Place portable changeable message signs (CMS) on the shoulder or median nearest the discontinuous lane when implementing a dynamic late merge system (DLMS) to manage a work zone.
A Minnesota DOT experience in work zone management.
Made Public Date


United States

Dynamic Late Merge System Evaluation


The Minnesota Department of Transportation (Mn/DOT) evaluated a new traffic control strategy for lane closures in work zones during two construction seasons. A dynamic traffic control strategy, called the Dynamic Late Merge System (DLMS), was used, in addition to static orange and black warning signs usually placed in advance of a lane closure in work zones. The system consisted of a Remote Traffic Microwave Sensor (RTMS) detector, which, under congested roadway conditions, activated three strategically placed changeable message signs (CMS, see Note 1) to provide lane use instructions to drivers.

The objective of this overall project was to develop, test, and evaluate a traffic control system that would dynamically incorporate the best aspects of early- and late-merge systems. Through the use of technology and a series of signs, drivers under free-flow conditions were encouraged to leave the discontinuous lane early in advance of the lane’s closing point, whereas drivers under congested conditions were instructed to use all lanes, including the discontinuous lane, until they near a designated merge point. This system was designed to improve work-zone safety and mobility while reducing driver frustration.

Note 1: Changeable message sign (CMS), variable message sign (VMS), and dynamic message sign (DMS) were observed to be used interchangeably in various literature.

Lessons Learned

A typical traffic control plan for work zones make use of static orange and black warning signs placed in advance of a lane closure. The static sign farthest from the closure warns drivers of the work zone ahead while subsequent signs post the speed limit, the site distance from the impending lane closure, and instruct drivers to take specific actions to merge with traffic in the adjacent lane. This approach is effective as long as the traffic demand is below the capacity of the work zone, and free-flow traffic conditions prevail on the roadway. When congestion does occur, varying presumptions on the part of drivers regarding merging protocol result in incompatible behavior, which in turn results in longer queues. During increased congestion, the DLMS strategy used by the Mn/DOT was proved to be effective in reducing queue length in work zones.

The DLMS is a fully automated traffic control system that utilizes Doppler radar and RTMS to collect data on the current state of traffic in work zones. In a DLMS set up, a combination of changeable message signs (CMS) and static work zone signs displays lane-use instructions for drivers in the section of roadway preceding a lane closure. The strategy encourages early-merge under free flow conditions and late-merge under congested conditions. Mn/DOT’s experience in using DLMS offers the following insights.

  • Implement DLMS to reduce queue length in highway work zones. In the Mn/DOT field test, the use of DLMS demonstrated efficient lane usage in advance of the taper point of the discontinuous lane and shortened the overall queue length in the work zone by 35 percent.
  • Install DLMS with adequate consideration to message posted on signs and in the gap between signs. Three CMSs coupled with Doppler radar were located at different positions throughout the advanced warning zone. The first two signs were placed approximately three miles and one mile from the lane closure, respectively. The last CMS was placed approximately 500 feet from the beginning of the lane closure taper point. An RTMS detector monitored speed and volume data approximately one mile from the lane closure. When the congestion level worsened, indicated by the average speed near the merge point falling consistently below 30 miles per hour, the activation of CMSs was triggered. The CMS farthest from the work zone displayed the message “STOPPED TRAFFIC AHEAD - USE BOTH LANES.” The next CMS sign read “USE BOTH LANES – MERGE AHEAD.” The sign closest to the work zone showed alternating messages of “TAKE TURNS – MERGE HERE.” When traffic speeds increased as congestion dissipated, the CMSs were turned off and the system returned to the typical static work zone traffic control mode that encouraged the early merge. For a pictographic understanding of the physical set up of signs used in the Mn/DOT DLMS field test, refer to Figure 4 in the source document (see Source information listed as part of this narrative).
  • Place portable CMS on the shoulder or median nearest the discontinuous lane when implementing a DLMS to manage a work zone. Mn/DOT found that placing CMS on the shoulder or median nearest the discontinuous lane makes the signs much more visible to the drivers who may have a tendency to leave the lane early instead of utilizing its full storage capabilities. The agency determined that the CMS advising merge at the taper point should be positioned adjacent to the last static merge message sign so there is no contradiction in instructions from signs at two different locations. This placement also gives an additional buffer zone of distance to complete the merging procedure, which could encourage drivers to use the discontinuous lane until they near the impending merge point.
  • Permit drivers to become comfortable with the DLMS by allowing an adjustment period and conducting an outreach program. Not every driver understood or obeyed the instructions to “USE BOTH LANES” to the taper point. Especially in the first days of the Mn/DOT’s DLMS operations, many instances of slow-moving continuous lane and empty discontinuous lane were present. Occasionally, a small group of drivers would follow the directions and use the left lane to the merge point but after their merging procedures, the lane would become vacant again. The frequent reason for these periods of an empty discontinuous lane was vehicles blocking the lane farther upstream. It was common to see a car or truck in the discontinuous lane content to travel at the same speed as the slow-moving continuous lane. This action prevented any other drivers from using the lane until the vehicle finally decided to merge. In a few instances, angry drivers used the shoulder or median to pass the blocking vehicle. As time progressed, it appeared that drivers became more comfortable with the concept of using or allowing other vehicles to use the discontinuous lane. Vehicles were still occasionally seen blocking the discontinuous lane but not nearly as frequent as before.

Mn/DOT’s deployment of a DLMS yielded some useful guidance for future such installations. Changeable message sign placement, wording of lane use instructions, typical driver reactions, and system activation thresholds were major factors that needed fine tuning during system deployment. The important insights from the Mn/DOT test were that: (a) the activation of the DLMS saw a 35% reduction of the typical work zone queue length, and (b) the usage of the discontinuous lane and the adjacent continuous lane nearly equalized as a result of DLMS. Despite these advantages, traffic throughput decreased slightly with DLMS, an anomaly attributed to the less than ideal driver adherence to lane use instructions. Additional analysis also revealed that the difference between the travel times with and without the DLMS was not statistically significant. This lesson learned promoted ITS goals of improving safety and mobility in work zones.

Dynamic Late Merge System Evaluation

Dynamic Late Merge System Evaluation
Publication Sort Date
Minnesota Department of Transportation

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