Is it time to prohibit left turns in busy intersections? This transportation engineer thinks so

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Whenever drivers make a left-hand turn in a busy intersection, they have to cross oncoming traffic. This creates temporary bottlenecks that slow down traffic — in some situations, even to a grinding halt. Left-hand turns can also be dangerous, being involved in nearly 60% of all crashes occurring at intersections and causing three times more pedestrian fatalities than right-hand turns. For these reasons, and more, some experts believe it is time to remove left-hand turns from our intersections, with research to back it up.

For more than a decade, Vikash Gayah, an Associate Professor of Civil Engineering at Penn State University, has been performing mathematical models and traffic simulations analyzing the impact of left turns. His research shows that — at least for grid-like street networks found in many U.S. cities — eliminating left-hand turns may greatly improve traffic overall.

Removing left turns could save fuel, time, and even human lives

According to Gayah, the main weakness of a two-way network with prohibited left turns is that it can lead to longer travel times when these networks are congested. Rather than making a quick left turn, some drivers would have to make three right turns to get in the right direction. However, Gayah’s research shows that, on average, drivers would only have to drive an additional block in prohibited left turn scenarios, but these delays would be more than offset by the smoother traffic.

These simulations are backed up by some real-life data, most notably from UPS. Think about it, when was the last time you saw a UPS truck make a left-hand turn? The reason why you can’t recall is that the company changed its delivery routes way back in 2004 in order to minimize the number of left-hand turns its drivers make. Carriers make left-hand turns only when the action is unavoidable.

Considering UPS’s massive fleet, this seemingly slight adjustment adds up to make a massive difference. Since it eliminated left-hand turns from its routes, UPS claims that every year it saves 10 million gallons of fuel, cuts 20,000 tons of carbon emissions, and delivers 350,000 extra packages since drivers spend less time stuck in intersections. Since vehicles are now equipped with GPS devices, onboard computers, and smartphones, avoiding left-hand turns has never been easier.

UPS isn’t alone. In the Commercial Driver’s License (CDL) manual, the authors address the following warning to truckers in Section 2 concerning left-hand turns:: “Before you start across a road, make sure you can get all the way across before traffic reaches you.”

But getting rid of left turns across an entire city sounds like an administrative nightmare and rather unrealistic. A more practical approach would be to identify and then remove left turns in individual locations where some form of intervention is most needed.

In a recently published study in March 2021 in the journal Transportation Research Record, Gayah and colleagues devised a computer algorithm that can evaluate which intersections should prohibit left turns to improve traffic.

The algorithm can be fed data from traffic simulations to pinpoint which intersections need to remove the left turns. Each city is different, but Gayah says that one general pattern is that left-turn restrictions are most effective at busier intersections in the centers of cities than at less busy intersections farther from the center.

“This is because the busier the intersection, the more people will benefit from smoother traffic flow. These central intersections also tend to have alternative routes available that minimize any additional distance traveled due to the restrictions. Lastly, fewer cars tend to turn left at these central intersections to begin with so the negative impact of removing left turns is relatively small,” Gayah wrote in an explanatory article about his research for The Conversation.

These are all solid arguments in favor of changing legislation regarding left-hand turns. In some situations, left-hand turns smoothen traffic and should be kept in place. However, there is ample evidence in favor of scrapping them from most busy intersections in large cities. In the meantime, consider these tips when making a left-hand turn:

  • Always use a turn signal;
  • Only make the turn if you can safely complete it;
  • Check your blind spots;
  • Know what is in front of and behind you;
  • Yield to pedestrians and vehicles who have the right-of-way;
  • Don’t stop suddenly without warning;
  • Avoid cutting corners;
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About Tibi Puiu

Tibi is a science journalist and co-founder of ZME Science. He writes mainly about emerging tech, physics, climate, and space. In his spare time, Tibi likes to make weird music on his computer and groom felines.

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