Speed were measured of cars and trucks on parts of different motorways where good conditions of visibility and road quality existed, over a 3-year period. Results show that speed distributions on nearly all sites for both cars and trucks could be approximated by a normal (gauss) curve, with a standard deviation of 0.17 times the average speed. There existed significant differences between average speeds for cars and for trucks, and for traffic going uphill and that coming downhill. A regression equation is given for average speeds. Also formulas for minimum sample size, and for the calculation of the 85 percentile speed in the establishing of speed zones. An annual increase in average speed was observed of 0.35 mph for cars and 1.05 mph for trucks, on level parts.
Abstract