In October 2016, the Texas DOT is scheduled to bid a $60 million construction project for two miles of State Loop 1 that contains the intersection of State Loop 1 and Slaughter Lane in South Austin. The agency considered conventional diamond and single-point urban interchanges, but chose a configuration that’s gaining popularity for its safety and efficiency: the diverging diamond interchange (DDI).

Roundabout vs. Diverging Diamond Interchange

Roundabouts are slowly gaining popularity in the U.S. It’s easy to see why: They’re faster than a stop sign or simple signalized intersection and don’t have signals that must be maintained.

However, while drivers navigate single-lane roundabouts well, they tend to become intimidated and slow down in multilane roundabouts. That’s why multilane roundabouts don’t compete well with interchanges, which handle higher traffic volumes.

It’s the left turn that reduces roundabout capacity. With its high-left-turn throughput, the diverging diamond interchange (DDI) provides a particularly appealing alternative.

Also known as double-crossover diamond interchanges, DDIs are grade-separated interchanges in which the non-freeway/cross-street traffic crosses to the left side of the street, proceeds across the freeway, and crosses back to the right side of the street.

In addition to processing 50% more left-turning vehicles than a standard diamond interchange with the same number of lanes, the configuration allows pedestrians and cyclists to be routed through rather than along each side of the interchange via a shared use path (SUP) protected by a concrete barrier. This eliminates the expense of building a turnaround bridge and enhances safety.

The Missouri DOT installed the first DDI in the City of Springfield in 2009. Since then, 62 have been built nationwide. All 50 state DOTs allow the configuration.

Less opportunity for trouble

According to a recent study comparing seven of the earliest DDIs installed in the U.S., the configuration experiences 33% fewer crashes than conventional diamond interchanges. Crashes that caused injury fell 41%.

The configuration's geometry is safer for several reasons.

The City of Austin, Texas, will have its first diverging diamond interchange (DDI) by 2018. Gold indicates pedestrian walkways. Pink indicates bike lane/shoulder paths.
Lockwood, Andrews & Newnam The City of Austin, Texas, will have its first diverging diamond interchange (DDI) by 2018. Gold indicates pedestrian walkways. Pink indicates bike lane/shoulder paths.
  • Conventional diamond interchanges have 26 vehicle-to-vehicle conflict points; as shown in the diagram at right, DDIs have 14. Ramps can’t be accessed by driveways or side streets; all property access is handled on the cross street, where traffic is slower. Developers of property adjacent to freeway ramps may see this as a limitation, but it enhances throughput and safety.
  • Because only one direction of traffic is crossed at a time, crosswalks are shorter.
  • The configuration doesn’t allow exiting ramp traffic to proceed through the interchange and onto the entrance ramp. Because this movement doesn’t exist, wait times at crosswalks are also shorter.
  • Bicycles can be accommodated on the right side of traffic lanes, which is safer for cyclists.
  • Cut-through islands guide pedestrians and cyclists to crosswalks.
  • Medians are wide enough to safely stage all non-motorized users when two-stage crossing is required.
  • DDIs have more traffic signal green time than conventional diamond interchanges because they have a two-phase rather than four-phase signal system. This lowers wait time for pedestrians, cyclists, and motorists giving pedestrians more time to clear crosswalks.

Achieving mutually beneficial multimodality

As with any interchange, there are several considerations for safely and cost-effectively integrating bicycles, pedestrians, and the disabled with vehicles.

Crosswalks. Like the roads approaching the interchange, determine crosswalk width based on number of users during peak times. Wider crosswalks that can accommodate two or more pedestrians walking side by side minimize the time required to cross multiple pedestrians and wheelchairs.

Other important safety and Americans with Disability Act (ADA) considerations:

  • Position crosswalk as perpendicular as possible to traffic lanes to minimize crossing time while keeping pedestrians visible to drivers.
  • Pedestrians may be unaware what direction traffic is coming from, so some engineers paint “LOOK LEFT” or “LOOK RIGHT” on crosswalk pavement or place directional arrows in traffic lanes close to crosswalks.
  • Use detectible warning surfaces on crosswalk approaches and provide audible accessible pedestrian signals (APS) with countdown signal heads.

Shorter crosswalks, more traffic signal green time, and fewer conflicts with vehicles make diverging diamond interchanges safer for cyclists and pedestrians.

Bike Lane or Road Shoulder? Avid cyclists prefer to be on the road pavement in a bike lane, shared lane, or shoulder. Others prefer the slower, and possibly safer, SUP.

Because some cross streets don’t have bike lanes on the interchange approach, some designers don’t place bike lane striping only within the interchange. Instead, they provide pavement for a future bike lane and stripe it as a road shoulder. Once bike lanes are constructed on cross streets, the shoulder can be striped as a bike lane.

For the intersection of State Loop and Slaughter Lane in south Austin, the TxDOT solicited input from bicyclists at a public meeting attended by more than 200 people.

As a result, the city’s first DDI:

  • Uses curbed SUPs within traffic islands to keep cyclists on the pavement rather than pedestrian ramps.
  • 10-foot-wide SUPs that cut through traffic islands to better accommodate bicycles and pedestrians.
  • Uses large SUP radii (minimum centerline radii of 12 feet) at the curves.

Other considerations:

  • Verify that large commercial trucks can get through the interchange without encroaching into the bike lane/shoulder. This helps with emergency response and incident management as well.
  • Verify that way-finding signage can be seen by the cyclist traveling through the interchange.

With most transportation agencies pushing to do more with less funding, DDIs function well and are likely to be more common in the future.

The Federal Highway Administration provides a wealth of information on DDI policy and planning, multimodal considerations, safety, design, and operational characteristics.