Portable Variable Speed Limit Systems

Late last year the FHWA Every Day Counts initiative held another wonderful webinar. This one covered two more work zone ITS products: variable speed limit systems and dynamic lane merge systems. This webinar was so chock full of information that I will discuss them one at a time. You can view the recorded webinar at: https://connectdot.connectsolutions.com/p1rhnco4915/?launcher=false&fcsContent=true&pbMode=normal .

Today let’s start with variable speed limit systems. Todd Peterson of FHWA began by explaining the basic structure and goals of a variable speed limit (VSL) system. These are portable, trailer-mounted signs that move as the work zone moves.

Josh Van Jura of Utah DOT described VSL experience in his state. Utah began with static (manually adjusted) signs. He says they have many interstates posted at 80 MPH. And for construction, especially where workers are exposed to traffic, they like to reduce those speeds significantly – sometimes as much as 30 MPH or more. He later said these systems are especially useful for slab replacement work where you have a relatively long work zone, but workers are present and visible only in one or two small locations. So the system gets the speeds down in advance of the work. It also drives down the speed variance.

UDOT VSL1 UDOT VSL2

So far they have chosen to use these systems where queuing is unlikely. They don’t want to solve one problem, only to cause another upstream. They have been pleased with the results and now want to move to dynamic VSL systems. They applied for and won a grant to finance their research. Kimley-Horn is doing the work.

Mr. Van Jura made an interesting point. He said that this project was especially susceptible to scope creep. As they discussed their wants and needs folks asked for CCTV, weather detection and more. Much of that was for the testing phase primarily. But even then those features would have driven the cost up significantly, so they chose to do without them.

The basic system uses sensors to measure speeds in and near the work zone. Those speeds are sent over a cellular link to a server where they are processed by an algorithm that then adjusts the posted speed limit to match the measured 85th percentile. The displayed speed limit is adjusted no more than every 10 minutes. Everything is date and time stamped showing the posted speed limit at that time at that location.

Todd Foster of VerMac asked what type of sensors they plan to use. As he said, doplar has occlusion issues but side-fire radar is a pain to set up. Josh said they plan to make that decision on a project by project basis. For two lane roads, they will use doplar. For projects on larger facilities they will weigh the need to relocate the sensors, the space available to place those sensors, and the costs associated with each to decide what should be required for that application.

A survey during the webinar asked the audience if their agency was considering using VSL systems in their work zones. 70% said yes. It then asked whether they were considering a regulatory system or an advisory system. 73.8% plan to use a regulatory system. Utah Highway Patrol cooperated fully with this UDOT initiative. Apparently other states don’t see that as an issue either.

We look forward to the final publication of Utah’s research. But it sounds like portable VSL’s should be another tool in our work zone ITS toolbox.

In our next post we will look at their discussion of lane merge systems.

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.