The impact of variable speed limits (VSL) on the aggregate traffic flow behavior on freeways is shown to bear, in case of appropriate VSL operation, similarities with the impact of ramp metering, in particular when addressing potentially active bottlenecks. While ramp metering holds traffic back in metered on-ramps, VSL may be used to hold traffic back on the mainline, i.e., be operated as a mainline metering device. To verify this approach, a quantitative model of the VSL impact is proposed which allows for VSLto be incorporated in a macroscopic second order traffic flow model as anadditional control component. The integrated freeway network traffic control problem involving ramp metering and VSL control measures, is formulated as a constrained discrete-time optimal control problem and is solved efficiently by a suitable feasible direction algorithm. An illustrative example of a hypothetical freeway stretch is investigated under different control scenarios, and it is shown that traffic flow efficiency can be substantially improved when VSL control measures are used to address potentially active bottlenecks, with or without integration with ramp metering.
Abstract