pounds at 7 miles and 54 pounds at 12 miles, showing what a rapid increase to resistance follows an attempt to go at even moderate speed over such a road. The authors make the following comments of the tests: There are three principal elements which determine the tractive-resistance curve for different speeds and a given vehicle, within the range of conditions covered by the tests. The first element is a constant resistance, the magnitude of which depends on the lack of resilience of the road surface and the tires, that is to say, on the energy losses due to displacement of tire material and road-surface material. This constant ele ment would be encountered upon a smooth, level road of the particular type considered, in the absence of impact, air and wind resistance. The second element is an increasing resistance with increasing speed due to impact losses. This resistance results from lack of smoothness of the road surface and varies approximately as the second power of the velocity at impact. The third element is an increasing resistance with increased speed. It is due to the air pressure against the front of the vehicle and varies approximately as the second power of the speed. The first element is called the displacement resistance, the second the impact resistance and the third the air resistance. The displacement resistance is very marked in Curve 10, for a graniteblock pavement with sand joints. The displacement resistance varies, not only with the type and surface quality of the road, but also with the type, dimensions and quality of the tires on the wheels. The same tires were used in the experiments by Kennelly and Schurig. The air resistance of a given vehicle at any given speed is the same for all classes of pavement. The impact resistance of a road depends not only on the type and character of the road surface and the sizes of its irregularities, but also on the type, dimensions and quality of the tires on the wheels, the weight of the truck and the quality of its springs. Increasing the gross weight of the vehicle by 12 per cent, through load, was found to have no effect on tractive resistance within the observed speed limits for smooth roads in good condition; but on rough roads, a distinct increase in tractive resistance with this extra weight was observed. RURAL PUBLIC ROADS OF UNITED STATES AT CLOSE OF 1915 Circular 63, United States Department of Agriculture. Prepared by the Division of Road Economics, Office of Public Roads and Rural STATE REVENUES USED IN 1914 IN EACH STATE FOR PUBLIC ROAD AND BRIDGE PURPOSES AND TOTAL OUTSTANDING BONDS FOR ROADS AND BRIDGES AT CLOSE OF 1914 From Bulletin 390, U. S. Department of Agriculture, Prepared by the Office of Public Roads and its State Collaborators $3,949,019.00 $71.22 $77.01 $1.840 $0.6900 5,544,048.00 49.92 67.79 3.280 0.2000 245,013.65 20.11 2.23 2.900 0.2400 556,398.82 46.86 4.54 1.690 0.7700 23,231,964.02 292.60 467.00 2.540 0.2080 5,215,490.78 102.75 107.00 2.360 0.6970 2,402,383.52 34.92 34.23 4.160 0.6800 14,334,245.98 165.99 351.84 3.000 0.2200 2,112,680.80 19.57 30.45 1.274 0.1769 5,310,466.76 144.23 55.54 7.890 0.5900 10,424,580.00 113.86 232.00 1.360 0.2050 446,496.05 205.76 418.50 0.820 0.0720 1,024,480.37 24.26 33.59 0.670 0.3510 1,217,809.42 12.64 15.84 2.080 0.3400 2,370,560.16 51.48 56.86 1.080 0.3780 9,920,079.11 76.92 37.80 2.540 0.3910 803,070.63 91.15 9.77 2.150 0.4000 1,023,941.01 71.86 112.22 2.870 0.4620 3,224,528.82 60.39 80.08 1.560 0.3720 7,944,717.38 187.25 118.87 6.950 0.7900 2,483,747.00 77.55 103.39 2.030 0.2120 9,880,240.50 130.50 178.81 4.230 0.4000 669,661.16 44.06 6.86 4.590 0.3700 $240,263,784.46 $98.22 $80.79 $2.620|$0.3500 $344,763,082.32 |