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
MondayJan.1Apr.2July3Oct.2
Sunday14151615
Saturday27282928
FridayFeb.9May11Aug.11Nov.10
Thursday22242423
WednesdayMar.7June6Sept.6Dec.6
Tuesday20191919

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 method[177] 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.