Wednesday, November 28, 2012

I-55 Managed Lane Project


IDOT is initiating the I-55 Managed Lane Project in the Chicago area. According to the project website:
"The scope of work for this project is anticipated to include the addition of one lane in each direction within the existing median of I-55 needed to accommodate implementation of a managed lane, which could include Express Toll Lane (ETL), High Occupancy Vehicles (HOV) lane, High Occupancy Toll (HOT) lane, Congestion Priced lane, or other feasible managed lane configurations as determined to be appropriate for a projected 2040 travel demand."

Based on the findings in the literature and my personal research work, managed lanes need to be carefully planned and designed considering common issues such as "induced demand" and "empty lane syndrome". Adding capacity to a road, when not properly managed, can induce demand which can eventually result in the same level of congestion despite the added capacity. On the other hand, a HOV/HOT lane may also suffer from underutilization or "empty lane syndrome." Also from the operational point of view, an HOV lane works better if passes through one or more recurrent bottlenecks. If an HOV lane ends prior to a bottleneck, its capability to reduce congestion by smoothing the flow of HOVs may be harmed. One example of such not properly designed HOV lanes is the HOV lane on I-5 NB in Portland, Oregon which ends about 1 mile prior to a recurrent bottleneck. In this case, the bottleneck-induced congestion backs up and blocks the outflow of the HOV lane and as a result, a queue forms in the HOV lane, just like the adjacent general purpose lanes. 

Identifying the locations of the recurrent bottlenecks on I-55 should be done prior to any initial design of the HOV/HOT lane on this corridor in order to make sure that the beginning and end points of the managed lane is properly selected.

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