On the Road

Usually, driving on the wrong side of the road is highly frowned upon and illegal.

Unless of course, you are crossing a freeway intersection that is a diverging diamond interchange – in that case, you will temporarily drive on the wrong side of the road!

While this might be something you have never witnessed or if you have you won’t forget – a diverging diamond interchange takes elements of a traditional diamond interchange and improves the concept by promoting safety and removing the need for left-hand turns.

Originally used in France in the 1970s, there are over 100 diverging diamond interchanges here in the United States with more being built each year.

Today, we will take a quick look at how the double intersection utilizes traffic signal phases and diverging traffic to promote intersection safety and efficiency.

First, what is a Diverging Diamond Interchange (DDI)?

A diverging diamond interchange (DDI) is an interchange that has two signalized intersections at each end (instead of one central intersection). In between the two signals, traffic crosses over to the left side of the roadway.

The central premise of the design is to keep traffic moving safely by allowing drivers to turn left onto and off the freeway simultaneously without stopping or crossing over opposing lanes of traffic. This offers unique safety benefits and actually reduces traffic volume.

Here is how it works:

  1. Motorists approaching a freeway interchange with a diverging diamond interchange from the arterial roadway have the ability to turn right onto the freeway ramp-like usual.
  2. However, to turn left or remain straight, following pavement markings, drivers crossover to the left side of the roadway and proceed straight OR can turn left to enter the freeway without having to cross opposing traffic lanes.
  3. Meanwhile, motorists exiting the freeway can turn right like normal or have the ability to proceed left without crossing any opposing lanes of traffic.
  4. Using the traffic signals at both ends with short cycles, cars approaching will wait at the signal only temporarily almost like a canal system in some cases while the oncoming traffic clears out.

 

Unlike a conventional diamond interchange where there are multiple points where vehicles may cross paths, the DDI reduces these conflict points and mitigates accidents by cutting them nearly in half according to the FHA study.

Diverging Diamond Interchange Benefits

According to the Federal Highway Administration, diverging diamond interchanges and their innovative design not only help with continuous traffic flow but also offer safer travel as well.

In an evaluation of a DDI in Springfield, Missouri by the Federal Highway Administration, they compared crashes before and after the install of the diverging diamond interchange and found:

  1. There were no more left-hand crashes (the DDI removes left-hand turns)
  2. Right-angle crashes were reduced 72%
  3. Total crashes were reduced at the intersection by 46%

Other benefits of a diverging diamond interchange include:

  • The use of two traffic signals with short cycle lengths reduces traffic delays
  • Can accommodate all vehicles (i.e., buses, trucks, cars, cyclists, etc)
  • Greatly reduces conflict points
  • Promotes efficient movement
  • Cost benefits by removing the need for extra lanes, turn lanes, etc
  • Can handle significantly more traffic volume

Let’s not forget that a diverging diamond interchange costs less than a traditional intersection because they can handle the same amount of traffic volume, but with the need for fewer lanes.

The downside involves motorists being confused as they exit freeway ramps or experience the double interchange for the first time. Some associated risks for pedestrians as they now have to potentially cross an additional crosswalk.

Additionally, skeptics claim the double signal takes away the element of free-flowing traffic, however, the short cycles have proven to be significantly more efficient in easing traffic delays.

The Verdict –

When transportation departments and authorities can save money while simultaneously increasing motorist safety, that is a huge win.

While temporarily driving in the opposite direction can be a bit of a temporary shock for first-time DDI drivers, mitigating the need for left turns and driving on the ‘Opposite side of the road” has actually improved safety for all!

This article is part of LYT’s “On the Road” series. See how LYT.ai can help make your intersections safer and cities more efficient with some of our products: https://lyt.ai/

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