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.