Dynamics of Variable Speed Limit System Surrounding Bottleneck on German Autobahn

Abstract

This paper presents findings of an ongoing empirical study focusing on identification and examination of several recurring freeway bottlenecks. It integrates the fusion of several traffic management and driver information data sources along an 18-km (11.2-mi) section of Autobahn 9 near Munich, Germany. These combined data sources further understanding of traffic dynamics and driver behavior before, during, and after bottleneck activation. The primary focus of this paper has been the investigation of variable speed limit and traveler information systems provided by means of overhead dynamic message signs so as to improve understanding of how these systems affect driver behavior and bottleneck formation and location. Toward that end, speed limit and information messages have been compared with actual traffic dynamics on the segment of Autobahn 9, and the analysis has found a strong correlation. It has been found that when drivers were warned of approaching congested conditions, the speed limit was reduced before bottleneck activation as a means of managing dense traffic. The system reduced the speed limit to control dense but still flowing traffic, and traffic continued to flow during congested periods at speeds between 30 and 40 km/h (19 and 25 mph). A comparison of sampled measurements of flow and speed with fundamental diagrams of speed–flow and flow–density indicated that speed limits were reduced. These findings will be connected to a before-and-after comparison of the system to be done to determine benefits and effects of such a system.