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Unusual new highway design will test drivers and pedestrians

Drivers near Stockton, California, are being warned to prepare themselves for a peculiar new road design apparently gaining favor with planners.

Called the “diverging diamond interchange” (DDI), the design was invented by a student at the University of Maryland for a term paper in 2000. The inventor of the design, Gilbert Chlewicki, is now known as the “father of the diverging diamond interchange.”

Studies claim the design appears to reduce crashes

The design was first built into a real road in 2009 in Springfield, Missouri. It has since been catching on with urban planners and highway engineers, with 120 either built or planned in the U.S.

A study analyzed 26 of the existing DDIs and documented a 37% decline in all traffic accidents at the interchanges. Major crashes resulting in death or serious injuries declined by 54% after converting to DDIs, according to the study.

Savings in construction costs, maybe more

An official in Manteca, where California’s first DDI will be built, told the San Francisco Chronicle that the total price for the project will be $26 million, roughly 72% of the cost of a traditional design. DDIs are said to require less land and lower construction costs.

The safety of motorists, pedestrians and bicyclists are paramount, of course, and the public has a right to hold design and construction companies and government accountable for injuries and deaths caused by faulty designs. Estimated savings or costs from settlements were not discussed in press accounts. Some cycling and pedestrian advocates have expressed reservations about the new design.

Diverging diamond interchange confuses drivers, at first

The DDI in Jan Joaquin County will allow cars to exit from, enter onto and cross the six-lane Highway 120 from the four-lane Union Road.

As the Chronicle explains, the DDI “requires drivers to veer at a 45-degree angle across the center divide, switching sides with opposing traffic and briefly motoring across as if they are in England.”

People are usually left skeptical that the arrangement could ever be safely navigated when judging only from written or verbal explanations, diagrams, and even video simulations. Designers promise that, not unlike roundabouts for many people, DDIs become second nature after they’re experienced just once, and are sometimes not noticed by drivers at all.

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