Variable Speed Limit Systems – Revisited

In a post on April 24th we discussed a recent webinar on variable speed limit signs. The speakers voiced their disappointment in the technology and found very little if any benefit from their use. But now a new study by the University of Missouri saw far better results for VSL signs used in work zones.

“Evaluation of Variable Advisory Speed Limits in Congested Work Zones” by Praveen Edara, Carlos Sun and Yi Hou found far better, but still mixed results when using Variable Advisory Speed Limit signs in work zones.

As we have known for some time, the VSL results depend to a great extent on the algorithm used. In the Missouri study the original algorithm used in the field resulted in shorter queue lengths and reduced speed differentials. In fact the maximum speed differential was reduced by a remarkable 10 MPH. However it also reduced throughput by 7 to 11% and travel time increased by 4 to 8%. So the results were mixed.

They then experimented through simulation. Using the same traffic data they tried two other algorithms. One smoothed over a one minute period and the other over a 5 minute period. The original field algorithm smoothed over a 30 second period.

The 5 minute smoothing algorithm still reduced throughput but only by about 1%. At the same time it reduced end of queue conflicts by 30% and lane changing conflicts by 20%. Speed variances remained low. And they saw medium to high compliance with these advisory signs.

Compliance is key, of course, and the Missouri experience was very different from previous installations in places such as Utah and Oregon. Even where VSL posted limits were enforceable, compliance only occurred when message signs explained the reason for the speed limit reduction or when law enforcement was present and actively enforcing those limits.

The reasons for this could be many. Maybe Missouri drivers are just more law abiding. It was not discussed in the study but perhaps MoDOT did a better job of explaining VSLs to the public before they were installed. Or perhaps the need for variable limits is more apparent to drivers in a congested urban work zone. But whatever the reason, it is clear we should not give up on variable speed limit systems just yet. More studies are needed, especially on the subject of the best applications for these systems and the algorithms driving them.

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