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What Goes Into Timing Traffic Lights?


As part of the cities project, we're tackling some gee-whiz questions about how cities work. Today, this question. How does the city decide the timing of traffic lights and pedestrian crossings? Turns out, the more forms of transportation you add to an intersection, the more complicated it is to time the lights. Melissa has gone outside here in Washington to learn some basics.


I'm here in the heart of Chinatown. We're at 7th and 8th Street Northwest and I'm joined by Gideon Berger of the Urban Land Institute who's going to help us out. Gideon, hi.


BLOCK: When we polled our staff about these issues, this was the most popular question, traffic lights. Now, obviously there's a conflict. You've got cars that want to drive through. You've got pedestrians who want to cross. What's the trick here?

BERGER: Well, for any city, the real challenge is trying to balance all those different nodes. And one of the interesting things that's changing in the world of transportation planning is now cities are taking a multi-modal approach, not just measuring how long it takes a certain number of cars to get through an intersection, but how many pedestrians are going through the intersection, how many people on a bus or on a train are going through the intersection and how many people are biking through the intersection.

So as they do that, they're making their jobs a little bit more complicated but they're also thinking about how people behave in the real world.

BLOCK: If you're trying to time the lights, right, for drivers, sequentially so the traffic can keep flowing, there's a pattern. They're going to start turning red in sequence, right, so you can kind of move through and they can turn red behind you, but you can still make connections without speeding too much.

BERGER: That depends on what kind of the street you have. This street, on H Street, this is a particularly challenging street to accommodate all the different needs because a couple blocks away is the entrance to an interstate. And normally on a feeder road, you might want to time the lights sequentially to allow the cars to quickly get onto the interstate.

That is a difficult thing to accommodate at this particular intersection because of all of the land use that's here. You have a lot of employment. You have a lot of entertainment. You have a Metro station. You have a lot of pedestrian traffic. So you probably wouldn't time the lights to allow the cars to gradually speed up and get faster as they get to the interstate.

So you're going to get some stacking here, particularly at rush hour, as a result of that. Now, stacking can be a good thing from a pedestrian safety perspective, 'cause it means the cars are going to be going slower.

BLOCK: Gideon, do you think - is there an ideal time for a pedestrian crosswalk; number of seconds?

BERGER: You know, I think the thinking on that is sort of evolving. Because as our society, as we become an older country, we're really starting to have to think about how long it takes older folks to get around. And so, there are standards that are put together by the traffic professionals, there are national standards. Every city has, if not codified standards, then guidelines that their public works or transportation departments enforce. They're different in every city. They can be different.

BLOCK: Yeah. Now, we've taken especially interesting intersection here because look what happens here. This is a diagonal cross pedestrian intersection. Traffic stops in both directions. I've never seen this before. You can cross diagonally across from one corner to another. What's going on?

BERGER: Yes, the technical word for it in is Barnes dance.

BLOCK: Barnes dance.

BERGER: Right, it's named after the pioneering traffic engineer Henry Barnes.

BLOCK: No kidding.

BERGER: It's a very powerful statement 'cause it's telling the people who are driving that the pedestrians really have priority at this intersection. And all the cars have to come to stop in all directions to accommodate all the pedestrian movements.

BLOCK: Well, look. There goes somebody across the diagonal cross right now.

BERGER: That's right, yeah. I've been watching it for about 10 minutes and most people have not been using it and...

BLOCK: Well, it's not well marked. You wouldn't necessarily know you can do that.

BERGER: That's right. There's definitely they're sending I think mixed signals to the pedestrians here. One of the more telling ones is the curb cuts on the corner. There are curb cuts to cross laterally and parallel, but there is no sort of diagonal curb cut as well. And also, the crosswalks, there's no diagonal crosswalks.

Henry Barnes use to say he could fix any traffic problem with a can of yellow paint and some common sense.


BERGER: And I don't know if that's completely true. But the idea behind it is, which is that by some simple, inexpensive repainting, re-striping you can really change the way an intersection functions.

BLOCK: Gideon Berger, good to talk to you.

BERGER: Thanks.

BLOCK: Gideon Berger is a former city planner. He's now with the Urban Land Institute here in Washington


CORNISH: You can follow the Cities Projects on twitter @nprcities. And we're posting photos you've sent from the heart of your cities at NPR.org/nprcities. Transcript provided by NPR, Copyright NPR.