The Minnesota DOT deployed and evaluated a dynamic late merge system (DLMS) for lane closures in work zones in the Minneapolis/St. Paul metropolitan area. The DLMS was comprised of microwave vehicle detectors, three dynamic message signs (DMS) placed 3.25 miles, 1.5 miles, and 500 feet from the beginning of the lane closure, and static orange and black warning signs placed in advance of the work zone. When congestion began to form, the DMS were activated to provide the following lane use instructions:
- STOPPED TRAFFIC AHEAD alternating with USE BOTH LANES (furthest from the taper).
- USE BOTH LANES alternating with PREPARE TO STOP, MERGE AHEAD, or MERGE 1.5 MILES AHEAD.
- TAKE TURNS alternating with MERGE HERE.
If traffic speeds fell below a designated threshold, the DMS would display the messages above as well as "SORRY FOR DELAY." If abnormally high speeds were detected, the DMS would display messages such as:
- REDUCE SPEED alternating with WORK ZONE AHEAD.
- REDUCE SPEED alternating with STOPPED TRAFFIC AHEAD.
The DLMS was deployed on I-35 near Lakeville, I-494 in Plymouth, and U.S. Route 52 in St. Paul. The I-35 deployment, which occurred from May to October 2004, included a ten-day "before" period when baseline condition data were gathered. Speed and volume data were collected via microwave vehicle detectors, loop detectors, and visual observations.
When the DLMS was activated in the "after" period, the percentage of drivers utilizing the discontinuous lane increased dramatically. The percentage of vehicles using the discontinuous lane was nearly 60 percent at a location a half-mile from the taper. Discontinuous lane use at the beginning of the lane closure increased from 25 percent during the baseline condition to approximately 50 percent. When traffic demand was heaviest, lane use fell to 30 percent at the taper location.
The DLMS reduced confusion over lane use and the correct place to merge, reduced aggressive driving, and equalized the queue lengths in both lanes. This lane use pattern minimized queue length, reduced the distance traveled in congestion, and improved driving conditions upstream of the work zone. By decreasing the speed differential of vehicles in adjacent lanes, the DLMS also created safer driving conditions.
Lane closures in work zones present a challenge, often impeding traveler mobility as well as safety. The Dynamic Late Merge System (DLMS) deployed by the Minnesota Department of Transportation (MNDOT) is a traffic control strategy designed to improve throughput and driver safety during such lane closures. This DLMS, which consists of three changeable message signs (CMS) and a remote traffic microwave sensor (RTMS) detector, can dynamically change its lane use instructions based on the current traffic demands. Under light traffic conditions, an early merge strategy can be utilized, but during periods of heavy demand, this can be altered to a late merge strategy.
MNDOT is in the second year of evaluating the deployment of DLMS. In 2003, MDOT deployed and tested the system in a single location; this was followed by the 2004 evaluation of DLMS at three additional sites: I-494 Northbound in Plymouth, I-94 Southbound in Plymouth, US 52 in St. Paul and I-35 around Lakeville. Five potential benefits of the new system were identified and used as measures of effectiveness to evaluate the DLMS. The potential benefits include:
- Shorten queue lengths before work zones
- Increase traffic capacity through work zones
- Reduce aggressive driving
- Decrease number of work zone related incidents
- Reduce travel time
This report presents MNDOT’s 2004 evaluation findings separately for each site and provides a set of lessons learned for future deployments of DLMS.
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