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August 22, 2014

Mini-roundabouts can be built for as little as $50,000

Journal Construction Editor

Photo courtesy of city of Mount Vernon [enlarge]
The mini-roundabout in Mount Vernon has an asphalt island that big trucks can drive over while turning.

With the budget noose tightening on road funding, the Washington State Department of Transportation will build three mini-roundabouts near Bellingham to save money.

This is the first time that WSDOT will use the abbreviated roundabouts, according to Chris Damitio, a WSDOT project engineer.

“It's a low-cost mobility and safety enhancement,” Damitio said. “We would love to do a full-blown roundabout.”

But a full-size roundabout costs about $1.5 million to $2 million, while a mini-roundabout can be built for about $100,000, Damitio said.

According to WSDOT, compact roundabouts have a smaller diameter — about 62 to 67 feet at the three Whatcom County intersections — compared to up to 120 feet for a full roundabout. They are similar to full roundabouts in that they have raised center islands to direct traffic, but those islands are about 3 inches tall instead of 6 inches or more with a full roundabout. They also have raised channelization to guide traffic into the circle.

Most vehicles go around a mini-roundabout's raised center island. The rear wheels of longer trucks and trailers may ride up onto the island to help them through the roundabout.

The project in Whatcom County will build the mini-roundabouts at the northbound and southbound Interstate 5 ramps at Slater Road, and at Pacific Highway and Slater Road. Bids will be opened on Aug. 26 and the work is estimated at $300,000. WSDOT is handling design and bidding and Whatcom County is paying for it. Construction is expected to start in mid- to late September.

Damitio said traffic backs up at stop signs on the ramps and on Pacific Highway. Slater Road doesn't have any traffic control devices in those areas.

The new roundabouts are expected to reduce collisions and their severities by slowing traffic on Slater from a current limit of 35 mph to 10-15 mph as cars enter the roundabouts.

The results of a United Kingdom study published in 2006 show a 30 percent reduction in crashes with mini-roundabouts compared to signalized intersections. A report from the Federal Highway Administration says the small roundabouts are common in the U.K. and France.

Damitio said construction in Whatcom County will be quick: the compacts will take about two weeks to build whereas full-size versions would take about two months.

WSDOT recommended the mini-roundabouts after examining several options, including making each intersection a four-way stop, installing traffic signals or adding full roundabouts. It found that four-way stops would create even bigger traffic back-ups, especially at peak hours.

Damitio said WSDOT is looking at other locations to build mini-roundabouts, but nothing is definitive. He said WSDOT engineers will be watching to see how the Whatcom County project works out.

There are about 120 standard roundabouts throughout the state, including about 35 on state highways. The first one was built in 1997.

One of the first mini-roundabouts in the state was built last summer at Cedardale and Anderson roads in Mount Vernon by Interwest Construction.

Esco Bell, Mount Vernon public works director, said drivers were initially uncomfortable with the roundabout but have since gotten used to it.

“It works fabulously,” Bell said. “It handles traffic capacity and is safer.”

Bell said there was a death at the intersection in 2013 before the mini-roundabout was installed that was attributed to a driver failing to stop.

There have been no deaths since the mini-roundabout went in and only four minor collisions. Bell said there have been no accidents this year.

“The educational component is really important to help everybody accept it,” he said.

After Interwest built the mini-roundabout, the city came back and installed a center island with yellow curbs containing an asphalt-filled interior.

City engineers heard about the mini-roundabout from Victor Salemann, an engineer at David Evans and Associates who now works at Transportation Solutions Inc.

“It was an opportunity that popped up right when we needed it,” Bell said.

One of the things that Bell likes about the mini-roundabouts is that they can be built within most intersections, foregoing the need to buy right of way property. He said they can be built for as little as $50,000 with a simple configuration and up to $250,000 with a more complex layout. He said you can't build a signalized intersection for less than $500,000.

The mini-roundabout at Cedardale and Anderson roads was built for about $150,000.

Bell said they are planning to build another mini-roundabout next year at South LaVenture Road and East Section Street, near Mount Baker Middle School and Little Mountain Elementary. He said this one will be more complex because it will need pedestrian improvements due to its proximity to the schools.


Benjamin Minnick can be reached by email or by phone at (206) 622-8272.

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