The I-270 corridor in Maryland is a primary commuter route and one of the most congested and unreliable corridors in the country. There is a high-occupancy vehicle (HOV)-only lane in both directions that is operational during peak periods. However, the demand in the corridor is much higher than the capacity available along I-270, and the HOV lane is not sufficient to manage the demand.
Previous research conducted for the Transform 66 Outside the Beltway project utilizing the capabilities within VISSIM traffic microsimulation software to simulate and analyze the impacts of CAVs showed that with higher proportions of CAVs along the corridor, there is potential for significant reduction in congestion. However, 100% fleet conversion to CAVs cannot be expected to happen for the foreseeable future, and the maximum benefits to roadway capacity can only be achieved when the entire vehicle fleet has made the conversion. But providing dedicated CAV lane(s) can work as an interim solution. This study was the next step in a line of research that was conducted to evaluate the impacts of different levels of CAVs mixed into existing lanes. Similar to HOV lanes, this study looked at providing a limited-access managed lane along the I-270 corridor for the exclusive use of a CAV fleet and evaluate its impacts on mobility in the corridor.
A base scenario with existing conditions was evaluated along with three additional scenarios with a CAV-only lane and varying levels of CAV proportions in the fleet along the corridor. The overall vehicular demand was kept consistent between the different scenarios. Detailed speed congestion diagrams, corridor travel time and throughput along the corridor were compared as measures of effectiveness to evaluate the impacts.
- There is a direct correlation between the proportion of CAVs in the fleet and the increase in capacity in the corridor.
- With the CAV-only lane and just 15 percent of the vehicles being CAVs, at least one upstream bottleneck is eliminated. With an increasing proportion of CAVs in the fleet, the benefits along the corridor tend to scale down as the CAV-only lane reaches its capacity and can no longer accommodate additional vehicles.
- There is a big reduction in travel times for up to a 30 percent proportion of CAVs, but beyond that more CAVs do not help reduce corridor travel times. The travel times are an average of all vehicles in the corridor. There is a more than 25 percent reduction in travel times with the CAV-only lane and 30 percent proportion of CAVs in the fleet.
- Considering an upstream bottleneck being eliminated, the increase in throughput could make the downstream bottleneck worse. However, in this case even with the elimination of an upstream bottleneck, the downstream bottlenecks reduce or remain the same with an overall increase in throughput along the corridor with the CAV-only scenarios. So even with the increase in throughput upstream, there are speed and travel-time improvements.
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