Variable Speed Limits on I-270 in St. Louis
Additional information and resources
What is the problem?
For the most part, traffic flows well along I-270. However, during periods of morning and evening traffic, there are sections of the highway which get extremely congested. This means that there is enough space on the road to handle traffic; however, some adjustments need to be made to traffic flow to allow it to flow smoother.
What is the solution?
If traffic slows down as they approach a congested area and all the drivers stay at a constant speed, traffic will get through the congested area faster. Imagine the highway as a funnel. Now, imagine the traffic which has to travel along the highway during a certain time as a container of rice. If you pour all the rice into the funnel at the same time, it gets congested at the bottom of the funnel and takes some time to work through the funnel. Now, if you slowly pour the rice into the funnel – keeping it at a steady pace – the rice moves through the funnel evenly and doesn’t cause congestion. In fact, even though the rice is entering the funnel slower, all the rice gets through the funnel (to its destination) faster.
Note: this metaphor is based on an idea from Paul Haase of Sammamish, Washington, in response to a challenge by Washington State Department of Transportation Secretary Doug MacDonald.
What does that solution look like in St. Louis?
The Missouri Department of Transportation is starting variable speed limits along I-270. When congestion starts building along stretches of I-270, MoDOT will use changeable speed limit signs to vary the speed limit on the road. Speed limits along I-270 could range from 60 mph during extremely light traffic, to as low as 40 mph during extreme congestion. This would also include congestion due to crashes along I-270.
When does this program start?
MoDOT will start using variable speed limits along I-270 once all the equipment is installed and tested. That work should be finished sometime in early May.
