Citation

Forbes TW, Wagner FA. Highw. Res. Board bull. 1962; 351: 1-17.

Copyright

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

DOI

unavailable

PMID

unavailable

Abstract

Time-headways between regular, small, and compact vehicles were used to study the effect of the latter on maximum traffic flow and therefore on highway capacity. Time-lapse photographic recording was the method used. From the records at each location, four complete 5 minute samples were read to determine the normal complete distribution of time- headways. The film was then searched for small and compact cars and, for each of these, 4 car groups were recorded to give headways before and behind the smaller vehicle and a comparison headway between two regular vehicles ahead. Records were made for peak and off-peak periods for six locations. Statistical analysis included comparison of the expected number of small vehicles with the actual number observed in the lower 15 and higher 15 percent of the time-headway distribution. Chi-square comparisons indicated no statistically significant differences from the expected number. After eliminating stragglers, time-headway averages for the three different types of cars were compared by means of fisher's t-function for lanes 1 and 2 at each location, combining lanes 1 and 2 and combining the three locations at which headways were similar. Time-headway differences between the different types of vehicles occurred in the different samples. However, these were not consistent in direction (headways between regular vehicles were not consistently larger or smaller than those including a small or compact vehicle). The differences were not statistically significant. It was concluded that drivers of both the smaller vehicles and the surrounding standard automobiles operated in both peak and off-peak traffic in a way similar to operation of standard cars. Therefore, no consistent increase or decrease of traffic headways or of highway capacity is to be expected from inclusion of small and compact vehicles in the traffic stream. There was some indication that time-headways may be of greater significance than distance-headways for certain purposes.