various widths, carrying a wide range of traffic volumes. Over 300,000 vehicles have been observed in these studies. A number of interesting facts are revealed by analysis of the data collected on a four-lane divided highway representing the highest standards of design, and on a two-lane road, of reasonably good design, both carrying traffic of the same character. The graphical presentation in plate 25 shows that the speed of vehicles begins to decrease with increase in traffic volume, even at very low volumes.
With a zero traffic volume there are no vehicles and consequently there can be no value for vehicle speed, but vehicle speed at zero traffic is approximated when a vehicle moves by itself, so separated from other traffic that the driver is not influenced by the presence of other vehicles on the road. His speed is then uninfluenced by that of other vehicles, and represents a free choice at the particular time on the particular road. The average speed for all vehicles using the road under that condition was, in the case of the two-lane width, 44 miles per hour, and on the divided highway 48 miles per hour. Increase of the total traffic volume to 500 vehicles per hour resulted in a decrease of the average vehicle speeds to 40 and 47.5 miles per hour respectively, and at a volume of 1,000 vehicles per hour in both directions, the speeds become 35.5 and 47 miles per hour. It is significant that there was no serious reduction in speed with increasing volumes on the four-lane road and also that at low traffic volume the traffic moved faster over the divided highway, with its invitation to faster and safer travel, than over the two-lane road.
DIFFERENCE IN SPEED OF SUCCESSIVE VEHICLES BEST INDEX OF CONGESTION
It has been found that the average speed of all vehicles does not. represent a final criterion of the freedom of movement or congestion. Results of various analyses show that the average difference in speed
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