The Missing Piece in DOT’s Left-Turn Safety Plan: Real Split-Phase Signals

DOT is ramping up the use of leading pedestrian intervals to reduce left-turn collisions, without committing to add signals that completely separate pedestrians and turning drivers. Photo: Brad Aaron

Split-phase traffic signals protect pedestrians and cyclists by separating them from turning drivers — people walking and biking across the street get their own signal phase, and drivers turning into the crosswalk get another. Research indicates that split-phase signals are highly effective at preventing traffic injuries and deaths. But when DOT revealed its strategy to reduce crashes caused by left-turning drivers, there was no commitment to increase the use of split-phase signals.

DOT is scaling up a similar intervention — leading pedestrian intervals, which allow pedestrians to enter intersections a few seconds before turning drivers get a green light. LPIs reduce injuries too, but not as much as split-phase signals, according to a 2014 DOT-funded study published in the journal “Accident Analysis and Prevention” [PDF].

The study analyzed crash data from 68 New York City intersections with either LPIs or split phases between 2000 and 2007, though the vast majority — 59 — had LPIs. Both types of signal adjustments performed better than a control group of intersections where turning drivers were permitted to proceed at the same time as pedestrians and cyclists. The improvement was more pronounced, however, at split-phase signals.

At intersections equipped with split-phase signals, pedestrian and cyclist injuries declined a precipitous 67 percent. At intersections with LPIs, pedestrian injuries declined 38 percent and bicyclist injuries 52 percent. (For the control group, the reduction was 25 percent for pedestrians and 44 percent for cyclists.) The data on split-phase signals was limited, however — it came from only nine intersections, with no locations in Manhattan.

Left-turning drivers cause 19 percent of serious pedestrian and bicyclist injuries in NYC, three times the rate caused by drivers turning right, according to DOT’s recent “Don’t Cut Corners” report [PDF]. To reduce left-turn collisions, DOT installed 417 LPIs last year, a major increase, and plans to add another 500 in 2016. DOT is also piloting the use of “split LPIs” that give turning drivers a blinking yellow light after the pedestrian head start, as well as design treatments like “enhanced daylighting” to get drivers to take turns more carefully.

In the Don’t Cut Corners report, DOT cites data from 104 intersections with LPIs, where fatal and serious injury crashes fell 56 percent. That is a very significant improvement. But if split-phase signals do an even better job, as the “Accident Analysis and Prevention” study indicates, why isn’t DOT installing more of them?

DOT Director of Strategic Initiatives Juan Martinez said the 2014 study relied on too small a sample and limited “after” data (two years worth) to draw firm conclusions about split-phase signals compared to LPIs. “They just didn’t have a lot of intersections to work with,” he said.

Martinez pointed to the 56 percent reduction in serious crashes as evidence of the efficacy of LPIs, arguing that split-phase signals are conducive to high-speed turns, which are more likely to result in severe injury and death if a collision occurs. “What it comes down to is speed,” he said. “How can we slow down the driver?”

The condition where DOT looks to add split-phase signals, he added, is where “you have a very heavy turn volume and a heavy pedestrian volume, and we don’t think we can either slow the turns or that we can protect pedestrians otherwise.”

Christine Berthet, who has prodded DOT to install more split-phase signals through her work at the Clinton-Hell’s Kitchen Coalition for Pedestrian Safety and Manhattan Community Board 4, said DOT is often reluctant to install them even in the conditions that Martinez described.

At 41st Street and Ninth Avenue, she said, DOT resisted CB 4’s call for a split-phase signal until a truck driver struck and killed pedestrian Shu Ying Liu at the intersection in 2013. More recently, advocates have requested a split-phase crossing at 57th and Eighth, another intersection with lots of pedestrians, but Berthet said DOT cited the heavy turn volume as a reason not to implement one.

Berthet questions why DOT didn’t publish data on the safety effect of split-phases in its Don’t Cut Corners report.

Martinez said DOT is working with a data analysis firm for a more comprehensive study on the effectiveness of different types of intersection treatments. There is no timetable for that study’s completion.

  • Jeff

    To be fair, the “Accident Analysis and Prevention” is only looking at “accidents”, which are a tiny percentage of overall crashes. I could see why the data wouldn’t be particularly reliable. I mean, can a crash really be called an “accident” if it could have been prevented with a split-phase signal?

  • Simon Phearson

    Here’s the thing. Do split phases protect pedestrians and cyclists? Sure, that seems to be what the evidence suggests. But will NYC’s DOT implement split phases in a way that wouldn’t further subjugate cyclists and pedestrians to second-class treatment? And, if not, wouldn’t that just result in more disregard of split phases, potentially undermining their safety benefit?

    Every split phase I deal with, as a cyclist, follows a pattern: Take a small slice of the green phase for through traffic and give it over to cyclists. Timing isn’t adjusted so that, say, through traffic gets less of a green and the lights cycle more frequently or the lights are timed for bike traffic, while the cycling-through phase is often incredibly short. (They seem to be timed so that the only cyclists who “get the green” are those already standing at the intersection.) So what happens is you miss most of your through-phases, as a cyclist, and spend a lot of time watching drivers not turn in front of you, while a lot of people drive past you. In nearly every case where I’ve seen it, cyclists respond in a predictable fashion: by running their split-red.

    Let’s focus on LPIs. LPIs protect people, they help moderate traffic, and cyclists can get part of the advantage of the LPIs with a suitable change in the law. It’s not perfect, but we can’t trust the DOT to implement split phases on a comprehensive basis without substantially inconveniencing pedestrians and cyclists, given their apparent competence and priorities.

  • J

    So I tend to be with Martinez here. When drivers the green turn arrow, they tend to turn left more aggressively and quickly, since they are told they have the exclusive right of way. I see this happen all the time. Split phase signals also mean pedestrians get less time to cross the street, as some of the time they currently get then goes to car turns. Less time to cross means more people cross against the signal, which, combined with faster more aggressive turns can be deadly.

    That said, I do think it’s worth studying in detail, so we make rational evidence-based decisions that lead to greater safety.

    In the mean time, efforts to reduce turning speeds are a proven way to improve safety, and we should continue to implement them.

  • snobum

    DOT recently installed a hybrid at 32nd/3rd. It has a red turn arrow for the first 5-10 seconds of the phase (while straight has the green) and then changes to a flashing yellow arrow. I think it has the same effect as an LPI, and calls a little more attention to the fact turning vehicles don’t have the right of way.

    LPI’s really should be standard at every intersection.

  • J

    Interesting. If there’s a way to do this, with a greatly reduced turn radius (slow turn speeds), then it may be worlds better than a split phase. If I recall correctly, this was the setup (minus the flashing yellow arrow) in many busy intersections in Amsterdam.

  • BrandonWC

    That’s some excellent snark.

  • That’s the “split LPI” that DOT is testing out at a few locations now.

  • Who cares if drivers turn at higher speed if they have an exclusive green light? They are not a risk yo anyone..
    Less time to cross but 99 % safe. You are making the case that pedestrian/cyclists flow is more important than their safety. the whole point of vision Zero is to Put safety at the top.

    And it is always easier to get more time in the future.

    I do not think we should increase risk for law abiding citizens, in order to save lives of scofflaws. When I jaywalk, I take my life in my hands, I do not expect the city to protect me.

    In MCB4 8 people have been killed in the pedestrian crossing with the walk signal by turning vehicles . The only treatment that is safe and feels safe is the split phase signals as installed at the major intersections on bike lanes. All seniors swear by them.

  • Not sure how it would be” worlds better “? what would be the benefits compared to those of split phases?

    if all these features are so good, how come they are not applied to vehicular traffic? Because they are not safe enough… Drivers are protected by a mutually exclusive green/red lights when they cross an intersection… But not pedestrians or cyclists … Why?

    Are we all ready to settle for less safety than drivers?

  • LPIs protect people for 7 seconds. The rest of the time you are accepting the second class citizen treatment : drivers get a full protected crossing while pedestrians and bicyclists do not.
    And who is more at risk to be killed?

    DOT installed an LPI. At 41 st street and 9 th. and we still got a neighbor killed there. How would you feel if it was one of your familiy’s member?
    If there is a walk sign, I want to be fully protected, just like when there is a green light drivers are fully protected.

    Inconvenience is the argument used by drivers who do not care about safety. Dot could implement split phase with bikes having green light during both the walk phase ( as it is today) and the car turn phase as it is the case in the mixing zones) . Nothing prevents them to do that. Except it would be more dangerous for cyclists.
    We should all advocate for such together. I would hate to have pedestrian safety compromised knowingly because cyclists think it is inconvenient .. I thought we were on the same team!

  • I think they mean crashes – not politically correct does not make the data less compelling. Double the safety… Huge.. How ca we say no..

  • Simon Phearson

    My point is that an approach to split phases that still puts an undue emphasis on driver convenience is one that will undermine its own safety goals. Convenience does matter, insofar as a split phase that makes pedestrians and cyclists feel like they’re stopped for no reason is one that they’ll ignore.

  • Bernard Finucane

    On another note, the intersection in the picture is a classic example of an intersection that would be made much safer by simply giving pedestrians more space and cars more guidance.


DOT Lays Out a Strategy to Make Left Turns Less Dangerous

DOT will be ramping up the use of intersection treatments to protect pedestrians and cyclists from left-turning drivers, the agency announced today. The initiative is paired with a DOT study, “Don’t Cut Corners” [PDF], that illustrates the disproportionate danger of left turns. Mayor de Blasio had announced in January that reducing the risk of left turns […]

Evidence That Split-Phase Signals Are Safer Than Mixing Zones for Bike Lanes

When DOT presented plans for a protected bike lane on Sixth Avenue, one point of contention was the design of intersections. How many intersections will get split-phase signals, where cyclists and pedestrians crossing the street get a separate signal phase than turning drivers? And how many will get “mixing zones,” where pedestrians and cyclists negotiate the same space as turning […]