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Speed Limits 101

Posted On: December 06, 2017

A common call we get at Chelan County Public Works is from frustrated residents who want to know what they can do about motorists driving over the speed limit in their neighborhood.

State law sets Washington’s basic speed law as well as the maximum speed limits for state highways, county roads and city streets. The law also authorizes agencies to raise or lower these maximum speed limits, but only when supported by an engineering and traffic investigation.

So the answer for callers, unfortunately, is that there is no easy answer.

The 85th Percentile

Research and experience show that effective speed limits are those that the majority of motorists naturally drive on a roadway. Research also shows that raising and lowering speed limits doesn’t substantially influence that effective speed limit.

Speed limits reflecting the speed most motorists naturally drive are selected, in part, by determining the “85th percentile speed.” That’s the speed that 85 out of 100 vehicles travel at or below on a designated road.

Data from the Washington State Department of Transportation shows that roadways with speed limits set at the 85th percentile speed have fewer collisions than roads where the posted speed limit is above or below what the majority of drivers naturally drive.

Along with the 85th percentile, engineers also consider other factors when setting speed limits, including roadway characteristics, roadside development and lighting, parking practices, collisions rates and traffic counts.

But if you lower the speed limit, people will drive slower

This is not true. People also don’t automatically drive faster when the speed limit is raised. Most drivers base their speed of travel on current road conditions. So studies have consistently shown there are no significant changes in that 85th percentile speed when speed limits are modified, according to the WSDOT.

In addition, most people believe that lowering a speed limit is as easy as putting up a new sign. When considering a speed limit modification, engineering and traffic investigations must first be completed by Chelan County Public Works. The information than must be presented to and reviewed by the Chelan County Board of County Commissioners, which decides whether to pass a new speed limit into law. So changing a speed limit – so it can be enforced – can be a lengthy process.

Why not install speed bumps on my road?

In Chelan County, where we enjoy four seasons, speed bumps are just not practical when it comes to snow removal and snowplowing practices. In addition, temporary speed bumps are time consuming to install and then remove each year. They also tear up the roadway when anchored into the road surface, and they can even become dislodged.

So speed bumps in Chelan County are neither practical nor financially efficient.

I’m going to put up signs in my yard, warning drivers to slow down

Privately owned signs cannot be placed in the county right-of-way, as it is against Chelan County Code (Chapter 11.92.050).

County right-of-way is an easement for public travel. It’s the county-controlled property that includes the roadway and, oftentimes, property that runs alongside the roadway. While the right-of-way is typically 60 feet wide (that’s 30 feet on either side of the road as measured from the center of the roadway), right-of-way varies throughout the county. It can be as much as 120 feet in some areas, and as little as 40 feet in others. It can also vary along one stretch of roadway.

If you don’t know where the county right-of-way is on your street, call Chelan County Public Works at 667-6415 so someone can answer that question for you. Or visit the Chelan County Code, Chapter 11.92, for sign guidelines at www.codepublishing.com/WA/ChelanCounty. Guidelines also prohibit residents from posting signs on utility poles.

For more details about sign regulations in the county refer to Chapter 11.92 of the Chelan County Code.

So what can we do about speeders?

If you believe you have chronic speeders in your neighborhood, call the Chelan County Sheriff’s Office on its business line at 667-6851. There may be some things the sheriff’s office can do, including doing an emphasis patrol to deter motorists from speeding in the area or setting up a traffic speed trailer along the roadway to check the speeds of travelers.

The sheriff’s office also may notify Public Works if it believes a posted speed limit may be inappropriate for the roadway. Public Works then would, in turn, consider beginning an engineering analysis.


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