SPECIALIST-RM — Integrated variable speed limit control and ramp metering based on shock wave theory

Abstract

Both from theory and from practice it is known that the variable speed limit algorithm SPECIALIST can resolve moving jams on freeways. However, the algorithm does not take on-ramps into account in its current form, while on-ramp traffic may have an impact of the effectiveness of the algorithm. In this paper we extend the theory of SPECIALIST to include on-ramps and ramp metering. The theory of SPECIALIST is formulated in the framework of shock wave theory and it results in a control scheme that describes the propagation of the fronts (boundaries) between the various traffic states, and describes the time and location where the speed limits need to be active. Here we extend this with the traffic states describing the additional flow from an on-ramp (either metered or not), and develop the resulting control schemes. Since the location of the on-ramp relative to the moving jam and to the control scheme can be categorized into four cases, we develop four different control schemes. The developed control schemes can be used to analyze offline traffic data to assess the number of moving jams that can be resolved. Resolvability basically depends on (1) whether the traffic situation (speeds, flows, etc.) is such that a control scheme can be constructed, and (2) whether the control scheme physically fits on the freeway stretch equipped with variable speed limit displays. As an example, we analyze the data of a stretch on the A2 freeway in the Netherlands for various speed limit values and maximum on-ramp flows. The results show that for the number of resolvable moving jams the fact that the on-ramp flows are taken into account is more important than the metered value of the on-ramp flow.