Characterizing Traffic Conditions in Urban Areas

A series of vehicular traffic experiments conducted in Austin, Texas, shows the reasonableness of the two assumptions in the two-fluid (moving and stopped vehicles) model of town traffic. The observational data support the assumption that the average speed in an urban street network is proportional to the fraction of the vehicles moving raised to a power and is also in agreement with the supposition that during relatively uniform periods the traffic is ergodic. An important consequence is that the average of the fraction of time stopped for a test vehicle circulating in a street network is approximately equal to the average fraction of the vehicles stopped in the system during the same test period. The parameters of the two-fluid model and the observed ranges of trip time and stop time per unit distance have been shown to be effective in assessing or rank ordering the relative quality of traffic service in a number of Texas cities; comparisons are made with various cities around the world. The two-fluid methodology appears to be useful in a preliminary “before”/“after” study during which signal timing changes were made. Finally, a preliminary analysis of aerial photographic data in two cities allows the determination of an additional two-fluid model parameter, p , in the relation stating that the fraction of vehicles stopped is given by the ratio of concentration to the jam or maximum concentration raised to a power, p . It is suggested that this parameter may be useful in describing the relative quality of various traffic systems.