Why Might Speed Signs Not Affect Traffic?

In 2005, the city of Bellevue, WA, assembled information from the previous 5 years testing radar speed signs at a range of locations. Not only did the report strongly show that these signs do reduce speeds and often remain effective even after 4 years of use in the same spot, they also recommended some reasons which explain why some signs aren\’t as effective and may not provide good traffic slowing effects. While this is not the fault of the signs themselves, it would seem to be the choice of sign location.

They found that the signs that didn\’t have much of an effect were the ones that had the most limited sight-distance for approaching traffic. A driver can\’t be influenced by an indicator he or she can\’t clearly see. While this is simple common-sense, it appears that one shared trait among the signs that didn\’t work was that they weren\’t visible very far away.

Signs that are approached at a high speed and do not become clearly visible until the driver is almost on them do less for traffic slowing than those the driver can see further away, even though it\’s far enough back the sign does not yet display that driver\’s speed. At least 300 feet of clear visibility was determined a good amount for effective usage of the signs.

The least effective signs were also the ones put in places where far fewer drivers sped to start with. When about 85 % of drivers in that area were already following the posted speed limit, the speed reductions were far less. This makes good sense, since folks are usually provident with their speed in those areas. Speed changes were much more dramatic in places where people generally surpassed the limit.

The study inspired the recommendation that the town continue to install radar speed signs for traffic control, and listed a few considerations so far as placement to help the town make the most use of this valuable traffic control tool.

A speed display sign that\’s not placed in the right area won\’t be as useful as it probably should be. Let the experts at trafficalmsystems.com help you determine which traffic calming devices are best for your neighborhood.