Citation

Ryan DP, Breuning SM. Highw. Res. Board bull. 1962; 324: 73-84.

Copyright

(Copyright © 1962, National Research Council (U.S.A.), Highway Research Board)

DOI

unavailable

PMID

unavailable

Abstract

To obtain known or new characteristics of traffic flow at high volumes, the relationship between speed, volume and density on an expressway were studied. The traffic flow analysis was separated into noncritical and critical flow, and the analysis limited on the determination of the boundary between the two types of flows and on the characteristics of noncritical flow. Noncritical flow is studied alone to obtain reliable data on that type of flow. Forty mph was found to be the boundary between critical and noncritical flow for the speed-volume and the speed-density relationships. A density cutoff was also chosen for the volume-density relationship, although the cutoff point is not so clearly evident for density as it seemed to be for speed. It was found that all three relations among speed, volume, and density are linear within noncritical flow in range of observations bounded on one side by low traffic flow and on the other by the boundary with critical flow. This linearity is lost at the boundary where flow becomes unstable. Therefore, it is proposed that the end of linearity of flow marks the boundary between noncritical and critical flow. The three relationships studied showed considerable difference in correlation coefficients. The coefficient of correlation for speed is the lowest, that for density the highest. This might point to a comparative independence of speed from the other measurements.