At 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.