Highways and Highway Transportation

by George R. Chatburn

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.

© Underwood and Underwood

TRAFFIC ON FIFTH AVENUE, NEW YORK CITY

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.