How Diverging Diamond Interchanges Reduce Traffic Light Phases

With multiple diverging diamond interchanges (DDIs) coming to Central Florida as part of I-4 Beyond the Ultimate, it’s important to look at the innovations they’ll bring to busy roadways along Interstate 4 (I-4). A DDI improves safety by eliminating left turns across traffic and increases capacity by allowing more motorists through the interchange as the traffic light cycle is reduced from four phases to two.

“There are no left-turn phases (onto the highway) from the road in a DDI because left turns occur while driving on the left side, making left turns easy,” said Gilbert Chlewicki, P.E., the original designer of the DDI. “Think of it this way: On a conventional road in the U.S. where drivers are on the right side of the road, right turns are easy and left turns are hard (requiring a dedicated signal phase so cars can cross the opposing traffic lanes). The DDI moves traffic to the left side of the road when left turns are needed.”

DDIs have two traffic signals, one on each side of the interstate. Drivers turn right onto the highway entrance ramp before encountering the first traffic signal, while those turning left to enter the freeway do so after traffic has shifted to the left side, prior to the second traffic signal. Both signals have just two phases, allowing traffic to switch sides while motorists going the other direction are stopped.

“There is no need for an exclusive left-turn phase from the major road, which is usually needed in a conventional diamond interchange,” Chlewicki said.

Without traffic signal phases solely for left turns, cycle lengths for red lights are typically much shorter in a DDI.

The reduced number of signal phases also enhances safety for motorists exiting the highway. Because traffic flow through a DDI is more efficient, traffic queues on exit ramps are much less likely to extend back to the highway than at a busy standard diamond interchange, eliminating a potential safety hazard.

For more information on the DDIs coming to I-4, visit