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.