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 = 335.12qr2
| 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.