After a traffic study, results are in that the speed cushions on Lake Washington Boulevard in Renton are working to reduce speeding.

The 10 sets of speed cushions installed along Lake Washington Boulevard, between Houser Way North and Burnett Avenue, have caused a significant reduction in speeding on the street, according to a 2024 traffic study.

Renton civil engineer Blake Costa said this portion of Lake Washington Boulevard has historically been a speeding area. Costa said data was taken on speeding in the area in 2022 and in 2024 after the implementation of speed cushions.

“In the traffic world, we use 85th percentile speed and that’s 85 percent of the drivers are driving at or below that speed,” Costa said. “That speed was 40 mph on a 25 mph road. So that was really high and the volume was really high for this kind of roadway.”

The project was completed in October 2023. Costa said there were about 7,300 vehicles per day on average before the project and stayed about the same at around 6,500. The study showed the average speed of cars on the road decreased from 37 mph to 25 mph after implementing speed cushions.

“Our real goal was to get the 85th percentile between 25 and 30 because that is our metric we use to decide whether or not we should start the traffic calming process,” Costa said.

The 85th percentile decreased from 40 mph to 28 mph, hitting the goal they had for the project.

“The biggest piece was the number of speeders that were travelling over 30 mph,” Costa said.

According to the studies, before the project, 6,121 vehicles were travelling more than 30 mph each day. After, the number of speeding vehicles was 442, a decrease of 93 percent.

“Now the majority of people are travelling close to speed limit. That’s where we want speeds,” Costa said. “From like a geometric design point of view, you’re supposed to design your roadway to a specific speed limit and with the traffic calming, that allows us to bring speeds closer to the speed limit.”

According to the National Highway Traffic Safety Administration, there were 12,151 speeding-related deaths in the U.S. in 2022, and speeding was a contributing factor in 29 percent of all traffic fatalities.

Costa said they follow FHWA guidelines for creating the speed cushions and they are basically elongated speed bumps. He said the ones they built for the project are about three to four inches high.

“They’re not as abrupt as a speed bump,” Costa said. “Speed bumps are more appropriate for really low speed, like less than 10 mph, for like a parking lot. You’d never want to put those on a street.”

Costa said they also have a wheel path cut-through that emergency vehicles can straddle, which lessens the likelihood of reduced response times from emergency services.

“That was the big piece that allowed us to start using these as calming devices because the previous mayors, city councils and emergency services were against them because they were concerned they would reduce their response time,” Costa said. “With these, it allows the vehicle to go through without having to really slow down.”

Costa said they are limited to where they can put the speed cushions because they don’t want them on arterials where cushions could cause issues to cars going too fast. He said they are limiting speed cushions to streets with speed limits of 25 mph or less.

“So essentially neighborhoods, that’s where we’re focusing this, where the people are most vulnerable: where they’re living and where kids are playing outside or in the street,” Costa said.

Costa said they have already added speed cushions to an area near Valley Medical Center, and the next projects will be on Aberdeen Avenue NE between NE 12th Street and NE 24th Street, and SW 5th Place between Hardie Avenue SW and SW 5th Court.

Speed cushions along Lake Washington Boulevard have reduced speeding on the street, according to a 2024 traffic study. Photo by Drew Dotson/ Renton Reporter