Coordinated ramp metering strategies have been suggested as more effective measures than local ramp metering when there are multiple congestion bottlenecks on the freeway, excessive ramp delays, and when the optimization of freeway and on-ramp performances requires the metering of several ramps. A number of studies compared the performances of the available coordinated traffic responsive ramp metering algorithms relative to each other and to other strategies. However, the results from these studies have not been well summarized to provide a good understanding of the relative performances of these algorithms. This paper discusses the available coordinated ramp metering strategies and review studies that evaluated and compared their performances. Based on the reviewed study results, it appears that traffic responsive ramp metering strategies can outperform Time of Day (TOD) strategies. In addition, it appears that coordinated traffic responsive algorithms can outperform local control algorithms, depending on several factors including traffic demand patterns, how well the coordinated algorithms are calibrated, and the particular coordinated algorithm under consideration.
