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Beginning in mid to late December, drivers will notice some new message signs on I-91 South as they approach the work zone of the I-91 Viaduct Rehabilitation Project in Springfield.

MassDOT is implementing a pilot program for a Dynamic Lane Merge System to improve traffic flow and increase safety where the lanes on I-91 South drop from two to one. MassDOT received a number of comments from drivers that expressed concerns about safety issues at the merge. The pilot program is expected to address these concerns and alleviate issues of safety and congestion.

Late Merge System

Diagram showing a “Late Merge System” with message signs and traffic cones.

A Dynamic Lane Merge System uses real-time message boards to tell drivers how and when to merge based on data collected by sensors positioned in traffic barrels along the road. The system is “dynamic” because it uses the traffic data to alternate between guiding vehicles into a “late merge” or an “early merge.”

A late merge works best for high traffic volumes and low average speeds due to congestion. It uses the maximum capacity of the roadway by instructing drivers to remain in their lanes until the merge point, at which point they will take turns merging to create a “zipper effect.”  Filling both lanes with vehicles and taking turns entering the merge point makes the late merge work.

Early Merge

Diagram showing an “Early Merge System” with message signs and traffic cones.

An early merge works best in low traffic volumes and high average speeds, and instructs drivers to merge well ahead of the merge point to avoid slowing down traffic.

When it is up and running, the Dynamic Merge System should help reduce congestion and increase safety for all drivers. You can read more about it in this fact sheet on the I-91 Viaduct Rehabilitation Project website.

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