An Implementation of Proactive Variable Speed Limits (VSL) Was Simulated To Reduce Critical Traffic Conditions by 39 Percent.

The Use of a Dynamically Adjusting Speed Limit Can Be Used To Manage Off-Ramp Queue Overflows and Reduce the Risk of Collisions.

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Variable speed limits for motorway off-ramp queue protection

Summary Information

A paper by researchers at the Queensland University of Technology proposes the use of Variable Speed Limits (VSL) to improve safety at congested off-ramps. VSL can be used to dynamically respond to queue congestion, reducing the speed of oncoming traffic and preparing drivers who are approaching the end of the queue.

In this particular paper, researchers offer a proactive VSL strategy that uses forecasted queue size to determine speed limits, with the goal of preventing a queue from spilling over the off-ramp to the mainline. The system uses input from a loop detector to assess the number of incoming cars and to update its predictions.


A series of simulations of an off-ramp were run to test the VSL strategy, using real-world data to inform the model. In order to model the likelihood of traffic collisions, the study looked at the incidence of low Time to Collision (TTC) values. This parameter represents the expected time for two vehicles to collide, provided they continue driving at their current velocity. Low TTC values indicate that a situation may be considered a "critical traffic situation." A commonly used cutoff value for this point is 3.5 seconds. In general, lower TTC values represent relatively higher probabilities of collision.


  • The incidence of TTC values between 0 and 1 seconds decreased by 29 percent when a proactive VSL was employed, compared to the base scenario of a static speed limit.
  • Overall, the incidence of drivers with a TTC of under 4 seconds decreased by 39 percent with proactive VSL, compared to the base scenario.
  • Travel time for exit traffic was found to significantly decrease by 2.8 seconds, or approximately 1.8 percent, when proactive VSL was in use. Travel time for mainline traffic was not found to significantly change.
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