Diverging Diamond Interchange - I-10: Houghton Road Traffic Interchange

The new I-10 Houghton Road traffic interchange was scheduled to be converted to a Diverging Diamond Interchange (DDI) in fall 2021.

Houghton Road DDI traffic flow

The animation below shows how vehicles, bicycles and pedestrians will navigate the new Houghton Road DDI. Please note, it shows one direction at a time to more clearly demonstrate the traffic patterns.

How Diverging Diamonds Work

A DDI is a diamond interchange in which both directions of traffic on a cross street (in this case Houghton Road) briefly switch to the other side of the road to better facilitate traffic flow. Vehicles are guided safely through the interchange using signals, signs and pavement markings.

Drivers moving from south to north through the interchange will use the following lanes depending on their destinations:

• Use the right turn lane to turn right onto eastbound I-10 toward Benson.
• Use the left lanes to turn left onto westbound I-10 toward downtown Tucson.
• Through traffic should stay in the right or middle lanes.

Drivers moving from north to south will need to use the following lanes depending on their destinations:

• Use the right turn lane to access westbound I-10 toward downtown Tucson.
• Use the left lanes to turn left onto westbound I-10 toward Benson.
• Through traffic should stay in the right or middle lanes.

Like most traffic interchanges, the DDI includes two sets of signals, one on the north side and one on the south side. The crossover points (to transition from the right side to the left side of the road and vice versa) occur at the signals.

Signal operations are simplified in a DDI because there is no need to allocate green light time specifically for left turn movements from the cross street to the on-ramps. Instead, left turns are made using free-flowing lanes after the cross-over.

The two videos below are from the recently-opened I-17/Happy Valley DDI in Phoenix. The videos show how traffic moves through that DDI interchange.

This animation from the Illinois Department of Transportation (IDOT) also provides a good example of how a DDI works. This DDI design is similar to the one under construction at I-10 and Houghton Road.

DDI Background

The Diverging Diamond Interchange was first proposed in the U.S. by the University of Maryland in 2000. Following an intensive study of the design by the Federal Highway Administration (FHWA), the first such intersection in the U.S. was completed in Springfield, MO in 2009.