St. Louis (KMOV) – Interstate 270/255 is one of the most congested roads in St. Louis, so in an effort to improve traffic flow, MoDOT implemented variable speed limits three years ago, but now plans to use the signs as advisory speed limit signs.
Earlier this spring, MoDOT announced the effort has been successful but changes need to be made to continue improving traffic flow and decreasing crashes. The digital speed signs will change to advisory speeds and that change process begins Friday. From this point forward, the enforceable speed limit on I-270 will be 60 mph except in posted work zones.
“Starting Friday, the digital speed limit signs will remain constant at 60 mph and our MoDOT crews will be installing new permanent black and white 60 mph signs along I-270,” said MoDOT Assistant District Engineer Tom Blair. “During the next two weeks, the digital speed signs will be changed to yellow with black lettering indicating advisory information and the digital speeds will flash from 55 mph to 10 mph to advise motorists of slowed traffic ahead.”
The I-270 variable advisory speeds will adjust from the posted speed limit of 60 mph 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.
In two locations on I-270, the digital signs will remain at 45 mph due to ongoing 24 hour works zones with reduced speed limits. Those two locations are southbound I-270 between Dorsett and Page and northbound I-270 between Gravois and Interstate 44.
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 speeds 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, according to Blair. 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 300 cameras and 400 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 here.
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 here.