Project number: MT-11-14

Funding source: Caltrans

Total cost: $90,000

Performance period: August 2010 to July 2011

Project description

Variable speed limit (VSL) systems, as one of the freeway control strategies among Intelligent Transportation Systems (ITS), have been studied since 1970s. However, the cases of field implementations are limited, mostly in Europe and the United States. The VSL systems are considered to reduce risk of crashes, to warn drivers of hazardous roadway conditions, to stabilize and smooth traffic flows, to dampen shock waves, to postpone or prevent congestions, and to reduce emissions and fuel consumptions. The safety benefits of using VSL have been reported in several field studies both in Europe and the United States. Currently, the incentive of using VSL has been mainly safety from the application point of view. However, benefits such as improved traffic flow rates, lower travel times, smooth speed and density distribution and possibly lower pollution have been conjectured in literature and in some cases analyzed using mainly macroscopic traffic models. A closer look at these promising results in the literature shows that using the same macroscopic model to design and test raises questions as to whether the simplicity of the models used for evaluation are responsible for these optimistic results given the fact that other studies using microscopic simulations fail to demonstrate improvements on travel times albeit for different VSL strategies. The question whether the VSL strategies or the macroscopic models used to analyze them or both are responsible for the large differences in traffic flow and travel time benefits reported, remains unanswered. Consequently, the problem of what are the most appropriate dynamic VSL controllers and what benefits can be guaranteed in a consistent manner under different traffic flow conditions is an open one.