Suggested Speed Limits Coming to I-270

Missouri transportation officials have announced that a two-year experiment with variable speed limits on Interstate 270 in St. Louis has achieved limited success in reducing traffic congestion, while transforming the road into one of the safer highways in the state.

Because of the results, the (MoDOT) will now have advisory speeds on stretches of I-270, inlcuding the stretch running through Town and Country. The signs will post a suggested speed between 55 mph and 10 mph, depending on traffic. Drivers will know when the speed is advisory because it will be flashing, not constant, on the electronic speed limit signs along the interstate.   

An analysis of four segments of I-270, before and after the current variable speed limits were installed in May 2008, found that although traffic volume went up by 10 percent, the overall effect on increasing traffic flow during peak hours was marginal. The study, conducted by the Missouri University of Science and Technology with consulting firm HDR, Inc. and Heartland Market Research, concluded that with further refinement of the system and increased driver compliance, the benefits could be much higher.

MoDOT Assistant District Engineer Tom Blair said the results of the study show variable speed limits can have impact, but further changes are needed before a decision will be made to expand or discontinue the system.

“There are some improvements in traffic flow as a direct result, but they are not as strong as we would have liked,” Blair said.

However, the study did find a marked drop in the number of accidents along the busy interstate, which Blair deemed the state’s “biggest and baddest” stretch of pavement. According to the report, the rate of accidents dropped by as much as 8 percent in 2009 compared to 2008, while other St. Louis metro area highways such as Interstate 70 and Interstate 55 saw an increase.

The decrease makes I-270 the safest highway in the St. Louis metro area, Blair said.

The study also conducted surveys of the public and law enforcement, finding that a majority do not believe variable speed limits make a difference. According to the study, 65 percent of St. Louis citizens surveyed think they should be eliminated.

“It doesn’t surprise us,” Blair said of the survey. “I think the improvements we saw in mobility are small enough that (drivers) don’t feel it in the seat of their pants.”

In order to increase its efficiency and address such public relations concerns, MoDOT will be rolling out the major change to from variable to advisory speed limits in early July. Advisory speeds are not enforcable, but are there to inform drivers up upcoming congestion and will range from 10 mph to 55 mph.

Currently, federal regulations prohibits enforceable speed limits to go below 40 mph on a highway, something Blair said has caused commuters to feel that the variable speed limit system is irrelevant.

“They think, ‘You say go 40 mph, and I am only going 20 mph--this is a joke,’” Blair said.

According to a MoDOT press release, the advisory limits would function as a recommendation of speed rather than a mandated limit, similar to the way advisory speeds are posted on curves and ramps.  

Although it may be counterintuitive at first to most drivers, Blair explained that if drivers slow down instead of speed up as they approach an accident or congestion, they will get through faster.

A video on MoDOT’s website uses a funnel and a jar of rice to show this theory in action. When the rice is all poured in at once, the funnel quickly becomes backed up because only a certain amount can get through the bottleneck at the bottom. However, if the rice is poured in at a slow, steady pace, it will move through this bottleneck smoothly without any grains coming to a complete stop.

“If we all try to speed up, we are all going to jam each other up,” he said.

Blair estimated that the advisory limits will remain in place for about a year, after which MoDOT will evaluate whether the program should be brought to other highways in the state. He said after an initial cost of $900,000 to install the 70 signs, there is minimal money needed to maintain the solar-powered items.  

Blair said he hopes the changes will improve the system and create more meaningful changes to the flow of cars along St. Louis’ busiest highway.

“At this time, we would love it to be more successful than what we can announce it is today,” Blair said. “We didn't see enough results to say that we are ready to expand.”