Improving Minnesota's Stratified Ramp Control Strategy

Minnesota's most recent stratified zone ramp metering (SZM) strategy, the successor of the ZONE metering algorithm, has been deployed in the Minneapolis-St. Paul area since late 2002. The SZM strategy is intended to maximize 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 restrictive maximum ramp waiting constraint. Consequently, improvements were sought. These focus 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 for delaying and decreasing the freeway congestion and results in smoother freeway traffic flow compared with the original SZM strategy without violating the maximum ramp delay requirement.