More than 60% of intersection traffic collisions involve left turns. Some US cities, such as San Francisco, Salt Lake City and Birmingham, Alabama, are restricting the turns to the left.
Dr. Vikash Gayah, professor of Civil Engineering at the State University of Pennsylvania and interim director of the Larson Transportation Institute, analyzes how the turns to the left at the intersections cause accidents, make traffic worse and consume more gasoline.
How dangerous are the left turns at the intersections?
Vikash gayah: When turning left, you must cross traffic in the opposite direction. With the green light, you should wait for a traffic space in the opposite of the left before turning to the left. If you calculate the time to turn, it could collide with traffic in the opposite direction or be hit by it. That is an angle shock, one of the most dangerous types of shock.
In addition, the driver of the vehicle that revolves to the left usually looks at the traffic that comes in the opposite direction. However, there is possible that there are pedestrians crossing the street towards which it turns. Often, the driver does not see them, which can also cause a serious accident.
On the other hand, the right turns need to join traffic, but do not enter into direct conflict with it. Therefore, the right turns are much safer than the turns to the left.
What are the statistics on the specific dangers of the left turns?
Gayah: Approximately 40% of accidents occur at intersections; 50% of these accidents result in serious injuries and 20% in deaths.
About 61% of accidents at intersections imply a left turn. The left turns are usually the least frequent movement at an intersection, so that 61% is a lot.
Why are the turns to the left inefficient for the fluidity of the traffic?
Gayah: When vehicles that revolve to the left await the space, they can block the passage of other lanes, especially when several vehicles wait to turn left.
Instead of the fixed green light, many intersections use the green arrow to allow the vehicles that turn left. But for this, all other vehicles at the intersection must stop. Stop the rest of the traffic only to take some left turns makes the intersection less efficient.
In addition, every time it is changed to another “phase” of traffic, such as the green arrow, the intersection has a brief period in which all traffic lights are in red. Traffic engineers call it “completely red time”, and that is when the intersection does not serve any vehicle. The completely red time is two to three seconds due to phase change, and that lost time accumulates rapidly, which further reduces the efficiency of the intersection.
What restrictions have been tested in different cities?
Gayah: When the city center does not have much traffic, in valley hours, allowing these turns is acceptable because that additional capacity to move vehicles at each intersection is not needed.
Some cities are implementing signals that prohibit turning left at the intersections of 7 to 9, which is the peak time of the morning, or from 4 to 6, which is the peak time of the afternoon. In San Francisco, for example, Van Ness Avenue restricts the turns to the left during peak hours.
But cities are not implementing these restrictions on a larger scale. Restrictions apply more along individual corridors or isolated intersections instead of practically the entire center, provided it is possible. This would make the center’s street network more efficient.
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Benefits of prohibiting left turns in urban areas
Roundabouts are a way to avoid left turns.
Gayah: The roundabouts are safe because it is no longer necessary to cross traffic in the opposite direction. All circulate in the same direction. The meeting point is located and then it goes out.
But restricting the turns to the left, in general, is more efficient. The roundabouts are not so efficient when there is more traffic. The roundabout is filled, which can cause a jam and no vehicle can circulate. Traditional intersections are less prone to traffic jams.
The roundabouts also occupy more space. Installing a roundabout could involve expanding the intersection. In some urban centers, that implies demolishing buildings or removing sidewalks. Restricting the turns to the left only requires a signal that says “do not turn left” or “not turn left at peak hours.” That’s all.
What are the benefits of prohibiting them in urban areas?
Gayah: In any case, eliminating these turns will result in higher travel distances. I will have to travel a greater distance to get where I need to go. In the worst case, I would have to go around the block. In fact, I am touring four more apples to get where I need to go.
But not all trips require around the block. In a typical urban center, each trip will be approximately a longer apple on average. That is not much additional distance. And that additional trip is more compensated with the fact that each intersection with turns to the left now mobilizes more vehicles. Which means that, on average, every time you are at an intersection, you wait less time. So you travel a slightly longer distance, but you reach your destination more quickly.
Does avoiding these turns improve fuel savings?
Gayah: Our research revealed that, although vehicles travel longer distances on average with restricted left turns, they consume less fuel (between 10 and 15% less for trip) because they stop less at intersections.
Therefore, UPS and other fleets direct their vehicles to avoid the turns to the left. There is less slowed and less stops.
Do you think prohibiting these turns could become widely accepted?
Gayah: It is a new strategy, so it is uncomfortable for some people. But when they reach their fastest destination, I think people will adopt it.
*Vikash V. Gayah He is an associate professor of Civil Engineering at Penn State.
This text was originally published in The Conversation.
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