Bay Area Simulation and Ramp Metering Study - Year 2 Report

As part of the California PATH program, the Paramics microscopic traffic simulation model was evaluated through a pilot application to the I-680 freeway corridor, one of the most highly congested facilities in the San Francisco Bay Area. The main objective of the project was to test the capability of the model to serve as a tool for evaluating alternative transportation planning and traffic management scenarios. HOV lanes were investigated first on a "simple" network, and later on the existing I-680 corridor. An Application Programming Interface (API) "plugin" designed by Quadstone, which influences driver behavior regarding HOV lane use, was utilized in an attempt to improve HOV lane simulation. From the existing freeway model to the freeway with an HOV lane, it was found that the total vehicle hours traveled decreased 28 percent and the overall average speed increased 40 percent. Several scenarios were also developed within the I-680 Paramics model as part of a sensitivity analysis, to assess the effects on the network of altering certain input parameters. Ramp metering was later tested on the calibrated network with the added HOV lane, using various supporting API modules for Paramics developed at PATH. The local actuated ramp control strategy Alinea was implemented and evaluated. It should be noted that Caltrans does not plan to implement the Alinea strategy as tested in the simulation. The report describes the approach used to fine-tune the control strategy, then presents and discusses the results that were obtained. It was found that the overall system performance did not improve after the implementation of ramp control, because the benefits on the mainline freeway did not outweigh the additional delays experienced on the on-ramps. The report identifies a number of factors that may explain why the benefits of ramp metering could have been underestimated in this particular study. The Paramics model, together with its supporting modules, was found to be an effective and reliable tool for modeling traffic operations on a large-scale and highly congested freeway corridor. It can be used to evaluate the impact of an HOV lane or investigate ramp control metering strategies, and to undertake sensitivity analyses for the various alternative scenarios in a timely and effective way. However, a number of limitations and shortcomings have been identified, contributing to a likely underestimation of the ramp metering benefits in this study. The most important factor is the absence of route diversion. Because no parallel surface street was modeled, the model did not capture the spatial diversion that is likely to occur when ramp metering is implemented. Another key issue is the ramp queue control process embedded in the Alinea control strategy: because of high demand levels on many on-ramps, the storage capacity is often reached during the simulation period, resulting in an override of the Alinea-optimized metering rate. This phenomenon prevents ramp control to reach the full potential of mainline freeway improvements. Finally, no modal response was considered in the scenario comparisons, which may have contributed to improving the overall system performance with a higher usage of HOV bypass and mainline lanes. These limitations will be addressed in future applications of Paramics to other Bay Area freeways.