Traffic Planners Battle the Left Turn

Diverging Diamond Springfield

A story on Talk of the Nation earlier this month piqued the interest of my inner urban planner and transportation geek.  It was about the efforts of traffic engineers to design intersections that minimize potential points of conflict and maximize the flow of vehicles.  Particularly, they talked about something called a “Diverging Diamond Interchange” or DDI.

Diamond Interchange

The classic diamond interchange is very inefficient and results in nearly two dozen points of potential conflict between cars going different directions.  Every left turn, whether to enter or exit the freeway, results in the rest of the traffic having to stop.

Michigan Left

One solution known as the “Michigan Left” (because it is used mostly in Michigan, I suppose) is a relatively dangerous solution that has drivers making a u-turn about 200 meters after the intersection, then making a right-hand turn at the intersection.  The disadvantage of these uncontrolled u-turns is that one thing drivers don’t do particularly well is judge the speed and distance of oncoming vehicles. 

87th Street

Another solution is the SPUI – Single Point Urban Interchange.  This design has all traffic coming together at a single intersection controlled by a single set of lights.  I’m familiar with this as it is the new design that was built near where I used to live in suburban Kansas City.  It is an elegant design but still has some problems, not the least of which is that the space in the middle of the intersection is very large and people can get lost.  I’ve observed this on several occasions, where cars have drifted into the wrong direction, especially at night when traffic is light and visual cues (like the headlines of oncoming cars) not so available.

If you have never had the pleasure of driving through an SPUI and are curious how they work, here’s a nice short video that shows an animation of traffic going through the intersection. 

Diamond 1

The latest innovation, which so far has only been built a few places in the United States, is something called the Diverging Diamond Interchange, or DDI.  Somewhat counterintuitively, the DDI involves the lanes of traffic switching sides on each end of the interchange.  The result is that there are only two controlled intersections and drastically fewer potential points of conflicts.  All left turns follow the natural, uninterrupted flow of traffic.  (Thanks to NPR for the graphics.)

Here is how it works:

Diamond 2

As you approach the traffic signal, the lanes of traffic curve slightly to the left, passing oncoming traffic (which waits at the light) at a 25-degree angle.

Diamond 3

As you travel through the signal, you are on the left hand side of the road.

Diamond 4

If you are turning left onto the freeway, you simply make a left-hand turn without waiting for a light.  Through traffic keeps on moving, not having to stop for cars waiting to make left-hand turns.

Diamond 5

After passing through the second intersection, traffic crosses back over to the right-hand side of the road.

Diamond 6

Exiting from the freeway to go left onto the arterial road, you merge and do not have to go through an intersection.

Diamond 7

Entering the freeway with a right hand turn is also a merge and does not require you to go through an intersection.  In all, the traffic flows much more smoothly. 

Here’s a short animation that shows the traffic flow.

There are multiple safety benefits.  Traffic keeps moving, reducing standstill time and the risk of rear-end crashes.  Right-angle crashes are eliminated for drivers turning left onto the freeway, as they no longer cross oncoming traffic.  Finally, fewer intersections and independent directions to cross those intersections means fewer collisions caused by people running lights.

All told, I’m sold on the Diverging Diamond Interchange.  Looks like a great way to improve traffic flow and reduce the risks of accidents.

One other option that is interesting is something called the Pinavia interchange, which I guess has been built in Europe.  It is an elaborate interchange that requires no intersections and is aesthetically pleasing, too.  Above is a brief animation to give you an idea of how it works.

Anyhow, that’s enough urban planning/transportation geekiness for today.  But isn’t it fantastic, the things you can learn on NPR?