MoDOT: I-270 Getting 'Advisory' Speed Limits

Mediocre results from the changeable, digital-display speed limits posted on Interstate 270  in St. Louis have prompted transportation officials to unveil "advisory" speed limits instead beginning in July.

The new, "advisory" speed signs will post a suggested speed between 10 mph and 55 mph, depending on traffic, although an official 60 mph speed limit will remain in place. Drivers will know when the speed is advisory because the sign will be flashing, officials said.

State transit officials made the announcement following the release of a study Wednesday that reports only limited success reducing traffic congestion while also decreasing the number of accidents.

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.

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.”

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.”