Hey, DOT, Smarter Signal Timing Could Resolve Bike-Car-Pedestrian Conflict

With the right traffic signal timing, city officials could maximize the successive green lights for all road users.

Separate green light phases for cyclists and left-turning motorists, like the one pictured here, wind up forcing cyclists to stop every two or three blocks. Photo: Gersh Kuntzman
Separate green light phases for cyclists and left-turning motorists, like the one pictured here, wind up forcing cyclists to stop every two or three blocks. Photo: Gersh Kuntzman

A pair of bike advocate-engineers think they have solved a lingering issue dogging DOT at intersections along protected bike lanes: how to maximize safety for pedestrians, cyclists and drivers without slowing them down with red light timing designed for cars.

The designers — Christine Berthet and Joe Realmuto — say the optimum signal timing on main arterials should be a 100-second light cycle, of which 70 seconds are green [PDF].

Berthet and Realmuto were motivated by DOT’s “Cycling at the Crossroads” report, released earlier this year, which concluded that separate green light phases for cyclists and left-turning motorists — known as “fully split phases” — were the best way to ensure safety for all road users, yet still had too many drawbacks to be widely implemented. (A similar intersection design, known as the “delayed turn” or split leading bicycle interval, builds on that model by giving cyclists their own phase as a head-start, then allowing drivers to proceed with a flashing yellow arrow.)

Enter Berthet, Manhattan Community Board 4 transportation committee chair, and Realmuto, an engineer and cyclist from Queens. The pair set out to find the optimal traffic signal timing to give motorists, cyclists, and pedestrians the highest possible number of green lights in a row.

Since the city’s existing signal timing schemes are based on typical driving speeds, Berthet and Realmuto pegged their simulation to 25 mph speed limit, assuming each block is 264-feet long and takes 7.2 seconds to travel by car. Assuming slower speeds for pedestrians and cyclists, Berthet and Realmuto then simulated variety of possible signal timing schemes, all of which gave drivers 45 straight green lights, to determine which would produce optimal “green waves” for cyclists and pedestrians as well.

A visual representation of Berthet and Realmuto's simulator. The three lines represent different vehicles traveling at different speeds.
A visual representation of Berthet and Realmuto’s simulator. The three lines represent different vehicles traveling at different speeds.

Based on the simulation, 70 seconds of green time and 30 seconds of red would yield a 45-block green wave for drivers, a 19-block green wave for cyclists, and 13-block green wave for pedestrians. That arrangement works best on Manhattan’s avenues, whose crosstown street carry less traffic and therefore necessitate shorter light cycles.

Berthet, a longtime safe streets advocate in the Chelsea and Hell’s Kitchen neighborhoods, has been pushing for more split-phase signals for years. DOT has demurred, citing its desire not to stop cyclists at too many green lights. But its intersection report foretold plans to “develop strategies to improve signal coordination that reduces bicyclist stopping and delay along corridors with several fully split phase intersections.”

Berther and Realmuto hope their simulation can serve as a model for DOT to replicate.

“It’s kind of a holy grail, in a sense,” Berthet said. “There is a sweet spot. You don’t have to compromise too much if you do it the right way. There’s no reason not to give the proper safety.”

  • Joe R.

    Pedestrians crossing the avenues *don’t* encumber turning vehicles for 23 seconds at every intersection all the time. I’d love to know where you’re getting this statistic from.

    You’re basically saying we’re stuck with light timing which frankly sucks for pedestrians and cyclists just so cars can make turns. Well guess what? This means part of the green phase on minor cross streets will be usurped by pedestrians and cyclists just ignoring the red light, which is what happens now in practice. The minor cross streets get a defacto shorter green cycle as a result. You can optimize for cars to the detriment of everyone else, or you can optimize for all three groups without severely negatively impacting cars.

  • sbauman

    Pedestrians crossing the avenues *don’t* encumber turning vehicles for 23 seconds at every intersection all the time. I’d love to know where you’re getting this statistic from.

    You helped derive it a few posts ago. It’s the total of the 13.6 sec walking time to cross the street plus the delay to start walking of a sidewalk queue that’s 5 people deep. Assuming a 2.5 second reaction time for each, the last pedestrian on the queue will start walking 10 seconds after the first. That comes to 13.6 + 10 or 23 seconds. By the time the last pedestrian crossing in the traffic direction clears the first traffic lane, that pedestrian will be replaced by the last pedestrian crossing from the opposite direction.

  • Joe R.

    We’re in touch with DOT now and I’ll let you know what they say about all this. For now though, I highly doubt queues 5 people deep are a normal thing, or at least that’s not something I see at every corner.

  • qrt145

    But New Yorkers have a negative reaction time. It is, after all, the city of Spider-Man! 🙂

    “But Peters, who studied crosswalks in only New York, found that that was six seconds too many — she found reaction times of negative three seconds. New Yorkers, whether they waited patiently on the sidewalk or had already maneuvered their way into the crosswalk, were moving before the walk signal even flashed; they looked instead at the traffic light and charged out as soon as it turned in their favor.”

    https://www.nytimes.com/2015/04/26/magazine/how-do-we-protect-new-york-citys-pedestrians.html

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