Development and Evaluation of a Framework for Selecting Operational Strategies for an Integrated Diamond Interchange - Ramp Metering Control System (SWUTC/04/473700-00025-1)

Diamond interchanges and their associated ramps are where the surface street arterial system and the freeway system interface. Historically, these two elements of the system have been operated with little or no coordination between the two. One drawback of operating the ramp-metering system and the diamond interchange system in isolation is that traffic from the ramp, particularly if it is metered, can spill back into the diamond interchange, causing both congestion and safety concerns at the diamond interchange. While flushing the ramp queues by temporarily suspending ramp metering has been the primary strategy for preventing queue spillback, it can result in freeway system breakdown, which would affect the entire system's efficiency. The aim of this research was to develop operational strategies for managing an integrated diamond interchange ramp-metering system (IDIRMS), and to develop a general framework for implementing such an integrated system. Integrated control strategies (ICS) were developed based on the two commonly used diamond interchange phasing schemes, basic three-phase and TTI four-phase. The ICS were evaluated using VISSIM microscopic simulation under three general traffic demand scenarios: low, medium, and high, as characterized by the volume-to-capacity ratios at the metered ramps. Preliminary system design and detailed functional diagrams were developed to guide traffic engineers for field implementation of the system. The results of the evaluation indicated that the integrated operations through an adaptive signal control system were most effective under the medium traffic demand scenario by preventing or delaying the onset of ramp-metering queue flush, thereby minimizing freeway breakdown and system delays. The integrated system would require enhanced detection and communication systems, but the system could be designed and implemented based on existing features equipped with most advanced traffic signal controllers.