Variable Speed Limit Systems OR Queue Warning Systems?

You may be familiar with FHWA’s Every Day Counts program that is designed to speed the adoption of promising technologies. In their third iteration known as EDC-3 they have chosen to look at two work zone technologies: variable speed limit systems and end of queue warning systems. The work on these is nearly complete. Expect the final documents early this coming year. This blog will no doubt comment on it when the time comes.

Meanwhile, in the September issue of the University of Minnesota’s CTS Catalyst newsletter they ran an article looking at the effectiveness of a variable speed limit system on I-94 in the Twin Cities. This is a permanent system, not one in a work zone. But data suggests that in this case, it is not helping to reduce crashes on that stretch of highway. They stressed that the final results are not yet in, but crash statistics to date show no change.

According to the article, the local MTO director, John Hourdos, has a few theories about why this is occurring. “issues include a simple time lag in the VSL system, a requirement that all lanes display the same speed limit, and the complexity of the I-94 commons area itself. In addition, the driving public simply doesn’t understand what the signs are telling them. “People do not know what the system really does,” Hourdos says. “There hasn’t been much education on it… and when they try to decipher it on their own, they get even more confused.””

This offers several lessons for work zones that we should consider here:
1) VSL systems are not well suited to conflict points along multilane freeways. In this case traffic in right lanes is trying to merge left while traffic in left lanes is trying to merge right. This results in very dynamic queuing often in just one lane.
2) Education is critical. Drivers didn’t understand the purpose of the system, nor the advisory nature of the changing speed limit.
3) VSL systems look at all lanes downstream. As a result many complained that it is not responsive enough. Slowing in one lane is not reflected in the posted speed limit as quickly as some feel it should be.

A fourth lesson we might draw from this discussion comes from a comparison of variable speed limit systems and queue warning systems. Both are ultimately intended to reduce crashes. Variable speed limit systems are used more as a corridor treatment, while queue warning systems are normally used in the smaller area in advance of a work zone where frequent and dynamic queuing is expected.

In urban areas where most traffic is local commuter traffic and where most stay in one lane, VSLs work well. But at decision points where conflicts occur as vehicles are merging in different directions, they apparently don’t work as well.

Queue warning systems, by their very nature are more responsive. And their messages are better understood by drivers.

Both systems are good tools, and they are similar in design. But keep the strengths and weaknesses of each in mind when choosing the best one for your application.