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: Weight in Traffic Rubber-tired vehicles tons value Large automobile trucks, loaded 9 5 Large automobile trucks, empty 4 4 Small automobile trucks, loaded 3 3 Small automobile trucks, empty 1¹⁄₂ 2 Pleasure automobiles 1³⁄₄ 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 |Motor| Light | Heavy | Pleasure | Was Taken |Cycle| Horse | Horse | Motor | | | | | Cars | | +-----+------+-----+------+-----+-----+ | | | | | | | | | |Empty|Loaded|Empty|Loaded|Light|Heavy| --------------------+-----+-----+------+-----+------+-----+-----+ 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 | | | | | | | | --------------------+-----+-----+------+-----+------+-----+-----+ --------------------+------------+------------+ Time Count | Light | Heavy | Was Taken | Motor | Motor | | Trucks | Trucks | |-----+------+-----+------+ | | | | | |Empty|Loaded|Empty|Loaded| --------------------+-----+------+-----+------+ 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 | | | | | --------------------+-----+------+-----+------+ --------------------+----------------------+------+------ Time Count | |Street|Hourly Was Taken | | Cars |Totals | Specials | | |-------+-------+------+ | | | | Over | | |10-tons|15-tons|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 _Wv_² _Fs_ = ----- 2_g_ 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, _Wv_² _F_ = -----, 2_gs_ 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 authorities[179] 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_ = 335.12_qr_² 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_ = _T__{_r_} - _T__{_a_} + 2_a_² - _ab_ - _b_² -------------------(_T__{_a_} - _T__{_b_}), 3(_a_ + _b_) where _T__{_r_}, _T__{_a_} and _T__{_b_} represent the tonnage originating on the sectors of radius _r_, _a_ and _b_ respectively. For the first mile, _a_ = 1, _b_ = 0. _H_ = _T__{_r_} - ¹⁄₃(_T__{_a_}). For the eighth mile, _a_ = 8, _b_ = 7. _H_ = _T__{_r_} - _T__{8} + ²³⁄₄₅(_T__{8} - _T__{7}) THEORETICAL AVERAGE TONNAGE OF SIX UNIFORMLY DISTRIBUTED MARKET ROADS[180] -----+-----+-------------------------------------------------------- | | UNIFORM YIELD PER ACRE OF | +-----------------+------------------+------------------- | | One-tenth Ton | One-fourth Ton | One-half Ton | +-----+-----------+------+-----------+------+------------ | | |Tons Hauled| |Tons Hauled| |Tons Hauled | | | per day | | per day | | per day | |Total+-----+-----+ Total+-----+-----+ Total+------+----- Maxi-|Aver-|Tons |Over |Over | Tons |Over |Over | Tons | Over |Over mum | age | per | 1st |8th | per |1st | 8th | per | 1st |8th Haul| Haul|Year |Mile |Mile | Year |Mile |Mile | Year | Mile |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] [Illustration: Graphic representation of distribution of traffic on roads located along section lines.] 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| Relative Sections |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. [Illustration: © _Underwood and Underwood_ GIVING A MACADAM ROAD AN APPLICATION OF TARVIA BINDER This is Followed by a Coat of Screenings and then the Road is Rolled Again.] [Illustration: © _Underwood and Underwood_ A ROAD OF MIXED ASPHALT AND CONCRETE BEING TESTED OUT] 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 -------- = 2.4 tons. 10 × 100 On a concrete, asphalt or brick pavement having a tractive resistance of 30 pounds per ton the team could draw 2 × 1200 -------- = 8 tons. 10 × 30 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 = 33,000_h_ 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 33,000_h_ 375_h_ _T_ = ------------- = ------. 5280_v_ _vt_ ------- · _t_ 60 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 375 × 20 _T_ = -------- = 7.5 tons; 10 × 100 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: ----------------------------------------+------------+--------------- | |μ = coefficient |Lbs. per Ton|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 analysis[182] based upon this fact leads to the formulas: For a horse-drawn load, _t_ - _g_ _L_ = ---------_H_. (1) μ + _g_ For a tractor, _P_ _L_ = ------- - _T_. (2) μ + _g_ For an automobile or truck, _P_ _L_ = -------, (3) μ + _g_ 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_ = _CAV_² 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: ---------+-----------------------+--------------------------- |Available Engine Effort| Car Speed| Per 1000 lb. of Car |Air Resistance Per 1000 lb. m.p.h. | Weight, Lbs. | 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. [Illustration: Graphical representation of the effect of grade on the load that can be drawn.] [Illustration: Graphical representation of the effect of road resistance on the load that may be drawn.] =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 Engineers[183] recommend the following maximum grades for various kinds of pavements: --------------------------------+------------- |Maximum Grade Kinds of Roadway | 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. [Illustration: © _Underwood and Underwood_ CROWNING A CALIFORNIA DIRT ROAD WITH TRACTOR DRAWN GRADER] =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 -----------------------+----------+-----+----+------+-------+-----+ |Ideal Road| | | | | | Qualities | for this |Best |Sand| | | | |Particular|Earth|Clay|Gravel|Macadam|Brick| | Location |Road |Road| Road | Road |Road | -----------------------+----------+-----+----+------+-------+-----+ Low first cost | 20 | 20 | 16 | 16 | 15 | 10 | Low cost of maintenance| 20 | 15 | 15 | 10 | 8 | 9 | Ease of traction | 10 | 1 | 4 | 6 | 8 | 10 | Non-slipperiness | 10 | 9 | 9 | 9 | 9 | 8 | Noiselessness | 5 | 5 | 5 | 5 | 4 | 1 | Healthfulness | 10 | 5 | 5 | 6 | 8 | 9 | Freedom from dust and | | | | | | | mud | 10 | 1 | 2 | 3 | 4 | 9 | Comfortable to use | 10 | 3 | 4 | 5 | 6 | 8 | Appearance | 5 | 2 | 3 | 3 | 4 | 3 | +----------+-----+----+------+-------+-----+ Total | 100 | 61 | 63 | 63 | 66 | 69 | -----------------------+----------+-----+----+------+-------+-----+ -----------------------+--------+-------+---------+----------+------- | | | | | | | |Creosoted| | |Concrete|Asphalt| Wood |Bituminous| Sheet Qualities | Road | Block | Block | Concrete |Asphalt -----------------------+--------+-------+---------+----------+------- Low first cost | 12 | 10 | 8 | 14 | 13 Low cost of maintenance| 8 | 8 | 10 | 8 | 10 Ease of traction | 10 | 9 | 9 | 9 | 10 Non-slipperiness | 5 | 5 | 5 | 5 | 5 Noiselessness | 1 | 2 | 4 | 2 | 2 Healthfulness | 9 | 9 | 8 | 9 | 9 Freedom from dust and | | | | | mud | 9 | 9 | 9 | 9 | 9 Comfortable to use | 8 | 9 | 9 | 9 | 9 Appearance | 4 | 5 | 5 | 5 | 5 +--------+-------+---------+----------+------- Total | 66 | 66 | 67 | 70 | 72 -----------------------+--------+-------+---------+----------+------- Tilson gives the following weights for city pavements having heavy traffic:[185] --------------------+----------+-------+-----+-----+-------+---------- | |Granite|Wood | | Sheet | Pavement Qualities |Percentage| Block |Block|Brick|Asphalt|Bitulithic --------------------+----------+-------+-----+-----+-------+---------- 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: --------------------+----------+-------+-------+------+-------+----- | |Granite| Sheet | | | Wood Pavement Qualities |Percentage| Block |Asphalt|Brick |Macadam|Block --------------------+----------+-------+-------+------+-------+----- Cheapness | 14 | 4 | 6¹⁄₂ | 7 | 14 | 4¹⁄₂ Durability | 20 | 20 | 10 | 12¹⁄₂| 6 |14 Ease of maintenance | 10 | 9¹⁄₂ | 7¹⁄₂ | 8¹⁄₂| 4¹⁄₂ | 9¹⁄₂ Ease of cleaning | 14 | 10 | 14 | 12¹⁄₂| 6 |14 Low resistance to | | | | | | traffic | 14 | 8¹⁄₂ | 14 | 12¹⁄₂| 8 |14 Non-slipperiness | 7 | 5¹⁄₂ | 3¹⁄₂ | 5¹⁄₂| 6¹⁄₂ | 4 Favorableness to | | | | | | travel | 4 | 2¹⁄₂ | 4 | 3 | 3 | 3¹⁄₂ Acceptability | 4 | 2 | 3¹⁄₂ | 2¹⁄₂| 2¹⁄₂ | 4 Sanitary qualities | 13 | 9 | 13 | 10¹⁄₂| 4¹⁄₂ |12¹⁄₂ +----------+-------+-------+------+-------+----- | 100 | 71 | 76 | 74¹⁄₂| 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 | | Plain | | Water- +---+---+---+ | Cement |Bituminous| bound |_V_|_W_|_X_|Brick|Concrete| Macadam |Macadam -----------------------+---+---+---+-----+--------+----------+-------- 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 -----------------------------------------+--------------------------- |Possible Types of Surfacing Item +--------+--------+--------- | _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: ---------------------+------+------+----------+--------+ | 1 | 2 | 3 | 4 | | | |Bituminous| | Item | | | Macadam |Portland| |Earth |Gravel| and | Cement | | Road | Road | concrete |Concrete| ---------------------+------+------+----------+--------+ First cost per mile |$1,000|$5,000| $10,000 |$20,000 | Annual Interest, 5 | | | | | per cent | 50| 250| 500 | 1,000 | Annual Maintenance | 250| 250| 500 | 100 | Life of surface, yrs.| 0| 5| 10 | 20 | Cost of resurfacing | $ 0|$2,500| $ 5,000 |$15,000 | Annual Sinking Fund | | | | | 3¹⁄₂ per cent | 0| 466| 427 | 530 | Annual Total Cost | 300| 966| 1,427 | 1,630 | Daily Cost, per mile | 0.82 | 2.74 | 3.90 | 4.45 | ---------------------+------+------+----------+--------+ ---------------------+----------+-------+------- | 5 | 6 | 7 | | | Item | Sheet | Brick | | Asphalt | Stone | Wood |Bitulithic| Block | Block ---------------------+----------+-------+------- First cost per mile | $30,000 |$40,000|$50,000 Annual Interest, 5 | | | per cent | 1,500 | 2,000| 2,500 Annual Maintenance | 100 | 50| 50 Life of surface, yrs.| 20 | 25| 25 Cost of resurfacing | $15,000 |$25,000|$35,000 Annual Sinking Fund | | | 3¹⁄₂ per cent | 530 | 884| 899 Annual Total Cost | 2,130 | 2,934| 3,449 Daily Cost, per mile | 5.84 | 8.03 | 9.46 ---------------------+----------+-------+------- [Illustration: _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.