The Effect of Short Cars on Flow and Speed in Downtown Traffic: A Simulation Model and Some Results

The flow of vehicles down one lane of a straight, signalized urban street with fixed cycle traffic lights at 500 ft intervals was simulated by means of a Fortran IV computer program. A car-following model and a simple model of lead-driver behavior were used to model driver responses. Flow and speed measurements were made for “levels of congestion” in which, on average, 5, 10, 15, and 20 cars were queued at each traffic light; 10 ft cars, 20 ft cars, and mixtures of these two lengths were simulated. With 15 cars queued at each light a 70 per cent increase inflow can be obtained by the substitution of 10 ft cars for 20 ft cars. Average speed increases by about 57 per cent, from 3.5 ft/sec to 5.5 ft/sec. That the number of vehicles queued per light remains constant has been assumed. Complete results of our simulation are presented in Table III and Figures 2, 3, and 4. Validation of the simulation model was provided by traffic flow counts and speed measurements on Walnut Street in downtown Philadelphia; correspondence between model and reality was close.