Minnesota's most recent Stratified Zone Ramp Metering (SZM) Strategy, the successor of the ZONE metering algorithm, has been deployed in the Minneapolis/Saint Paul area since late 2002. The SZM strategy aims at maximizing freeway throughput while keeping ramp waiting times below a predetermined threshold. Preliminary evaluation results confirmed the overall effectiveness of the SZM strategy but also revealed that the freeway performance was compromised by the very restrictive maximum ramp waiting constraint. Consequently improvements were sought and presented in this paper. They focused on a better determination of the minimum release rate for each ramp and its integration with the overall SZM strategy. Both the current and the enhanced SZM strategies are tested in two freeway sites under various demand scenarios through a state of the art microscopic simulator. The simulation results indicate that the enhanced SZM strategy is effective in postponing and decreasing the freeway congestion as well as resulting in smoother freeway traffic flow as compared to the original SZM strategy without violating the maximum ramp delay requirement.
Abstract