Variable Advisory Speeds

About: Variable Advisory Speeds

Variable Advisory Speeds Inform Drivers of Congestion and Lane Closures on I-270

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 drivers 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 uses variable advisory speeds along I-270/I-255.  When congestion starts building along stretches of I-270, MoDOT uses changeable speed signs to vary the advisory speed on the road.  Variable Advisory Speeds along I-270/I-255 could range from 60 mph during extremely light traffic, to as low as 10 mph during extreme congestion. This would also include congestion due to crashes, work zones or weather conditions. If the advisory speed posted is less than 60 mph, the speed will flash continuously.

Why is it an advisory speed?

The variable speeds on I-270/I-255 began as variable speed limits in 2008. After a study of the system in 2010, it was determined that enforcement of the variable speed limits had been minimal and many people were either angry or confused about the potential of enforcement. The variable speed limits had helped to reduce some congestion and crashes. In 2011, the decision was made to change them to advisory. The speeds will include a yellow and black sign indicating advisory saying “Advisory When Flashing.” The speed reduction is intended to advise motorists to slow down because there is slowed or stopped traffic on the road ahead. These may be due to upcoming congestion, work zone lane closures, weather conditions or stopped traffic. Static Advisory speeds are very common. Typical examples would be posting advisory speeds on a ramp or a curve.

Will the message signs explain why the advisory speed is lowered?

MoDOT will provide messages on the Dynamic Message Signs in conjunction with the reduced speed

When motorists see the advisory speed is reduced, the Dynamic Message Signs will give them information on why (i.e. for work zones, for weather road conditions, for crashes and for recurring congestion).