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

Mixing zones, rendered above, are DOT's standard treatment for left-turns on corridors with protected bike lanes. Image: DOT
Mixing zones are DOT’s standard treatment for intersections where motor vehicle traffic turns across the path of protected bike lanes, but they are not as safe as intersections where pedestrians and cyclists get exclusive signal time. Image: DOT

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 drivers simultaneously?

DOT tends to opt for split-phase signals only at major intersections, like where Sixth Avenue crosses 14th Street and 23rd Street. At other cross streets with turning conflicts, the mixing zone is the go-to treatment. Manhattan Community Board 4 wants to change that, asking DOT to include more intersections with dedicated crossing time for pedestrians and cyclists in the Sixth Avenue project.

The evidence backs up CB 4’s assertion that split-phase signals are safer. Data from previous protected bike lane projects in Manhattan show that the reduction in injuries on streets that mostly received split-phase treatments was more than double the improvement on streets that mostly received mixing zones.

A 2014 DOT report [PDF] analyzed three years of before and after crash data from Manhattan’s protected bike lanes. The last section of the report shows the change in total crashes with injuries on 12 protected bike lane projects — six with primarily split-phase treatments (segments of Eighth Avenue and Ninth Avenue below 23rd Street, and two unconnected segments of Broadway in Midtown), and six with primarily mixing zones (segments of First Avenue, Second Avenue, Columbus Avenue, Broadway, and Eighth Avenue above 23rd Street). We don’t have access to the raw numbers DOT worked with, but the aggregate data strongly suggests that split phase treatments are significantly safer.

On average, crashes with injuries declined 30 percent on the six “split-phase” redesigns and 13 percent on the six “mixing zone” redesigns.

On eight protected bike lane segments, DOT also analyzed cyclist injury risk, a measure that, roughly speaking, divides the number of cyclists killed or severely injured on a given street segment by the number of cyclists counted on that street. Two of those segments, on Ninth Avenue and Broadway, were primarily split-phase projects while the others were primarily mixing zone projects. While we don’t have access to DOT’s raw data, the average improvement on the split-phase segments was about 51 percent, compared to a 30 percent improvement on the mixing zone segments:

Image: DOT
Image: DOT

Mixing zones cost less than split-phase intersections and require removing fewer parking spaces, which means they can be implemented faster and with less pushback than split-phase intersections. In November, DOT officials also told CB 4 that they were wary that, in cutting pedestrian crossing time, split-phasing would lead to increased disregard for walk signals.

Last Wednesday, DOT came back to CB 4 without having added any split-phase intersections to the Sixth Avenue plan. The committee passed a resolution calling on DOT to install two additional split-phase signals on Sixth Avenue in the next year, and DOT’s Ted Wright said the department could study their feasibility in the future.

The data suggest that pedestrians, cyclists, and motor vehicle occupants all get a clear safety benefit from split-phase signals, however. Even if split-phase signals aren’t included in the first iteration of a project to ensure timely implementation, it should be standard practice to retrofit existing protected bike lanes with them over time. More injuries and deaths will be prevented, and more people will feel safe biking on city streets.

  • Alex Brideau III

    I tend to use the term “perceived safety” which, as in all things, is separate and distinct from actual safety. But perceived safety can be very attractive and, interestingly, can eventually have a much greater impact on actual safety than the “perceived” moniker would suggest.

  • Joshua Putnam

    But unless you’re going to channelize intersections with movable walls, separate signal phases are clearly the more important protection, since the majority of serious conflicts are concentrated in intersections and the approaches to intersections, not mid-block.

    Yes, separated signal phases do rely on other people obeying the signals, but they actually have a pretty good record of doing that where signals are timed properly.

  • Alex Brideau III

    I agree that separate signal phases provide a good amount of safety.

    Physical barriers, while not perfect, do add a great amount of perceived safety, which (when combined with actual safety) is key to getting more bikes on the road. 🙂

  • MatthewEH

    No kidding. Have you ever tried to ride down the split-phase segment of Broadway from Columbus Circle to Times Square, or again from Herald Square to Madison Square? If you observe the bicycle-red phase of the split and ride at civilian cycling speeds, you can get 2 blocks before hitting the next red.

    So, if you make the logical choice to proceed on green left-turn arrows, there are some conflicts. Even so, I still find the split-phase design to be lower stress overall.

    I think what’s really needed here is for the turn-only part of the split-phase to become a yield/flashing yellow for cyclists rather than a hard red. Yield to cars that are turning, but if nobody’s coming and you can proceed without usurping right of way, go for it.

    I know they did a ticketing sting for cyclists offending in this way at one corner of my old office back in the day. (8th Avenue at 15th Street.)

  • MatthewEH

    Hah, yes, I made the same comment above.

    Note that there actually have been ticketing stings for this offense in the past. 🙁

  • MatthewEH

    Yep. I’ve noticed this problem is particularly awful on 9th Ave southbound at 41st and 39th. Feh.

  • MatthewEH

    All the upvotes for this, yes.

  • Joe R.

    When you do the math you can easily see why cyclists would choose to go on the left turn arrows. Besides the fact stopping every two blocks is annoying as heck (seriously, ask a pedestrian if they would like to repeatedly be forced to wait 30 to 45 seconds after walking for maybe 30 seconds), it kills average speeds to the point of making cycling just about useless. If you average 10 mph for those two blocks you’re in motion then it takes you 36 seconds. Now add another 30 seconds while you’re stopped for the signal, if not more. This brings your overall average speed down to around 5 mph. I haven’t personally rode on that segment of Broadway, but I have encountered my share of streets here in Queens where a cyclist strictly obeying the law will be reduced to average speeds well into the single digits, even if you try to go 20 mph in between lights. I’m convinced the people who plan these bike lanes never actually use them. Travel speed should be second to safety as a criteria of usefulness. A perfectly safe bike lane is virtually useless if a cyclist legally using it can’t average more than walking speed.

    I think what’s really needed here is for the turn-only part of the split-phase to become a yield/flashing yellow for cyclists rather than a hard red.

    In general that should be the case citiwide with traffic signals, other than in the rare cases where very poor sightlines would make yielding unsafe. DOT can do this yesterday rather than waiting for the legislature to pass an Idaho stop law. Just put a bike signal under all existing traffic lights. When the light is green, the symbol is green. When the light is red, the symbol is flashing yellow.

  • Charlie

    In Massachusetts, MassDOT has decided to use the term separated bike lane when referring to one that is physically separated from other traffic.

  • Charlie

    Split phases may be safer, but they greatly reduce the green time for bikes. I think it’s fine to have green for the turning (and through) traffic at the same time as the green for bikes. However, the intersections should be more of a separated design, where bikes stay separated and the geometry forces drivers to slow down and yield to them.

  • jsallen

    The term “perceived safety” is misleading, because there is no such thing. What it actually means is that people think they are safe, whether they actually are safe or not. Let’s not forget the story of the Pied Piper.

  • jsallen

    Doubletalk.

  • jsallen

    I think that you mean a left-turn lane on the left side of a one-way street, as is common in Manhattan. The problem is the same as with a right-turn lane on the right side of a street. Passing in the through lane clser to the center of the street avoids the risk of collision with a turning vehicle.

  • Alex Brideau III

    If you prefer, we can use the term “perception of safety” instead.

  • Alex Brideau III

    Of course, you cite some good examples of situations where some people’s perception of safety do indeed lead to poor choices, and actually can turn of other riders as well.

    However, at least in my case, I’ve heard arguments against (parking-/bollard-/curb-) protected bike lanes as they don’t eliminate conflict areas with cars. That may indeed be true, but they do create a higher level of comfort for those like me who don’t prefer to bike in the current climate of shared-use lanes or door-zone bike lanes. And once I and other casual riders like me take to the streets in greater numbers, it helps other road users to be aware that bikes are to be expected, and not an anomaly.

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