How Effective Are Radar Signs?

radar-sign-animatedThe City employs many methods to deter speeding vehicles: building speed humps, requesting police enforcement, and promoting education efforts. These efforts have been used for many years in the City of Bellevue and are tried-and-true.  Another approach to deter speeding vehicles is the use of stationary radar signs; you know, those signs that display your speed when you drive by (and flash when you are exceeding the speed limit).

You drive by them and they are meant to get your attention, but do they actually get you to slow down?  In Bellevue, the answer is yes.

Let’s take a look at how/why stationary radar signs are installed and how effective they are in keeping speeds near the posted limit.

Neighborhood Traffic Safety Services, the group in the City that manages this blog, is also responsible for installing and maintaining both elementary school flashing beacons and stationary radar signs throughout the City.  Currently, 31 stationary radar signs can be found on streets in Bellevue and most elementary schools have flashing beacons that help establish when 20 mph school zones are active.

The When, Where, and How of Radar Sign Installation

Radar signs can be a less obtrusive measure than installing physical elements, such as speed humps.  They are relatively low-cost tools that require minimal community support (65% of responding households in the proposal area must support the sign before moving forward with the project).  To help prioritize where signs should be installed, a rating system is used that takes into account the average speeds of vehicles, average daily traffic, number of accidents, proximity to community destinations such as schools and parks, and the presence of pedestrian facilities.  As funding allows, signs are installed in locations with the highest priority.    

Effectiveness of Radar Signs

Once a location has been equipped with a radar sign, just how effective will it be in reducing vehicle speeds?  In Bellevue, they are categorically effective in reducing vehicles speeds.* Note that over time, the effectiveness in reducing speeds actually has gotten better!

The below chart shows the reduction in vehicle speeds over three periods of time: 1 to 3 years, 2 to 5 years, and 6 to 8 years with the sample sizes of 31, 22, and 10, respectively.  After 1 to 3 years, 19% of the 31 radar sign locations in the study saw reductions of 85th percentile speeds of 0 to 4%, 42% of locations experienced 5 to 9% reductions, with 36% of locations experiencing 10% or more in speed reductions.  One location, or 3% of the sample size saw speeds slightly increase.


The trend over time reveals that speeds at radar sign locations continue to decrease with 60% of locations experiencing a 10% or more speed reduction after 6 to 8 years of installation.  Clearly, radar sign installations are not fleeting in their effectiveness; they have prolonged utility in reducing vehicle speeds. 

Radar Signs Decrease Speeds, but do People Like Them?

Since 2000, installing radar signs has been a robust element of the City’s traffic calming toolkit. Year after year, residents see the utility in these signs and often take to the Radar Sign Request form to formally ask for one in their neighborhood.  Once the form is received, it will be evaluated based on the aforementioned criteria. 

Radar Signs Coming Soon!

In the coming months, we will be installing radar signs on SE 34th Street near West Lake Sammamish Parkway and on 164th Place SE/SE 38th Street.   Check out the Projects tab on the blog for more information.

Additional Resources:

 *Data used to determine effectiveness from the 2009 Stationary Radar Sign Program Report. Reduced speeds represent decrease in 85th percentile speeds—the speed at which 85% of vehicles are traveling.  

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