Variable Speed Limit Signs

VSL UtahAt the recent National Rural ITS show held in Utah, a workshop was presented on the subject of variable speed limit signs. The speakers included Lynne Randolph from the Southwest Research Institute who talked about VMS use in Texas, Brian Christensen of Horrocks Engineers and Glenn Blackwelder of Utah DOT who discussed the local VMS installation on I-80 between Salt Lake and Park City, and Josh Crain of DKS who talked about the use of VMS in Oregon.

This technology remains one that makes good sense in theory, but whose complete affect on traffic is still not fully understood. This workshop filled in some of the blanks but there is more work to be done.

In short, a variable message sign system measures traffic speeds as conditions change and then displays the 85th percentile speed rounded down to the nearest 0 or 5 on regulatory speed limit signs upstream. The idea is to smooth traffic flow and reduce the speed variance that occurs as drivers begin to respond to changes in weather, traffic volumes and construction work.

The speakers shared some interesting lessons learned. Utah’s installation seemed to be the most successful. They have a total of 15 VSL signs through Parleys Canyon, a very steep climb from Salt Lake City east to Park City. There are 8 signs eastbound and 7 signs westbound. The other deployments discussed had far fewer signs or in Oregon’s case, they are single installations spread over the state.

Utah also posts VARIABLE SPEED LIMIT SYSTEM AHEAD signs at each end of the canyon. For these reasons they are far more visible and, I suspect, gets better results.

Utah did find that the white LED displays washed out behind a sign covered in the standard white reflective sheeting. To overcome this they added a black band around the speed display. You can see in the photo above that this change greatly improved the contrast so the signs are now easy to read in all light conditions.

The four speakers seemed to agree that drivers were more inclined to respect lower VSL speed limits when weather or construction activity justified the change. They did not respond as well when limits were lowered due to congestion downstream. Drivers need to know why they should slow before they respond.

Utah’s signs are enforceable. Texas’ signs are not. Oregon has installed some of each. They have developed a decision tree that weighs local conditions and the reasons for the installation to determine which type should be used in each situation.

It was a very informative workshop. I encourage you to download it when it becomes available.

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.