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‘Diverging diamond’ design chosen

‘Diverging diamond’ design chosen

This is a schematic drawing of how a diverging diamond design works.


POSTED April 12, 2017 9:33 a.m.
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City officials remain hopeful that the new Service/Mitchell/Highway 99 interchange is completed by 2023 and with it one of the first "diverging diamond" designs in California.

On Monday the Ceres City Council voted unanimously to recommend the unorthodox design after the state and the city have dismissed over 20 designs over the past several years.

City Manager Toby Wells suggested that passage of SB1, with a slate of new gas and vehicle tax increases will make more funding available to make the $100 million project a reality.

The city had only two design concepts that proved workable, however the other one did not significantly change the current conflict with traffic moving on the southbound freeway off-ramp at Mitchell Road with cross traffic coming on the southbound on-ramp. It also did nothing to improve circulation to west of the freeway, said Wells.

The diverging diamond concept is unique but not new, Wells noted.

"This is very common, very prevalent in Europe," he said on Monday. It's also a growing phenomenon in the United States because of its safety factor.

The nearest diverging diamond is operational near the Reno Airport south of I-80 and off of 580. One is being evaluating for Union Road in Manteca, which Wells called simpler than the more complex one for Ceres because of the railroad configuration.

While innovative, he said the design "makes sense" because of the constraints of land and land uses.
"We're trying to find ways to move as much traffic as we can through that interchange as safely as we can," said Wells.

In a traditional interchange there are 26 points of conflict with only 14 with the diverging diamond. The result is "significantly reducing both the number of accidents and severity of accidents" through slower speeds and reduced possibility of broadside crashes.

Ceres officials prefer the diverging diamond design as the best to handle traffic volumes into 2040. That's why triple left-hand turn lanes are being designed for the intersection of Service and Mitchell roads. That equates to five or six times to movement potential than exists today. The design also takes into consideration the limitations caused by the railroad tracks that run parallel just to the west of 99.

Wells said the diverging diamond design will allow full freeway access at Service Road.

"One of the key things that it does is providing us that direct access to west of 99," he said. Motorists will be able to get on and off the freeway in all directions at Service Road. That's important because of the regional commercial uses that are being designed into Ceres' new General Plan.

The next step is to circulate for public comment the detailed environmental study by June with a preferred alternative selected with construction designs occurring in 2018 with construction starting in 2020 and taking three years.

Funding for the project would come from multiple sources, including redevelopment bond proceeds, traffic impact fees, $30 million from new Measure L road tax revenue and SB1 to free up state funds with the possibility of federal highway funds.

The interchange design would also eliminate the current way motorists exit southbound 99 at Mitchell Road. The southbound off-ramp and southbound on-ramps cross each other, so drivers coming off the freeway must stop and wait for a break in southbound Mitchell Road traffic headed toward the on-ramp. Under the diverging diamond design, the only freeway access at Mitchell Road to remain would be the southbound on-ramp and the northbound freeway on-ramp.

The diamond design concept is well explained in several Youtube.com videos. Normally a vehicle travelling westbound over a freeway overpass would be on the north side while eastbound motorists would be on the south side of the overpass. The diverging diamond flips that pattern, mostly because it allows for less traffic conflicts, better and increased traffic flows and better access to the freeway. Proper signage are a "critical component" in keeping motorists from becoming confused about movement, said Wells.

Diverging diamond interchanges have worked successfully in Missouri, Florida, Minnesota, Wyoming, Nevada, Idaho, Texas, Utah, Kansas, Pennsylvania, Mississippi, Tennessee, Ohio, New York, Georgia and other states. Europe has used the design for about 40 years, said Wells. In all, 51 locations in the United States use the design.

The interchange design is key for the development of the area along Mitchell Road near Highway 99. The Mitchell Road Shopping Center with the Walmart Supercenter has been approved north of Service Road but a triangle piece to the south can be developed once it's known where right of way is delineated.

The city has been planning a new Service/Mitchell/99 interchange since 1997. The original design called for couplets - where Mitchell Road was southbound and Moore Road was the northbound movement - but it was scrapped for an expensive 2002 design plan which Caltrans ultimately rejected in 2009. The city dusted off the interchange project in 2011 and consulted Caltrans about better designs.

The Ceres City Council was introduced to the design in November 2014 and expressed excitement for it.

The refined design calls for the Brickit Court industrial area to be accessible by Don Pedro Road and not the current route via El Camino Avenue.

 

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