© _Underwood and Underwood_ A ROAD OF MIXED ASPHALT AND CONCRETE BEING TESTED OUT] The effect of character, weight, and density of traffic has been frequently mentioned and will again be referred to in what follows. There is no doubt a relationship between materials and design and the character and amount of traffic. A cinder road may be perfectly acceptable for a park drive where the traffic is light, but absolutely worthless under heavy commercial trucking. Resistance to traffic varies with different road surfaces. A smooth hard surface offers a very great deal less resistance than does a rough or soft surface. To illustrate, a horse is said to be able to pull directly on the traces one-tenth his own weight without being overworked. With a resistance of 100 pounds per ton (earth road in medium condition) a team of horses weighing 1200 pounds each could draw over a level road 2 × 1200 -------- = 2.4 tons. 10 × 100 On a concrete, asphalt or brick pavement having a tractive resistance of 30 pounds per ton the team could draw 2 × 1200 -------- = 8 tons. 10 × 30 In other words the load that can be drawn is inversely as the tractive resistance. Here speed was not considered. It was the natural walking gait of the horse about three miles per hour. If the speed is greater the load must be cut down proportionally. With a truck the direct pull is the effective power of the engine in foot-pounds per minute divided by distance in feet per minute; and the load that can be drawn is the direct pull times the tractive resistance. Thus if a truck may exert _h_ effective horse power = 33,000_h_ foot-pounds per minute, and the speed is v miles per hour, the load _T_, in tons, that may be hauled on a road having a tractive resistance of _t_ pounds per ton, is 33,000_h_ 375_h_ _T_ = ------------- = ------. 5280_v_ _vt_ ------- · _t_ 60 Therefore a truck of 20 effective horse-power will haul over a road whose tractive resistance is 100 pounds per ton at a speed of 10 miles per hour a load of 375 × 20 _T_ = -------- = 7.5 tons; 10 × 100 and on a smooth road with a tractive resistance of 30 pounds per ton at the same speed, 25 tons, or the same load 7.5 tons may be drawn at a speed of 33¹⁄₃ miles per hour. It must be remembered that when the speed is increased the tractive resistance is likewise increased. The air resistance is in about the ratio of the square of the velocity, so that 33 miles per hour would be too great in the last case.