between successive vehicles is a much more positive index of congestion. In periods of light traffic when passing is generally unrestricted, any desired difference in speed between successive vehicles may be maintained. As the traffic volume increases the opportunity for passing diminishes, lines of vehicles form and, accordingly, the difference in speed between successive vehicles becomes less, even though the average speed of all vehicles may not be affected greatly. Plate 26 shows that such difference in speeds is a much more sensitive index of congestion than is the average speed of all vehicles. This graph shows, for roads of the different widths, the mean difference in speed between Here again, a successive vehicles under various traffic volumes. straight-line relation exists between the traffic volume and the speed differences, again indicating that there is an effect on the freedom of movement even with relatively light traffic volumes, and that there is no point at which congestion suddenly occurs.
Beginning with a speed difference of 6.4 miles per hour at zero volume, this figure on the two-lane road drops to 3.5 at 1,000 vehicles per hour in the two directions, while the initial speed difference on the divided highway of 8.3 miles per hour drops only to 7.1 at 1,000 vehicles per hour. To restrict the freedom of movement on the divided highway to a speed difference of 3.5 miles per hour requires a total of over 4,000 vehicles per hour, or over 2,000 vehicles per hour in each direction.
DESIRE FOR SPEEDS FASTER THAN THOSE NOW GENERALLY POSSIBLE
Investigation of the distribution of vehicle speeds, on the two-lane road studied, shows that no vehicles traveled over 60 miles per hour even in traffic as low as 200 vehicles per hour in both directions, and only 7 percent exceeded 50 miles per hour. On the divided highway,
