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How this wild Manteca interchange design actually made traffic safer in Nevada

It might look like you're driving on the wrong side of of a Diverging Diamond Interchange, but you're not. It's designed that way to make things safer and quicker.

MANTECA, Calif. — While it might be reminiscent of a Hot Wheels ad, the new Diverging Diamond Interchange [DDI] that will be built in Manteca actually makes traffic move safer and quicker through an intersection.

The interchange style is a first for California, but our neighbors in Nevada recently opened their fourth interchange. 

“It can be a little bit of a shock to some people because I think the famous quote that people like to use is ‘It’s like you’re driving on the wrong side of the road,’ but you’re not. It’s designed that way,” said Devin Cartwright, Project Manager with the Nevada Department of Transportation.

While the interchange has caused concerns for some in California, Cartwright heard a lot of the same concerns when he designed Nevada’s first DDI in 2013. Despite the concern, the end result was a drop in both the number of crashes and a drop in the severity of the crashes.

RELATED: First of its kind interchange in California will be built in Manteca

How a DDI works

Credit: Courtesy: Mark Thomas
The diverging diamond intersection reduces potential accident locations from 26 to 14.

A DDI has drivers enter from the right side of the road, cross to the left side as they go through the interchange, and cross back again to the right. This crisscross pattern keeps traffic moving through the intersections and provides free left-turn movements for drivers heading onto and off-of the freeway.

“It can be confusing at first, but really, it’s kind of a very simple operation," Cartwright said. "If you want to make a left hand turn at the interchange, you stay in the left hand lane. If you want to make a right hand turn at the interchange, you stay in the right lane.”

A diamond interchange, where ramps come from the freeway onto a street with traditional intersections at both the ramp and end of the highway, doesn't always have the ability to move traffic efficiently. 

While the intersections are coordinated, they still need three separate timing operations to function. With a DDI, things get simpler because there are only “through movements,” and it gives free right-hand and left-hand turn movements to get people onto the freeway.

“You only really have to wait through one signal to get either direction on the highway as opposed to a traditional interchange where you have to go through two.”

Why the DDI works

Credit: Courtesy: Mark Thomas
Manteca will construct a Diverging Diamond Interchange to help with traffic flow and efficiency at Union Road and HW-120.

Even if the design seems intimidating, the engineering helps keep people safe and traffic efficient. Those elements are part of the reason why Manteca is putting one at Union and HW-120 and why Nevada put their first DDI at Moana in Reno.

Cartwright said their first DDI was home to some severe accidents. He added that intersections with freeway ramps tend to have high speeds combined with crossing or rear-end accidents, some of which were fatal. 

“We have seen the benefits just with the number and severity of crashes being reduced by about half,” said Cartwright. He added that a DDI can generally result in a reduction of about 50 to 60 percent in the number and severity of accidents.

Part of that has to do with the design slowing down traffic to about 25 mph or lower. The curves and other design elements help slow traffic so that people have more time to think about their actions, according to Cartwright. In instances where a crash does happen, they usually happen at a lower speed.

“Still, being slower speed, it doesn’t mean that it moves less traffic,” he added. “It’s actually more efficient at a slower operation speed, because the traffic signals are able to run longer per direction.”

More cars can go through at a slower speed as opposed to taking longer at higher speeds while waiting at a light.

“It is going to feel a little bit different for folks for the first couple of weeks, but we found that people [in Nevada] caught on very quickly.”

Manteca has already begun construction on their DDI, and Ceres, a city near Modesto, is currently in the design stages of one at Service Road and HW-99. 

Daniel Padilla, City Engineer for Ceres, confirmed the project is about halfway finished with plans, specifications, and estimates. They don't expect to see construction on their DDI for about two to three years. 

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