Road test of a freeway model

A few years ago Newell reworked classical traffic wave theory into "A Simpl ified theory of kinematic waves in highway traffic" (Newell, 1993a,b). The simplifications - use of cumulative count curves instead of flows for most of the calculations and a triangular flow-density relation to describe traf fic flows - were sufficient to allow him to apply the theory (Newell, 1993c ) to complex situations with multiple bottlenecks and multi-destination flo ws. This paper tests Newell's model by comparing its predictions with condi tions observed at three freeway test sites. The test data from San Francisc o Bay Area freeways is old, but extraordinarily detailed, so provides the n ecessary input and observed densities for comparison. In the tests, the mod el did a very good job of predicting the growth and decay pattern of large queues and the effect of traffic entering and leaving the roadway within a congested area, but had difficulties dealing with light or sporadic congest ion. However, the predicted travel times were quite accurate even for light ly congested roadways. The estimation of roadway capacities needed as input proved to be a major problem. The duration of queuing - both in the model and the real world - is very sensitive to the maximum rate at which vehicle s can enter bottlenecks, and neither standard estimation tools nor the data set provided estimates of sufficient precision. This would seem to be a pr oblem for any freeway model, but for many purposes there is no need for the level of accuracy sought here, and for others better data would be availab le. Overall, the results were very encouraging: the model requires very lit tle calculation time and delivers excellent results for the severely conges ted freeways that are of the most practical interest. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.