Dr. Vikash Gayah: Banning left turns at intersections
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Among traffic collisions that occur at intersections, more than 60 percent involve a left turn, and some U.S. cities are now restricting such turns.
On May 8, 2025, SciLine interviewed Dr. Vikash Gayah, a professor of civil engineering at Penn State University. See the footage and transcript from the interview below, or select ‘Contents’ on the left to skip to specific questions.
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Introduction
[00:19]
VIKASH GAYAH: My name is Vikash Gayah. I’m a professor of civil engineering at Penn State University, and I’m also the interim director of the Larson Transportation Institute at Penn State, and my research focuses on transportation operations—so how to move vehicles and people as smoothly as possible on surface streets—and transportation safety—so how to reduce the number of crashes and fatalities associated with crashes.
Interview with SciLine
How much more dangerous are left turns at intersections, compared to right turns, and why?
[00:43]
VIKASH GAYAH: A left turn is more dangerous because when you’re making a left turn you have to cross oncoming traffic. And if you’re making that left turn in a permitted fashion—so that’s where you have a green ball, and the left turning vehicle has to sort of wait for an appropriate gap in the oncoming traffic—you’re sort of leaving that decision up to the driver. And if they misjudge that decision, they can either hit the oncoming traffic or be hit by the oncoming traffic. That’s an angle crash, where the vehicles kind of strike at an angle, and that tends to be one of the most one of the most dangerous types of crashes. And then when the left-turning vehicle is making that turn, they’re looking, typically, at the oncoming traffic, the oncoming vehicles. But at the same time, typically, pedestrians are going to be crossing the street that they’re turning on to. And it’s very easy and common that the left-turning vehicle sort of ignores that or doesn’t see the pedestrians, and then that can lead to very dangerous collisions between the left-turning vehicles and the pedestrians. You contrast that to right turns, and right turns are not sort of—they’re merging into traffic, but they’re not conflicting directly with traffic. So the right turns are much, much safer than left turns.
Are there any statistics on the unique dangers of left turns?
[02:06]
VIKASH GAYAH: Approximately 40% of all crashes occur at intersections, and that involves 50% of the crashes that involve a serious injury, and about 20% of the crashes that involve a fatality. And then about 61% of the crashes that occur at intersections involve a left-hand turn. And when you think about the fact that left-hand turns are sort of the smallest volume of movement at an intersection, that 61% is actually quite a lot.
Why are left turns so inefficient, in terms of traffic flow?
[02:41]
VIKASH GAYAH: Sure. So if we think back to that permitted movement—where the left turning vehicles are waiting for a gap—when they’re waiting for that gap, they’re blocking the the other lanes next to them from moving. And if you have a left turn lane, and there’s many vehicles waiting for a gap, that can actually spill over and block the through movements from going. So that tends to be very inefficient. The other way of serving left turns is what we call a protected phase. That’s the left arrow, where we give dedicated time for the left-turning vehicles to move. But in order to do that, we have to stop all other movements at the intersection, and that’s stopping the intersection from serving the through movements, which are sort of the bulk of the traffic at an intersection in order to give dedicated time for the left turns, which tends to be sort of the smallest number of movement. And so we’re kind of making the intersection less efficient by stopping all other traffic just to serve a few left turns. And then adding that extra phase, that protected phase, when we go through the phases, every time you switch a phase of traffic—so that means you stop one movement and allow another movement to go—the intersection actually has a period of time where all of the lights are red. We call that an all-red time. And that’s time when the intersection is actually not serving any vehicles. And that is the most wasted time. It’s time when the intersection is essentially doing nothing. And so these ways to serve left turns, again, they’re not only—the left turns are not only dangerous, but they also—the way to accommodate the left turns tend to make the intersection less efficient as well.
What does your research show about the potential traffic benefits of eliminating left turns in urban areas?
[04:22]
VIKASH GAYAH: Any way you cut it, eliminating left turns will result in longer travel distances. I have to travel a longer distance to get to where I need to go. And that worst case of circling the block—I’m traveling four extra block lengths in order to get to where I need to go. But not all trips will require that circling the block. And when you look at it in a downtown where origins and destinations are just sort of distributed everywhere, on average, each trip will be about one block length longer, so just one block length longer, which is not a lot of extra distance. And that extra travel distance is more than offset by the fact that each intersection is moving more vehicles, which means that every time you get to an intersection, you wait less time on average. And so when you combine those two things, the fact that you’re waiting less time at each intersection but you’re traveling through one extra intersection on your trip, it turns out to be more than worth it. And actually, even though you travel longer distance, you actually get to where you need to go much more quickly because you’re moving faster. You’re spending less time stopped at intersections.
Does avoiding left turns improve fuel efficiency? Does UPS’s routing approach, which avoids left turns, really save millions of gallons of fuel each year?
[05:35]
VIKASH GAYAH: Sure, so yeah, we saw that as well, and we looked into it, and it’s true that UPS and other fleets sort of route their vehicles to avoid left turns. And the reason they do that is because it’s a lot of less idling and less stopping time for the vehicles. And these are usually larger vehicles that, when they’re idling, they’re wasting fuel, or just the act of stopping and starting again requires a lot of fuel to use, and so it’s actually more fuel-efficient to travel again a little longer distance but stopping less.. And it turns out to be the exact same thing. So in our work, we also looked at the fuel efficiency as well, and we found that restricting left turns actually—even though we’re having vehicles travel longer distances on average, so we’re increasing the amount of vehicle miles traveled—because these vehicles are stopping fewer times, and they’re spending less time waiting at intersections, they actually, as a whole, we use a lot less fuel than if we were to allow the left turns.
How much fuel is saved by avoiding left turns, according to your research?
[06:38]
VIKASH GAYAH: Yeah, I think it was on the order of 10-15% per trip, or even even aggregated: 10-15%.
When and where should cities restrict left turns for maximum benefit?
[06:53]
VIKASH GAYAH: What we found is that when the traffic network or the downtown is not very busy—so in the off peak periods—allowing left turns is totally fine because you don’t need that additional capacity, or that additional ability to move vehicles at each intersection. If you’re doing that, it’s not going to be used by any vehicle. So that’s the when and the where: You know, in a similar vein, when you look at—our work looked at, sort of, if we couldn’t sort of blanket apply this strategy, are there certain intersections where we want to restrict left turns versus others? And what we’ve ended up finding is that you want to restrict left turns at the intersections that are the most busy, where the additional capacity or the additional ability to move vehicles to the intersection would be the most used. And so those are going to be the intersections that are carrying the most vehicles. Restricting left turns at those locations makes those intersections much more efficient and allows traffic across the entire network to move move more smoothly and more efficiently.
What types of restrictions have been tried in different cities, and what have been the outcomes?
[08:00]
VIKASH GAYAH: Some cities already have signs in place that say, at intersections, no left turns, seven to nine, which is the morning peak period, or four to six is the afternoon peak period. So they’ve stumbled upon that. Maybe we want to restrict left turns during these busiest times of the day. But these cities are doing them in sort of a more isolated way. So, for example, San Francisco, Van Ness Avenue restricts left turns along this arterial during the peak periods. Some intersections in Salt Lake City or Birmingham, Wilmington, in Michigan—they’re stumbling upon the idea that maybe we want to strict left turns during these busy periods. But what they’re not doing is doing like sort of a network-wide or a widespread implementation. It’s more along individual corridors or isolated intersections that are sort of problematic. And what we’re trying to promote is that we should be restricting left turns across essentially the entire downtown, if possible, in order to make that downtown street network more efficient.
Roundabouts are one approach to avoiding left turns. What are the implications of building them?
[09:05]
VIKASH GAYAH: So roundabouts are actually very safe as well because, again, they’re getting rid of that need to cross opposing traffic. So in order to do that, you slow down, you enter the roundabout, and everyone circulates in the same direction. You kind of find where you need to go—where you need to exit, and then you exit there. So it’s much safer from that perspective, although roundabouts tend to be a little bit less safe for bicyclists and pedestrians just because of the additional locations where they need to potentially cross the road—actually that might increase their crossing distance or just make them more exposed to vehicles. So that’s a little that’s a little bit up in the air. But on the efficiency side, we actually compared our strategy to serving a downtown with a system of roundabouts, and what we found is that in general, the restricting left turns tends to be more efficient than applying roundabouts, sort of in a widespread way. And a part of the reason for that is roundabouts, while they are efficient in sort of light traffic conditions, when it gets to be more busy—when the roundabout gets full—what we can get is a situation where the entire roundabout is full of vehicles, and that essentially gridlocks the roundabout, where no vehicle can move. Eventually the vehicles kind of figure their way out. But you can imagine a circle that’s completely full of vehicles: If it’s completely full, no vehicle can move before the vehicle in front of them moves. But then they’re all—a vehicle is in front of all vehicles, and it just is going to get stuck. We call that gridlock. And so roundabouts are actually highly prone to gridlock, where traditional intersections are not. And then, just from a more—a different perspective, roundabouts, while they’re efficient and in some ways and safe in some ways, they also just require larger spatial footprints. So you can imagine, in a downtown area, in order to install a roundabout at some intersections, you actually have to expand the intersection and create this circle. And in some downtowns, that would require actually tearing down buildings or removing sidewalks. It could require significant infrastructure work and significant amounts of space. Restricting left turns does not require amount of a lot of space or a lot of infrastructure costs. It’s just a sign that says no left turns or no left turns during the peak periods. That’s it.
Do you think banning left turns could be a widely accepted strategy?
[11:24]
VIKASH GAYAH: Yeah, I mean, I think the general idea is that it is an uncomfortable strategy, the strategy that’s that’s new. And if I were to drive into a city and I couldn’t make a left, I would feel a little bit uncomfortable. Hey, where do I need—how do I get to where I need to go? In this day and age we all have sort of, like, mapping services, Google Maps, or navigation systems in our phones. That helps alleviate some of that stress, which I think is really nice. And so it’s just an easy way to feel comfortable with the strategy. And then once people get used to it and realize, wait a minute, you’re taking away my ability to make a left, which I used to be able to have. When you sort of get used to the fact that, OK, I have to travel a little longer, but now I just get to where I need to go much, much more quickly. I think it’s a strategy that people really latch onto.
