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Ramp Metering and the Two-Capacity in Freeway Operations
TIAN Zong-zhong
2006, 6(3):
11-20 .
Previous studies have demonstrated a unique operational feature for freeway operations, which is often referred to as the two-capacity phenomenon. The two-capacity phenomenon suggests that freeways can obtain higher flows under freely-flowing (unqueued) conditions than under queued conditions. At an isolated ramp metering location, it is due to the existence of the two-capacity phenomenon that signifies the effectiveness of ramp metering in reducing overal delays for both the freeway and the ramp traffic. This point is first illustrated using a numerical example. The paper further explores whether microscopic simulation models (VISSIM in this case) could be calibrated to replicate the two-capacity phenomenon as well as to analyze some characteristics associated with the phenomenon. It is important to note that microscopic simulation models, when used for evaluating ramp-metering operations with a focus on delay measures, have the capability of replicating the two-capacity phenomenon. Otherwise, erroneous and invalid conclusions might result. From this study, we found that most microscopic simulation models, such as VISSIM, use traditional car-folowing theory, which does not automaticaly exhibit the two-capacity phenomenon. However, with a special coding and parameter selection, the two-capacity phenomenon can be properly replicated in VISSIM. Using the VISSIM model, we conducted analysis on the characteristics of ramp metering and the two capacity phenomenon. We found that ramp metering diminishes the diference between the two capacity values, especialy when queue flush policies， adopted. While the freeway capacity under freely-flowing conditions proves to be difficult to estimate, it should have a value no less than the capacity under queue-discharge conditions.
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