Capacity effects of variable speed limits on German freeways

Variable speed limits are applied on a number of heavily trafficked freeway sections in Germany. Besides the impact on road safety, variable speed limits harmonize traffic flow at high volumes and hence influence freeway capacity. The paper reports empirical findings obtained for several freeway sections with different speed control conditions in Germany. The capacity of each cross section is determined by analyzing the speed-flow relationship as well as by applying methods for stochastic capacity analysis. The results show that the main effect of variable speed limits is a significantly reduced variance of the capacity distribution function. Hence, the application of variable speed limits leads to a lower risk of a traffic breakdown at moderate volumes. In highway capacity guidelines, this effect can be considered by applying different thresholds of LOS E for freeway sections with and without variable speed limits.