ST. LOUIS -Interstate 270/255 is one of the most congested roads in St. Louis. In an effort to improve traffic flow, MoDOT started a pilot program on this roadway to include variable speed limits. Since spring 2008, speed limits have been adjusted to keep traffic running smoothly through crashes and recurring congestion. A recent study of the I-270 variable speed limits shows they have provided a benefit to traffic flow but need some changes to make them more effective.
"Since installation of variable speed limits in 2008, I-270 has the lowest crash rate of all St. Louis interstates, and some congestion has been reduced," said MoDOT Assistant District Engineer Tom Blair. "Following a review of the program, MoDOT will be changing I-270's variable speed limits to become variable advisory speeds."
By July 2011, I-270 variable advisory speeds will adjust from the posted speed limit of 60 miles per hour down to 10 mph depending on the congestion on the road. Advisory speed limits are not enforceable. However, the purpose is to caution drivers and inform them of issues further down the road. MoDOT's message signs will also include information regarding the congestion or lane closures ahead and increased travel times. The speed signs will indicate the posted 60 mph speed limit and will inform drivers that the speed is advisory when flashing, between 55 mph and 10 mph.
"We will continuously advise drivers of how fast or slow their vehicle should be moving in order to safely get to their destination, in the shortest amount of time," said Blair. "The advisory speed limit is similar to having advisory speed limit signs posted on curves and ramps. It is the suggested speed to safely travel that stretch of road."
Variable advisory speed limits are designed to pace the traffic leading up to recurring rush hour congestion or from congestion caused by closed lanes due to a crash or work zone. If drivers ignore the speeds and hurry to the point of the congestion, it actually creates more congestion and is slower to get through. If drivers obey the advisory speeds, the overall pace of traffic will get through the congested area more quickly.
"MoDOT's Gateway Guide traffic system of cameras and speed sensors knows when and where traffic problems happen. The message signs and advisory speeds are designed to warn you so you can slow down and prepare for congestion and potential lane reductions," Blair said.
A video explaining the changes and the theory behind variable advisory speeds is available on MoDOT's You Tube account at http://www.youtube.com/watch?v=DzXP-6wagzI&feature=channel_video_title. The full report conducted by the Missouri University of Science and Technology with HDR and Heartland Market Research is available on MoDOT's innovation library at: http://library.modot.mo.gov/RDT/reports/Ri08025/or11014rpt.pdf.