Get More Out of Variable Speed Limit (VSL) Control: An Integrated Approach to Manage Traffic Corridors with Multiple Bottlenecks
Applications of model-based Variable Speed Limit (VSL) control have shown promise in resolving capacity drops and time delays for a single location with recurring bottleneck conditions. Researchers studied the application of VSL controls to a broader scenario, freeway corridors with multiple segments and multiple bottlenecks. With the objective of reducing fuel consumption and greenhouse gas emissions, three VSL control scenarios were developed and compared: static speed limit (no VSL), flow-based control, and density-based control. The flow-based fuel consumption centered VSL control strategy was designed based on the characteristics of shockwaves between adjacent bottlenecks to suppress the negative shockwave effects and maintain a fuel-efficient speed in the controlled segments. The second density-based strategy was designed to keep the density of freeway segments around an optimal level by managing the difference between optimal density and actual density.
The three VSL control scenarios were tested through microsimulation of a ten-mile freeway section of eastbound Interstate 80 near Davis, California, containing four junctions with critical bottleneck conditions and severe congestion occurring during afternoon peak hours. Aggregated travel time, fuel consumption, and carbon emissions (CO, CO2, and HC) from the model were selected as the measures of impact.
- Density-based VSL produced a greater reduction in fuel consumption and carbon emissions with less travel time increase than the other two VSL control strategies. With the optimal target speed set to 50 mph, the density-based VSL control yields a nearly 14 percent reduction in fuel use and CO2 emissions, with a 9 percent travel time increase.
- The performance of the density-based VSL strategy is not sensitive to the choice of optimal target speed, as long as it falls in the fuel-efficient range defined by the model (35-50 mph).
- Flow-based VSL mitigated downstream congestion, but transferred congestion upstream. Overall performance was no better than that of the static speed limit scenario, and was worse than density-based VSL.