ..... | ..... | .......... | |
| Over 15 tons | ..... | ..... | .......... | |
| Total | ..... | ..... | .......... | |
| Tonnage per foot width of pavement | .................... | |||
| Tonnage per foot width of roadway | .................... | |||
SUGGESTED FORM OF TRAFFIC CENSUS SHEET
Traffic Census Sheet County Number........ Station No....... County......
State Highway Department of New Jersey................................ 192...
............................................... Road at..........................................
Exact location................................................................................
Count taken..............from...........to.............from...........to..............
| Time Count Was Taken | Motor Cycle | Light Horse | Heavy Horse | Pleasure Motor Cars | Light Motor Trucks | Heavy Motor Trucks | Specials | Street Cars | Hourly Totals | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Empty | Loaded | Empty | Loaded | Light | Heavy | Empty | Loaded | Empty | Loaded | 10- tons | 15- tons | Over 15-ts. | ||||||||
| 6 | a. m. | to | 7 | a. m. | ||||||||||||||||
| 7 | a. m. | to | 8 | a. m. | ||||||||||||||||
| 8 | a. m. | to | 9 | a. m. | ||||||||||||||||
| 9 | a. m. | to | 10 | a. m. | ||||||||||||||||
| 10 | a. m. | to | 11 | a. m. | ||||||||||||||||
| 12 | noon | to | 1 | p. m. | ||||||||||||||||
| 1 | p. m. | to | 2 | p. m. | ||||||||||||||||
| .. ... ... ... .. | ||||||||||||||||||||
| 3 | a. m. | to | 4 | a. m. | ||||||||||||||||
| 4 | a. m. | to | 5 | a. m. | ||||||||||||||||
| 5 | a. m. | to | 6 | a. m. | ||||||||||||||||
| Total | ||||||||||||||||||||
Of above motor vehicles.........carried foreign licenses as follows.........
................................... Weather.....................................................
Type of pavement....................... Condition of pavement...................
Width of roadway........ Width of pavement........ Traffic.... Narrow...
Tires................... Special............... Inspector..................................
Notes............................................ Checked by...............................
Destructive Factors—From the above it appears that there is a general opinion that there should be some common measure for the destructive effect of vehicles upon road surfaces. As yet no unanimity of opinion has crystallized. While density of traffic influences the surface wear of the road crust—considerably in the case of earth and gravel, less for macadam and asphalt, and still less for brick and concrete—the actual weight of the wheel load seems to have a much greater destructive effect. The impact due to speed and irregularities of the road surface, the resiliency of the tires, the proportion of sprung to unsprung weight, and the shoving forces of the wheels all have their effects which are usually in some way connected with either the weight or the speed, or both, of the vehicle. The many experiments now being carried on by the United States Bureau of Public Roads, and the several states may furnish data from which a practical measure will some day be devised. Mr. Older, Chief Highway Engineer of the State of Illinois, under whose direction the comprehensive investigational and endurance tests under way in that state are being carried on, recently stated to a party of visitors, of which the author was one, that in his opinion weight, including impact, is the prime factor in the destruction of a pavement. Wear is of very minor importance, temperature and weather is of considerable importance.
Road surfaces must be considered as bodies acted upon by forces. Some day the stresses produced by these forces will have been analyzed, then will it be possible to standardize the importance of the several vehicle loads. At present it is known that the weight of the load and the weight of the pavement itself are under some circumstances sufficient to produce cracks in the pavement and disruption of the road crust. Bearing tests and bending tests are being devised to measure the effects of such loads. Road crusts, earth, gravel, macadam, asphalt, brick, concrete, are to varying degrees elastic bodies and when loaded they give, as an elastic band stretches, a spring shortens, or a bow bends, until the internal stresses reach a limiting point where the crust is broken or permanently distorted. It is well known that the effect on an elastic body of a suddenly applied load is twice as destructive as the same load gradually applied. And when the action is an impact the destructive effect may be very great indeed, depending on the physical properties of the impinging bodies. But however the load is applied, whenever the internal stresses reach the limiting strength of the material of which the road crust is composed it will go to pieces. The sudden application of the load by fast driving is a sort of impact. The stresses produced by this impact are now being studied. Much good is expected to come toward the solution of the problem of destructive vehicle influence from these researches.
Another effect of speed is noted on the more or less viscous materials of which road surfaces are composed. The pushing of the wheels against the surface causes wrinkles which continue to grow until the wrinkles become waves entirely across the pavement. Such waves may also be produced by expansion and contraction due to changes in temperature, but are probably always accentuated by wheel pressure. Side thrust of wheels often produces longitudinal waves in viscous road crusts.
In the classifications given no one seems to have considered the proportion of sprung and unsprung weight in the motor car. There can be no doubt but that the resiliency of the springs relieves the pavement of very much of the shock of impact. This is illustrated by an attempt to drive a nail into a springy board. It can hardly be done because the springiness of the board uses up, absorbs, the work of impact. A mechanical statement is, the work of impact equals the change in kinetic energy, or algebraically stated
Fs = Wv2⁄2g when the entire energy has been absorbed. Here F is the acting force and s the distance through which it acts, Fs, is the work done by the force F. W is the weight of the ram or moving body (vehicle, wheel load), v the velocity of impact and g the acceleration of gravity, a factor that enters the equation in the expressing of mass in terms of weight. Solving this equation for F there results,
F = Wv2⁄2gs, which shows that the smaller s is the greater the force of impact F. When s is made long by means of a spring the force F becomes smaller. This is illustrated by the old method of catching a baseball without gloves—the hands were allowed to go backward so that the work of stopping the ball was spread over a greater distance, the impact force thus becoming so small it did not sting the hands.
The effect upon the road, and also the vehicle, is like that of the hammer which hits a nail on the anvil. The nail is flattened, pounded to pieces very soon. But if the nail were not placed upon the solid anvil but upon a slab of springy steel, it might be pounded all day without doing it much harm, the spring at all times absorbing the shock. So with the weight of the vehicle largely sprung the damage to the roadway is comparatively small. Therefore, it would seem, as though a fair classification would take into account the springs of the vehicle.
The pneumatic tire, and the cushion tire and wheel, each act as springs and shock absorbers in varying degrees. In some of the censuses, pneumatic or solid tires were noted, and very many of the earlier noted whether rubber or steel tires were used.
Just how far all these things should be taken into account is questionable. Whether or not just as good results would not come for even a simpler classification is not yet determined. It might be that only the heavy loads and their frequency is all that need be considered if the destructive effect of traffic alone is aimed at.
The great amount of pleasure riding and the tremendous desire for such riding should be considered in laying out a system of roads and in the selection of a type of road, therefore all passenger cars and motor cycles should be counted and given an influence number.
Other Methods of Estimating Amount of Traffic—The amount of road traffic may be roughly estimated from the area served by the highway. Upon a map is outlined the tributary territory and its area measured by any one of several means. The area may be divided into small squares of known size and the number of squares counted; it may be divided into strips and the length of the strips measured with a scale and thence the area computed, or a planimeter may be used. Having found the area the unit tonnage is estimated from a knowledge of the character of the crops raised and the industries in the territory from which the haulage is calculated. The average haul may be determined, if desired, by finding approximately the center of gravity of the area and measuring its distance from the market. If the market place is at the center of a circle surrounding it and the products are uniformly distributed over the circle the mean distance is two-thirds the radius of the circle.
The tonnage, arising from farms, which is transported over the roads varies with the kind of crop, the fertility of the soil, the amount of stock fed, or kept for dairying, and numerous other local conditions. Studies made by various authorities179 indicate that the marketable products vary from 1⁄10 to 1⁄2 ton per acre. If a circular area with market place at the center is served by six uniformly distributing radial roads a mathematical analysis will show that the tonnage upon each one of these roads, one-sixth that from the whole circle, will be
T = 335.12qr2
| where | T | = | total tons per year, |
| q | = | yield of marketed crops in tons per acre, | |
| r | = | maximum haul-radius of the circle. |
Dividing T by the number of working days per year (usually taken as 300) gives the average daily haul into the market. The average length of haul may be taken as 2⁄3 r. The haul over any zone whose edges are concentric with the circle is considered to be all that originating in the area outside the zone plus that originating within the zone times the mean distance from the inner edge of the zone. The result of the analysis gives this equation, for the haul over any zone having an outer radius a, and an inner radius b,
H = Tr - Ta + 2a2 - ab - b2 3(a + b) (Ta - Tb), where Tr, Ta and Tb represent the tonnage originating on the sectors of radius r, a and b respectively.
For the first mile,
a = 1, b = 0.
H = Tr - 1⁄3Ta.
For the eighth mile,
a = 8, b = 7.
H = Tr - T8 + 23⁄45(T8 - T7)
Theoretical Average Tonnage of Six Uniformly Distributed Market Roads180
| Maxi- mum Haul | Aver- age Haul | Uniform Yield per Acre of | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| One-tenth Ton | One-fourth Ton | One-half Ton | ||||||||
| Total Tons per Year | Tons Hauled per day | Total Tons per Year | Tons Hauled per day | Total Tons per Year | Tons Hauled per day | |||||
| Over 1st Mile | Over 8th Mile | Over 1st Mile | Over 8th Mile | Over 1st Mile | Over 8th Mile | |||||
| 1 | 0.67 | 33 | 0.07 | 84 | 0.17 | 168 | 0.34 | |||
| 2 | 1.33 | 134 | 0.40 | 335 | 1.00 | 670 | 2.01 | |||
| 3 | 2.00 | 302 | 0.96 | 754 | 2.40 | 1,508 | 4.80 | |||
| 4 | 2.67 | 536 | 1.74 | 1,340 | 4.36 | 2,681 | 8.71 | |||
| 5 | 3.33 | 838 | 2.75 | 2,094 | 6.87 | 4,189 | 13.74 | |||
| 6 | 4.00 | 1206 | 3.98 | 3,016 | 9.95 | 6,031 | 19.90 | |||
| 7 | 4.67 | 1642 | 5.43 | 4,106 | 13.58 | 8,211 | 27.15 | |||
| 8 | 5.33 | 2145 | 7.11 | 0.85 | 5,362 | 17.76 | 2.13 | 10,724 | 35.52 | 4.25 |
| 9 | 6.00 | 2714 | 9.00 | 2.75 | 6,786 | 22.51 | 6.88 | 13,572 | 45.02 | 13.75 |
| 10 | 6.67 | 3351 | 4.13 | 4.87 | 8,378 | 27.82 | 12.18 | 16,756 | 55.63 | 24.35 |
| 11 | 7.33 | 4056 | 13.47 | 7.22 | 10,138 | 33.68 | 18.05 | 20,279 | 67.35 | 36.10 |
| 12 | 8.00 | 4826 | 16.04 | 9.79 | 12,064 | 40.10 | 24.48 | 24,128 | 80.20 | 48.95 |
| 13 | 8.67 | 5663 | 18.83 | 12.58 | 14,158 | 47.08 | 31.45 | 28,316 | 94.15 | 62.90 |
| 14 | 9.33 | 6568 | 21.85 | 15.59 | 16,420 | 54.63 | 38.98 | 32,840 | 109.25 | 77.95 |
| 15 | 10.00 | 7540 | 25.09 | 18.83 | 18,850 | 62.73 | 47.08 | 37,700 | 125.45 | 94.15 |
The table shows the theoretical average tonnage on each of six uniformly distributed radial roads. It is taken from Bulletin 136, U. S. Department of Agriculture. Since roads do not run in practice in this manner the results can only be used for comparison in confirming estimates.
Mr. E. W. James, of the Bureau of Public Roads, U. S. Dept. of Agriculture, makes an analysis of the distribution of traffic over the roads of a township located along the section lines of the United States land survey. The market place is taken at the center of the township.181
His analysis assumes the lay of the country makes all roads equally traversable and that the traffic seeks the nearest highway thence to the main traveled road east and west or north and south through the market center. This analysis shows that 4.8 per cent of the total mileage carry 39.3 per cent of the traffic; that 9.5 per cent of the roads carry 71 per cent of the traffic. In his opinion this analysis corroborates the observation of engineers to the effect that 20 per cent of the roads carry 80 per cent of the traffic. Of course the most important roads, measured in traffic, are the ones nearest the market, 15-22, 15-16, 16-21, 21-22. Following these naming only one of the four symmetrical roads, in the order of importance are 14-23, 14-13, 13-24, 13-x, 14-15, 11-12, 12-x, 12-13, 1-x, 11-14, and 1-12.
| Road between Sections | Relative Importance | ||
|---|---|---|---|
| 15 | - | 22 | 100 |
| 14 | - | 23 | 60 |
| 14 | - | 13 | 25 |
| 13 | - | 24 | 20 |
| 13 | - | x | 15 |
| 14 | - | 15 | 13 |
| 11 | - | 12 | 7 |
| 12 | - | x | 7 |
| 12 | - | 13 | 2 |
| 1 | - | x | 2 |
| 11 | - | 14 | 1 |
| 1 | - | 12 | 1 |
The same objections to this method hold as to the preceding. Local conditions always affect the travel on roads; hills, valleys, soil, drainage, nearness to other cities, railways, streams, and location of farmhouses, schoolhouses, churches, and factories, all enter into the estimate. A reconnaissance and the good judgment of the observer must supplement any method of formal procedure.
The Selection of a Suitable Type of Road—The highway plan should, if it has been carefully and scientifically made specify the type of roadway as well as the location of the highway. However, when the improvement is to be paid for by a special tax on the abutting land, it is customary to allow the taxpayers to have something to say about the type. Road engineers often object to this as being unscientific and unsound, on the theory that the layman is ignorant of the properties and behavior of road materials and that only an expert can make the proper selection. The author’s observation is, however, that hard-headed business men and farmers who have passed through the experiences of rough knocks are no more likely to make a mistake in the selection of a road type than is the young engineer fresh from the halls of college, or the engineer whose experience has prejudiced him in favor of particular types of road surfacing. The best and fairest of engineers cannot agree, then why not give the man who must pay the fiddler an opportunity to dance?
It will be well, nevertheless, for the engineer to suggest a type, or types, of roadway with his reasons for its or their suitability. If he can show that one type is superior to another the tax-payer will usually follow his advice, and agree to the type suggested. The final decision must rest with the road officials. They should know the requirements of the road, whether, for example, it is to be largely commercial or used largely for pleasure; whether durability or noiselessness is a determining factor; or whether a pleasing appearance and convenience to the inhabitants living along the way are of greater importance than directness and low grades. The decision must be made after taking all things into consideration even to the whims of the property-holders. The best road for a given location is the one which at a reasonable cost will give over a long period of time a service which is most satisfactory to the majority of its users. What is a reasonable cost and what is satisfactory service are debatable questions and usually must be compromised to a greater or less extent.
An ideal road is one that is cheap to construct and maintain, one that is durable, presents light resistance to traffic but is not slippery, is comfortable to travel and not annoying to users or dwellers along its side, and one that is easily cleaned and is sanitary. No road can contain all these qualities to the same degree, neither are they all of equal importance, but each should be given some weight in the selection.
Perhaps the first and most important item to be considered is the economic one of cheapness in construction and maintenance. In making a decision between two types of pavement the first cost will probably have more weight than will the ultimate cost. The fact that a higher priced article will last longer and in the end prove to be a saving has little charm for the man who has not the ready money to pay for the article. He will content himself with the cheaper until he can afford the better. If a community cannot pay for a certain type of road, no matter how desirable that may be, that type cannot be used. Types of roads must be selected which will utilize the materials most available. It would seem to be unwise for brick to be shipped from the Middle West to New England, or granite blocks from New England to the Middle West. Gravel, being plentiful in many states, is being used, and rightly so, more than any other road material notwithstanding the durability of a gravel roadway is less than that of many other types.
Durability is an important factor from an economical standpoint, as it enters vitally in the long-run cost of a pavement. It is also of importance on account of the infernal nuisance of having a roadway full of pot holes and rough places, to say nothing of the inconvenience to users of frequent repairs. Road officers are no more given to regarding the adage “a stitch in time saves nine,” than are other people, consequently non-durable roads are usually more or less out of order.
Durability depends upon the materials used in construction and their manipulation, proportioning, and other treatment; the character weight and density of traffic; system or lack of system in making repairs; the opening up of pavements for water, gas, and sewer or other purposes; building operations along the street; cleanliness; the absence or presence of street-car tracks; climate and possibly other factors.
Materials and Design—The physical properties of materials—their tensile, compressive, and shearing strengths, their elasticity, brittleness, etc.—while important elements in the durability of pavements, the design of the pavement, its thickness, the proportioning and mixing of parts, the laying, as well as the subgrade and its treatment are all elements that count very much also. No matter how good a material it can easily be spoiled in the handling. Some materials like vitrified brick and stone will last indefinitely on a little-used street while others like asphalt and creosoted wood block are much better for considerable wear. The use of definite and often meticulous specifications is to insure good materials and proper manipulation of the same, while the plans are carefully prepared ahead, so that durability and satisfaction may result.
The effect of character, weight, and density of traffic has been frequently mentioned and will again be referred to in what follows. There is no doubt a relationship between materials and design and the character and amount of traffic. A cinder road may be perfectly acceptable for a park drive where the traffic is light, but absolutely worthless under heavy commercial trucking.
Resistance to traffic varies with different road surfaces. A smooth hard surface offers a very great deal less resistance than does a rough or soft surface. To illustrate, a horse is said to be able to pull directly on the traces one-tenth his own weight without being overworked. With a resistance of 100 pounds per ton (earth road in medium condition) a team of horses weighing 1200 pounds each could draw over a level road 2 × 1200⁄10 × 100 = 2.4 tons. On a concrete, asphalt or brick pavement having a tractive resistance of 30 pounds per ton the team could draw 2 × 1200⁄10 × 30 = 8 tons. In other words the load that can be drawn is inversely as the tractive resistance. Here speed was not considered. It was the natural walking gait of the horse about three miles per hour. If the speed is greater the load must be cut down proportionally. With a truck the direct pull is the effective power of the engine in foot-pounds per minute divided by distance in feet per minute; and the load that can be drawn is the direct pull times the tractive resistance. Thus if a truck may exert h effective horse power = 33,000h foot-pounds per minute, and the speed is v miles per hour, the load T, in tons, that may be hauled on a road having a tractive resistance of t pounds per ton, is
T = 33,000h⁄5280v60·t = 375h⁄vt.
Therefore a truck of 20 effective horse-power will haul over a road whose tractive resistance is 100 pounds per ton at a speed of 10 miles per hour a load of
T = 375 × 20⁄10 × 100 = 7.5 tons; and on a smooth road with a tractive resistance of 30 pounds per ton at the same speed, 25 tons, or the same load 7.5 tons may be drawn at a speed of 331⁄3 miles per hour.
It must be remembered that when the speed is increased the tractive resistance is likewise increased. The air resistance is in about the ratio of the square of the velocity, so that 33 miles per hour would be too great in the last case.
Experiments to determine the tractive resistance due to the surface vary considerably, for it is impossible to secure like conditions of surface smoothness and cleanliness, to say nothing of hardness. The tractive resistance will with some materials vary with the temperature. That of sheet asphalt, for example, may be twice as much in summer as in winter. The tractive resistance may not be directly proportional to the load although it is customary to express it in pounds per ton. It is conceivable that a heavy load because it sinks into the road crust may require a greater number of pounds to move it than a light load that does not greatly sink in. This also leads to the effect of width of tire and diameter of wheel. Many experiments have shown the tractive force to be less with wide than narrow tires, due, no doubt, to the unequal sinking into the road crust. Likewise wheels ought, for the same reason, to show less resistance for large diameters; in fact some engineers give it as varying inversely as the diameter of the wheel.
The results of tests, while varying much, show in a general way, the direct pull necessary to draw a load at slow speed on the level in well-lubricated wagons to be approximately as follows:
| Lbs. per Ton | μ = coefficient of Resistance | |
|---|---|---|
| Upon Steel rails | 10 | 1⁄200 |
| Sheet asphalt, good condition | 20 | 1⁄100 |
| Asphaltic macadam or concrete, good condition | 20 | 1⁄100 |
| Concrete, good condition | 20 | 1⁄100 |
| Brick, good condition | 20 | 1⁄100 |
| Broken stone water-bound macadam, good condition | 30 | 3⁄200 |
| Gravel, good condition | 30 | 3⁄200 |
| Sand clay, good condition | 60 | 3⁄100 |
| Earth, best condition | 67 | 1⁄30 |
| Earth, medium condition | 100 | 1⁄20 |
| Earth, poor condition | 300 | 3⁄20 |
Resistance Due to Grade—The resistance due to grade is just as marked as that due to surface. The work necessary to draw a load up an inclined plane is the same as that of drawing on a level along the base of the plane and lifting it directly up to the height of the plane. A mathematical analysis182 based upon this fact leads to the formulas:
For a horse-drawn load,
L = t-g⁄μ +g H. (1)
For a tractor,
L = P⁄μ +g - T. (2)
For an automobile or truck,
L = P⁄μ +g, (3)
| where | L | = | weight of load drawn, including weight of vehicle (subtract weight of vehicle for net load); |
| H | = | weight of horse; | |
| T | = | weight of tractor; | |
| P | = | effective tractive force exerted (available engine effort); | |
| μ | = | coefficient of road resistance; | |
| g | = | grade (gradient) = tangent of angle of incline, nearly the same for small angles as the sine of the angle of incline, that is, the height of the incline divided by its length; | |
| t | = | the direct pull of the horse divided by the weight of the horse; | |
| h | = | horse-power = work of 33,000 ft.-lb. per minute. | |
| v | = | velocity in miles per hour. |
Equation (3) indicates that the load, including its own weight, that a truck or an automobile can draw varies directly as the horse-power exerted effectively, and inversely as the velocity. Also it decreases as the coefficient of road resistance, μ, and the gradient g increases.
The resistance coefficient, μ may include axle or internal resistance of the vehicle plus road surface resistance plus air resistance. The axle resistance is nearly a constant, the road resistance likewise, but the air resistance depends upon the speed v, varying approximately as the square of the velocity. W. S. James, in the Journal of the Society of Automotive Engineers, June, 1921, uses the formula
F = CAV2
| where | F | = | the wind force in pounds; |
| C | = | a constant, varies from.003 to.004; | |
| A | = | frontal area of automobile in square feet approximately 26; | |
| V | = | velocity in miles per hour. |
His researches show that the available engine effort P of equation (3) or horse power h is not quite constant but varies with the speed. His table follows:
| Car Speed m.p.h. | Available Engine Effort Per 1000 lb. of Car Weight, Lbs. | Air Resistance Per 1000 lb. of Car Weight, Lbs. |
|---|---|---|
| 15 | 107.3 | 4.9 |
| 16 | 105.2 | 6.8 |
| 20 | 107.6 | 8.8 |
| 25 | 106.0 | 13.4 |
| 30 | 103.9 | 19.2 |
| 35 | 101.2 | 26.0 |
| 40 | 98.0 | 34.1 |
| 45 | 94.1 | 43.4 |
| 50 | 86.8 | 53.8 |
Returning to Equation (3) which has been plotted in two different ways on page 260, it may be seen that the load that can be hauled up a grade decreases with the per cent of grade very rapidly for the roads having a small coefficient of resistance and very much less rapidly for larger resistances. For example, on steel rails, resistance 10 pounds per ton, μ = 1⁄200, a 1 per cent grade reduces the load to one-third the load that may be hauled on the level, and a 5 per cent grade reduces it to less than one-tenth of the same load. With a good asphalt, brick or concrete road, resistance 20 pounds per ton, μ = 1⁄100, a 1 per cent grade reduces the load to one-half, while a 5 per cent grade reduces it to about one-sixth the load that can be drawn on a level road. While for an earth road in bad condition or a dry sand road, 300 pounds per ton resistance, μ = 3⁄20, a five per cent grade only reduces the level grade load by one-fourth. This shows clearly that the better the road surface the less the grade must be in order to benefit by it. The plots on page 260 show the same thing in different ways, and also that the maximum load that can be hauled with a given force at a constant speed is greater, no matter what the grade, on the better types of roads than on the poorer, but that the very great advantages due to hard roads come with the better type of roads. Incidentally this plot shows that the load that may be hauled, other things being equal, on steel tracks, is very much greater than that that can be hauled on the best hard surfaced road with same power, therefore it will never be possible to haul loads on highways as cheaply as on railways unless the operating expenses on the highways can be made materially less than on railways.
Slipperiness—Road surfaces which become slippery not only decrease the tractive effort of horses and motors but are very dangerous also. Non-slipperiness ought then to be given weight in the selection of the type of roadway. Observations in London in 1873 by Heywood on slipperiness of pavements indicated granite-block most slippery, then asphalt and wood-block. Greene, in 1885, analyzing a series of observations made in the principal cities of the United States, gave the order of slipperiness as wood-block, granite-block, and sheet-asphalt.
Slipperiness increases with grade. A special committee upon road materials of the American Society of Civil Engineers183 recommend the following maximum grades for various kinds of pavements:
| Kinds of Roadway | Maximum Grade Per Cent |
|---|---|
| Gravel | 12 |
| Broken stone | 12 |
| Bituminous surface | 6 |
| Bituminous macadam | 8 |
| Bituminous concrete | 8 |
| Sheet asphalt | 5 |
| Cement concrete | 8 |
| Brick, cement grout filler | 6 |
| Brick, bituminous filler | 12 |
| Stone-block, cement grout filler | 9 |
| Stone-block, bituminous filler | 15 |
| Wood-block | 4 |
This would indicate that in the belief of the committee slipperiness is about in the inverse ratio of the grades. Those on which the steepest grades are allowed being the least slippery.
Climatic conditions affect slipperiness. Roads which are non-slippery in dry weather may be very slippery in wet weather. Pavements having a small amount of clay or earth on them are quite slippery when dampened, but after a hard rain may be much less slippery. Earth roads that have been thoroughly dragged are much more slippery immediately after a small shower than after a hard or soaking rain. Stone blocks and brick are worse after they have worn turtle-backed. Ice and sleet render all pavements slippery, but some more than others.
Sanitariness—The sanitariness of a road is the measure of the effect it has on the health of its users and the dwellers along its side. A dusty road is ordinarily an unsanitary one because of the germs of disease carried on the dust particles and which may be widely spread by the wind. An earth or gravel road when not dry or dusty is a sanitary road. A concrete or asphalt pavement when clean is very sanitary, but because dirt and debris brought upon it soon becomes ground into dust may become more unsanitary than an earth road. Mud, when clean, if that expression may be allowed, is sanitary, but when mixed on the road with the droppings of animals, sputum and other unclean things may become very unsanitary.
Noisiness—Noisiness is a real source of disease, especially mental disorders. The less noisy types of pavement are usually laid in front of hospitals.
Acceptability—The acceptability of a roadway depends in addition to the things mentioned on its looks, appearance, esthetics; on the degree of heat and light which it reflects; upon its springiness and comfortableness to travel over as well as its easiness upon horses’ feet and rubber tires.
Some Types of Roads and Their Qualities—Earth Roads.—The good qualities are: low first cost, not slippery, noiseless, easy on horses’ feet and on rubber tires, comfortable when in first-class condition. The poor qualities are: high tractive resistance, not durable, high cost of maintenance when traffic becomes dense, requiring constant attention to be kept in good condition, difficult to clean, muddy in wet weather, dusty in dry weather, choppy when dust blows away, rut easily, wear down rapidly under heavy traffic especially in windy localities, uncomfortable except when in prime condition. Adaptability: Satisfactory for light or medium traffic when properly drained and constantly maintained. It will probably pay to put in better roads when the traffic amounts to more than 400 vehicle-tons per day.
Sand-clay Roads.—The good and poor qualities are about the same as for earth roads. In fact they are earth roads with a selected mixture of sand and clay. They are more durable, harder and smoother than the ordinary earth road. They are appropriate for a light or medium traffic and are especially adaptable for sandy stretches or over clay or gumbo soils. The cost will depend upon the availability of materials; the cost of maintenance should be no more or very little more than earth roads. They should be good up to 800 vehicle-tons per day.
Gravel Roads.—The good qualities are: moderately hard, compact, and smooth, not slippery, noiseless, easy on horses’ feet, and not very hard on tires, not muddy, are comfortable, and low in first cost. Poor qualities: rut rather easily and require constant attention to keep them in first-class condition, dusty in dry weather. Gravel sometimes becomes loose on top and rolls under fast moving vehicles, causing skidding. When not thoroughly compacted gravel roads have high tractive resistance. They are particularly well adapted to country roads under medium traffic, especially where gravel may be obtained at a reasonable cost near at hand. At the present time more miles of gravel roads than of any other type of surface are being constructed in the United States. This is because of their low first cost and general satisfactory character for medium traffic.
Macadam Roads.—Moderate first cost and when well compacted smooth but not slippery. They require new dust continually to keep the stones cemented together. Under rubber tires the dust is not worn off the stones and what little there is on the roadway is picked up and spread to the winds. If covered with tar or asphaltic oil the stones cement together and form excellent roadways under medium traffic, where there are no extremely heavy trucks to cut through the surface. Traffic up to 1200 vehicle-tons per day is accommodated well by these roads.
Bituminous macadam roads are ordinary macadam roads impenetrated with bituminous materials. When well made they are excellent roadways, and unless extremely heavy trucking comes upon them ought to prove satisfactory for medium to moderately heavy traffic.
Bituminous Concrete Roads are made of broken stone mixed with a bituminous cement before laying and rolling. They, like bituminous macadam, are smooth, non-slippery, easy riding, have small tractive resistance and the first cost and cost of maintenance are moderate. Such roads have proven very satisfactory where the traffic is dense but not composed of real heavy units. On account of their dustlessness and general sanitary character as well as for their durability they are deservedly popular.
Brick Roads.—Vitrified paving brick give a hard durable surface, reasonably smooth and not slippery. The cost of maintenance is low and the appearance is good. Brick roads are expensive as a heavy concrete foundation is necessary, and they are noisy. They are well adapted for heavy hauling.
Concrete Roads.—This type of roadway is rapidly forging to the front. With the exception of gravel it leads in mileage of hard-surfaced roads. When made of good concrete sufficiently thick it has proven itself to be durable, hard, smooth, of small tractive resistance, comfortable, and not particularly expensive in first cost or maintenance.
With horse-drawn iron-tired vehicles it is doubtful if it would prove as durable as some other types but for rubber tired motorized vehicles it seems to be extremely well adapted. There is no doubt but that this type will continue to be popular. It has a tendency to crack under the action of temperature and moisture. It is customary to fill these cracks with tar, pitch or asphalt, giving an appearance which some people think not pleasing. The pavement is rigid and noisy, therefore objectionable for some localities.
Creosoted Wood Block Roads.—Wood blocks treated with creosote to preserve them from decay make an excellent pavement. They are smooth, durable, noiseless and sanitary, have small tractive resistance and are comfortable to ride upon. The principal objection is their habit of “bleeding” in the summer time. The sticky oil tar that oozes out is very objectionable, as it adheres to shoes and is tracked into houses. The first cost is considerable, but maintenance is low for many years after laying. Wood block roadways seem well adapted for bridge floors, for stable and shop floors, and for heavy teaming when placed on a substantial concrete foundation. They seem to last better for a moderate or semi-heavy use; when left idle they are more subject to decay.
Asphalt Block Roads have proven satisfactory for both country and city roads where the traffic is reasonably heavy. They are laid on both cement concrete and asphaltic concrete bases. They are smooth, easy riding, have light tractive resistance, are not very noisy, and are sanitary. The dark color is rather pleasing.
Sheet Asphalt Roads and Streets, considering their cost, durability, smoothness, ease of riding, low tractive resistance, and general acceptability, are among the most popular roads. What has been said of sheet asphalt will apply to asphaltic concrete of the Topeka specification and bitulithic types. The road is better for use. The asphalt and sand surface has the habit of swelling and cracking when not used. The proportioning and laying of a sheet asphalt surface is a particular job and requires a person of technical knowledge and experience to do it properly. Sheet-asphalt pavements seem well adapted for city streets and roads where there is a medium or dense traffic. With a firm foundation it stands up well under the heaviest traffic. Its popularity is truly deserved. The pavement under some conditions of moisture is inclined to be slippery but when dry is not. Neither is it very noisy.
Miscellaneous.—There are numerous other types of roads that have their proper uses in many localities. Burned clay, shell, furnace slag, coal slack, cinders, plank, corduroy, hay, bagasse, and possibly other materials have and will continue to be used with more or less success. The proper places for their use will depend upon local conditions which every good engineer always takes into account before deciding upon a type of roadway.
Comparison of Roads—In order to compare the relative merits of different types of roads weights are usually given to the different qualities entering into the roadway that they may be compared with a predetermined ideal. It must be remembered that such tables apply only to the particular road for which they are made out. No two can be exactly alike.
Here is one adapted from the author’s work on “Highway Engineering.”184
Comparative Table of Several Types of Roadway for Some Particular Locality
| Qualities | Ideal Road for this Particular Location | Best Earth Road | Sand Clay Road | Gravel Road | Mac- adam Road | Brick Road | Con- crete Road | Asphalt Block | Cre- osoted Wood Block | Bitu- minous Con- crete | Sheet Asphalt |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Low first cost | 20 | 20 | 16 | 16 | 15 | 10 | 12 | 10 | 8 | 14 | 13 |
| Low cost of maintenance | 20 | 15 | 15 | 10 | 8 | 9 | 8 | 8 | 10 | 8 | 10 |
| Ease of traction | 10 | 1 | 4 | 6 | 8 | 10 | 10 | 9 | 9 | 9 | 10 |
| Non-slipperiness | 10 | 9 | 9 | 9 | 9 | 8 | 5 | 5 | 5 | 5 | 5 |
| Noiselessness | 5 | 5 | 5 | 5 | 4 | 1 | 1 | 2 | 4 | 2 | 2 |
| Healthfulness | 10 | 5 | 5 | 6 | 8 | 9 | 9 | 9 | 8 | 9 | 9 |
| Freedom from dust and mud | 10 | 1 | 2 | 3 | 4 | 9 | 9 | 9 | 9 | 9 | 9 |
| Comfortable to use | 10 | 3 | 4 | 5 | 6 | 8 | 8 | 9 | 9 | 9 | 9 |
| Appearance | 5 | 2 | 3 | 3 | 4 | 3 | 4 | 5 | 5 | 5 | 5 |
| Total | 100 | 61 | 63 | 63 | 66 | 69 | 66 | 66 | 67 | 70 | 72 |
Tilson gives the following weights for city pavements having heavy traffic:185
| Pavement Qualities | Per- cent- age | Gran- ite Block | Wood Block | Brick | Sheet As- phalt | Bitu- lithic |
|---|---|---|---|---|---|---|
| Cheapness | 14 | 8 | 8 | 13 | 14 | 12 |
| Durability | 21 | 21 | 16 | 12 | 15 | 15 |
| Easiness of cleaning | 15 | 10 | 14 | 15 | 14 | 14 |
| Light resistance to traffic | 15 | 13 | 14 | 15 | 11 | 12 |
| Non-slipperiness | 7 | 7 | 4 | 6 | 5 | 6 |
| Ease of maintenance | 10 | 10 | 8 | 6 | 6 | 6 |
| Favorableness to travel | 5 | 2 | 5 | 3 | 4 | 4 |
| Sanitariness | 13 | 9 | 13 | 10 | 12 | 12 |
| Total | 100 | 80 | 82 | 80 | 81 | 81 |
| Less cheapness | 72 | 74 | 67 | 67 | 69 |
The Forest Service of the U. S. Department of Agriculture presents the following table:
| Pavement Qualities | Per- centage | Granite Block | Sheet Asphalt | Brick | Mac- adam | Wood Block | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Cheapness | 14 | 4 | 6 | 1⁄2 | 7 | 14 | 4 | 1⁄2 | |||
| Durability | 20 | 20 | 10 | 12 | 1⁄2 | 6 | 14 | ||||
| Ease of maintenance | 10 | 9 | 1⁄2 | 7 | 1⁄2 | 8 | 1⁄2 | 4 | 1⁄2 | 9 | 1⁄2 |
| Ease of cleaning | 14 | 10 | 14 | 12 | 1⁄2 | 6 | 14 | ||||
| Low resistance to traffic | 14 | 8 | 1⁄2 | 14 | 12 | 1⁄2 | 8 | 14 | |||
| Non-slipperiness | 7 | 5 | 1⁄2 | 3 | 1⁄2 | 5 | 1⁄2 | 6 | 1⁄2 | 4 | |
| Favorableness to travel | 4 | 2 | 1⁄2 | 4 | 3 | 3 | 3 | 1⁄2 | |||
| Acceptability | 4 | 2 | 3 | 1⁄2 | 2 | 1⁄2 | 2 | 1⁄2 | 4 | ||
| Sanitary qualities | 13 | 9 | 13 | 10 | 1⁄2 | 4 | 1⁄2 | 12 | 1⁄2 | ||
| 100 | 71 | 76 | 74 | 1⁄2 | 55 | 80 | |||||
Crosby gives three sets of ideal crusts for country roads: V for main roads, carrying a fairly heavy mixed traffic, W, secondary roads carrying moderate traffic, and X on minor roads with light farm travel almost wholly.186
| Components | Ideal | Brick | Plain Cement Con- crete | Bitumi- nous Mac- adam | Water- bound Mac- adam | ||
|---|---|---|---|---|---|---|---|
| V | W | X | |||||
| First cost, cheapness | 15 | 15 | 15 | 8 | 10 | 10 | 15 |
| Maintenance, cheapness | 25 | 25 | 20 | 25 | 20 | 20 | 10 |
| Durability | 7 | 7 | 7 | 7 | 5 | 5 | 3 |
| Ease of maintenance | 8 | 10 | 10 | 7 | 8 | 8 | 10 |
| Cleanliness | 5 | 5 | 5 | 3 | 3 | 5 | 2 |
| Low tractive resistance | 10 | 5 | 5 | 5 | 4 | 4 | 4 |
| Non-slipperiness | 10 | 10 | 10 | 4 | 7 | 5 | 10 |
| Sanitariness | 5 | 5 | 5 | 4 | 4 | 5 | 3 |
| Noiselessness | 5 | 5 | 5 | 3 | 3 | 5 | 4 |
| Acceptability | 5 | 5 | 8 | 2 | 3 | 4 | 5 |
| Favorableness to travel | 5 | 8 | 10 | 3 | 5 | 6 | 8 |
| Total | 100 | 100 | 100 | 71 | 72 | 77 | 74 |
Anderson gives the following economical table to assist in arriving at a proper type of surfacing:187
Method of Making Economical Comparison of Road Surfaces
| Item | Possible Types of Surfacing | ||
|---|---|---|---|
| A | B | C | |
| Estimated life of surface with proper maintenance, years | 4 | 8 | 12 |
| Original construction cost per mile | $ 8,000 | $15,000 | $30,000 |
| Annual charges for interest, depreciation and resurfacing | 2,364 | 2,528 | 3,797 |
| Cost of maintaining surface per mile, average, annual | 1,000 | 750 | 200 |
| Total cost per mile at end of 12th year, period | 40,368 | 39,336 | 47,964 |
| Value of road surface per mile at end of 12th year period | .... | 7,500 | 12,000 |
| Net outlay per mile of road | 40,368 | 32,836 | 35,964 |
The choice of selection here is evidently between B and C, with the figures so close together that the one with the least number of uncertainties would probably be adopted if economy is the determining factor.
Another method of making economical comparisons is shown in the table and plot following:
| Item | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Earth Road | Gravel Road | Bitumi- nous Macadam and concrete | Portland Cement Concrete | Sheet Asphalt Bitulithic | Brick Stone Block | Wood Block | |
| First cost per mile | $1,000 | $5,000 | $10,000 | $20,000 | $30,000 | $40,000 | $50,000 |
| Annual Interest, 5 per cent | 50 | 250 | 500 | 1,000 | 1,500 | 2,000 | 2,500 |
| Annual Maintenance | 250 | 250 | 500 | 100 | 100 | 50 | 50 |
| Life of surface, yrs. | 0 | 5 | 10 | 20 | 20 | 25 | 25 |
| Cost of resurfacing | $ 0 | $2,500 | $ 5,000 | $15,000 | $15,000 | $25,000 | $35,000 |
| Annual Sinking Fund 31⁄2 per cent | 0 | 466 | 427 | 530 | 530 | 884 | 899 |
| Annual Total Cost | 300 | 966 | 1,427 | 1,630 | 2,130 | 2,934 | 3,449 |
| Daily Cost, per mile | 0.82 | 2.74 | 3.90 | 4.45 | 5.84 | 8.03 | 9.46 |
Plot showing cost of several types of roads under varying traffic density. When the traffic density of road No. 1 (Earth and sand clay) becomes greater than 300 or 400 vehicles per day the curve would turn up because the maintenance costs would be increased. Similarly for Nos. 2 and 3 for 1600 to 2000 vehicles per day.
Selected References
“American Civil Engineers’ Pocket-Book,” Sec. 15, Art. 4, John Wiley & Sons, New York.
“American Highway Engineers’ Handbook,” p. 1360, John Wiley & Sons, New York.
American Society of Civil Engineers, Proceedings, 1918, p. 2327.
Anderson, Andrew P., “Modern Road Building and Maintenance.” Hercules Powder Co., Chicago.
Automotive Industries, “The Motor Bus Field as a Market for Trucks,” Vol. XLV, pp. 627-628, Sept. 29, 1921; “Weight of Trucks,” May 18, 1922.
Blanchard and Drowne, “Textbook of Highway Engineering,” Chap. II, John Wiley & Sons, New York.
Bullard, General Robert Lee, “The Motor Truck’s Importance on the Battle Front of France,” National Automobile Chamber of Commerce, New York.
Chatburn, George R., “Highway Engineering—Rural Roads and Pavements,” pp. 22-28; John Wiley & Sons, New York.
Collins, J. A., “Transportation Surveys for Rural Express Routes,” Good Roads, March 17, 1919.
Cornell Agricultural College Bulletin No. 205; Ithaca, New York.
Crissey, Forrest, “Our New Transportation System,” Saturday Evening Post, December 16, 1922, p. 14.
Crosby, W. W., “The Scientific Selection of Pavements,” Municipal Journal, May 29, 1913.
Dalton, James C., “Highways Must Be Made Self-supporting,” Automotive Industries, May 25, 1922.
Good Roads.—“Benefits of a National Highway System,” A committee report of the American Road Builders Association, Jan. 19, 1919.
Haydock, Winters, “The Pittsburgh Traffic Count,” Proceedings of the Engineering Society of Western Pennsylvania, Vol. XXVII, pp. 477-513.
Hirst, A. R., “Laying out Wisconsin Trunk Line Highways,” Good Roads.
Horine, M. C., “Economics of Motor Transport,” Journal of the Society of Automotive Engineers, May, 1922.
James, E. W., “Distribution of Traffic on a Rectangular System,” Engineering Record, Vol. LXXIV, p. 439.
Johnson, A. N., “The Traffic Census,” Public Roads, Dec. 1920, Appendix; also p. 16.
“Traffic Census and its Use in Deciding Road Width,” Public Roads, July, 1921, p. 7.
Jadwin, Colonel Edgar, “Relation of the War Department to Improved Highways.” Bulletin No. 25 of the Texas Engineering Experiment Station, Agricultural and Mechanical College of Texas, May 1, 1922, p. 40.
MacDonald, “Classification and Uses of Highways,” Engineering News-Record, Vol. LXXXIII, pp. 984-985, 635.
Massachusetts Highway Commission Report, 1912.
Simonds, Frank H., “History of the World War,” Vol. I, p. 118, Vol. V, p. 115. Doubleday, Page & Company, New York.
New Jersey State Highway Commission, Committee Report on Traffic Census—Engineering News-Record, Vol. LXXXVI, p. 338.
Taylor, Colonel B., “Similarity of Military and Commercial Motor Transportation,” National Automobile Chamber of Commerce, New York.
United States Bureau of Public Roads, “A Study of the California Highway System,” Public Roads, pp. 124, 136-138, 196-197, 200-209.
United States Census Reports.
United States Department of Agriculture, Bureau of Statistics Bulletin 49. Bureau of Forestry Bulletin.
Footnotes
173 Highway is sometimes used in the sense of greater importance and road in that of less, as in the expression “highways and roads.” Baker in his “Roads and Pavements” uses roads to indicate unpaved highways.
174 See Engineering News Record, Vol. LXXXIII, p. 985.
175 “Economies of Motor Transport,” by Merrill C. Horine, Engineer International Motor Company, New York City, in the Journal of the Society of Automotive Engineers, May, 1922.
176 See Simonds’ “History of the World War,” Vols. I and V.
177 “Am. Civ. Eng’s. Pocketbook,” Sec. 15, Art. 4, Wiley & Sons, N. Y.
178 Report of Third International Road Congress, 1913.
179 Bulletin 205, Cornell Agricultural College; Bulletin 136, U. S. Department of Agriculture; Bulletin 49, Bureau of Statistics, U. S. Dept. of Agr. Reports of the 1910 U. S. Census.
180 From Bulletin 136, U. S. Department of Agriculture.
181 Engineering Record, Vol. LXXIV, p. 439.
182 See “Highway Engineering,” by G. R. Chatburn, pp. 22 to 28, Wiley & Sons, New York, publishers.
183 Am. Soc. C. E. Proceedings, 1918, p. 2327.
184 “Highway Engineering—Rural Roads and Pavements,” by George R. Chatburn, John Wiley & Sons, New York.
185 “American Highway Engineers’ Handbook,” p. 1360, Wiley & Sons, New York.
186 “The Scientific Selection of Pavements,” by W. W. Crosby, in Municipal Journal, May 29, 1913.
187 “Modern Road Building and Maintenance,” by Andrew P. Anderson.