More than 60% of traffic collisions at intersections involve left turns. Some U.S. cities – including San Francisco, Salt Lake City and Birmingham, Alabama – are restricting left turns.
Dr. Vikash Gayah, a professor of civil engineering at Penn State University and the interim director of the Larson Transportation Institute, discusses how left turns at intersections cause accidents, make traffic worse and use more gas.
Dr. Vikash Gayah discusses why left turns should be banned at some intersections.
The Conversation has collaborated with SciLine to bring you highlights from the discussion, edited for brevity and clarity.
How dangerous are left turns at intersections?
Vikash Gayah: When you make a left turn, you have to cross oncoming traffic. When you have a green light, you need to wait for a gap in the oncoming traffic before turning left. If you misjudge when you decide to turn, you could hit the oncoming traffic, or be hit by it. That’s an angle crash, one of the most dangerous types of crashes.
Also, the driver of the left-turning vehicle is typically looking at oncoming traffic. But pedestrians may be crossing the street they’re turning on to. Often the driver doesn’t see the pedestrians, and that too can cause a serious accident.
On the other hand, right turns require merging into traffic, but they’re not conflicting directly with traffic. So right turns are much, much safer than left turns.
What are the statistics on the unique dangers of left turns?
Gayah: Approximately 40% of all crashes occur at intersections − 50% of those crashes involve a serious injury, and 20% involve a fatality.
About 61% of the crashes at intersections involve a left turn. Left-hand turns are generally the least frequent movement at an intersection, so that 61% is a lot.
Why are left turns inefficient for traffic flow?
Gayah: When left-turning vehicles are waiting for the gap, they can block other lanes from moving, particularly when several vehicles are waiting to turn left.
Instead of the solid green light, many intersections use the green arrow to let left-turning vehicles move. But to do that, all other movements at the intersection have to stop. Stopping all other traffic just to serve a few left turns makes the intersection less efficient.
Also, every time you move to another “phase” of traffic – like the green arrow – the intersection has a brief period of time when all the lights are red. Traffic engineers call that an all-red time, and that’s when the intersection is not serving any vehicles. All-red time is two to three seconds per phase change, and that wasted time adds up quickly to further make the intersection less efficient.
Roundabouts reduce the need for left turns, but they don’t work everywhere.
Pete Ark/Moment via Getty Images
What restrictions have been tried in different cities?
Gayah: When a downtown is not very busy – in the…
