It’s called the Diverging Diamond Interchange.
Some highway experts call it efficient. Some drivers call it confusing.
Whatever it’s called, the new design is coming to State Road 836, one of the busiest highways in South Florida, this summer.
The DDI, as it’s known, is part of the ongoing $799 million overhaul of the Dolphin Expressway, which links Miami-Dade’s western suburbs with Interstate 95 near downtown.
The newly designed interchanges at Northwest 57th and 27th avenues will offer a twist when entering and exiting the expressway — one that bucks the tradition of drivers staying on the right side of the road. The design marks new efforts by transportation planners to improve the flow of traffic on the congested highway, part of the network of toll roads operated by the Miami-Dade Expressway Authority, also known as MDX.
“The DDI format is proven across the country to relieve traffic congestion, improve expressway access and increase safety for drivers and pedestrians,” MDX said in a statement.
There are mixed opinions about the DDI design.
A 2010 Missouri Department of Transportation study at the junction of Interstate 44 and the Kansas Expressway in Springfield called it a “huge success,” but noted that initially there was confusion and inconvenience for some drivers.
“The drivers inconvenienced the most by the installation of a DDI are those going through on the cross route because they must cross over to the left side of the road and then back again to reach their destination,” the study said. “Since motorists are driving on the left side of the road, the interchange may initially be confusing to some and may not readily be embraced by the public.”
A Fortune article in June that reported five months of DDI data from Missouri showed a 60 percent reduction in collisions.
To understand how the new design works, note first how traditional American highway interchanges work.
In exiting a traditional interchange, drivers take a ramp that leads to a square intersection with another road where there is a red light or a stop sign. If drivers are turning left, they must wait for traffic to stop to enter safely and continue on the right side of the roadway.
But in a Diverging Diamond Interchange, and it does look like a diamond from above, traffic exiting the expressway does not have to cross opposing traffic to make a left turn as in the traditional interchanges.
Instead, drivers come to a traffic signal and wait for a green that allows entry into the flow of traffic on the left side. Just like London. But after crossing beneath the expressway, drivers come to a second traffic signal where they wait for the green to proceed to the right side of the road, like usual, and continue on their way.
“The DDI interchanges will reduce congestion caused by expressway traffic in these areas,” said Juan Toledo, MDX deputy director and director of engineering.
The new interchanges are scheduled to be completed in the summer. Over the next several months, MDX will promote the changes to prepare drivers for what’s to come.
“Driving through the DDI is not complicated at all, but it is different when done for the very first time, and these are the first DDIs to be built in South Florida,” Toledo said. “We are working vigorously to ensure that the community is prepared.”