Variable speed limits have safety benefits and a homogenizing effect, but a German study found no increase in freeway capacity.

A with/without study analyzing variable speed limits on a three-lane freeway near Munich, Germany.

Date Posted
10/28/2016
Identifier
2016-B01070
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Empirical Assessment of Traffic Management Effects of a Variable Speed Limit System on a German Autobahn: Before and After

Summary Information

This paper presents findings of an empirical study focusing on the traffic flow effects of a variable speed limit (VSL) system on a three-lane German Autobahn. It integrates loop detector data as well as VSL and driver information data provided by means of overhead dynamic message signs along a 16.3-km section of Autobahn 99 near Munich, Germany. These data sources allow for a "with and without" analysis reflecting traffic flow and driver behavior in the presence of as well as in the absence of the VSL system deployment. In addition to speed, the primary features of interest were the spatial-temporal extend of the queue (congestion), the flow changes caused by identified bottlenecks, the distribution of flow across lanes, the percent trucks per lane, as well as the flow homogeneity between lanes.

Benefits

The analysis recorded bottleneck flow reductions that were balanced across all lanes in the case of an active VSL system. In case of an inactive VSL system the flow reductions were slightly lower and primarily occurred in the center and the shoulder lane while there was even a small flow increase in the median lane. Further examination indicated that the gain in homogeneity and safety in the "with" case is at the cost of capacity. Toward that end, possible reasons for the differences between the "with" and "without" cases have been found. Those are based on the fact that a truck overtaking ban is in effect when the driver information system is active.

  • The capacity of this freeway was not increased by applying the VSL system; there was actually a slight decrease in measured flow. However, the safety and homogenizing effect of a VSL system were obvious. Consequently, the total yearly efficiency of the freeway might be higher due to avoided crashes and queues.

Empirical Assessment of Traffic Management Effects of a Variable Speed Limit System on a German Autobahn: Before and After

Empirical Assessment of Traffic Management Effects of a Variable Speed Limit System on a German Autobahn: Before and After
Source Publication Date
01/14/2013
Author
Simone Weikl, Klaus Bogenberger (Munich University of the Federal Armed Forces), Robert L. Bertini (Portland State University)
Publisher
Transportation Research Board
Goal Areas
Results Type
Deployment Locations