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LA In Sync: Traffic Signals To Cut Travel Times

losangeles.cbslocal.com

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Re: LA In Sync: Traffic Signals To Cut Travel Times

#11

Post by dkl »

purplelobster wrote: post #6

Something that puzzled me when I came to the US was that traffic lights (including a majority of pedestrian lights) just cycles through a certain schedule. Where I come from, traffic lights have sensors in the ground that manages the traffic. Also, all pedestrian crossings have very responsive buttons that truly work. The crossing light won't come on unless someone presses the button. In the US, I can rarely tell if…

Cities are more and more doing this (what you specified for pedestrian lights outside the US). My problem: there's a very large intersection that I walk through daily that changed to "only allow walk when the button is pushed" and 9 times out of 10 when I walk up to it and push the button when it normally would have been set to "walk", nothing happens. Then, the next traffic cycle I get the "walk" light. However, I've also noticed that from the time I push the button until when the cross traffic starts, 90% of the time I would have had time to walk across the street. So, I've started ignoring the button and just walking when "the time is right." In this case, it has made me less safe. Twice in a few months I've been caught in the middle of the street when the light turns.

I realize that in this case, the walk portion is probably just not tuned properly, but I wonder how often that happens.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#13

Post by lutusp »

purplelobster wrote: post #6

Something that puzzled me when I came to the US was that traffic lights (including a majority of pedestrian lights) just cycles through a certain schedule. Where I come from, traffic lights have sensors in the ground that manages the traffic. Also, all pedestrian crossings have very responsive buttons that truly work. The crossing light won't come on unless someone presses the button. In the US, I can rarely tell if…

As it happens, you have the choice to sense and respond to traffic, or have synchronized traffic signals in a complex grid like a city center. You can't really have both.

To have synchronized signals in a city-wide two-dimensional grid of intersections, you must ignore the fact that some intersections have many cars and some have none. If you give up on the idea of synchronizing traffic, then you can have traffic sensors and working pedestrian crossing buttons.

> In the US, I can rarely tell if the buttons are just props or if they actually do anything.

It turns out that in many places, especially those that have synchronized signals, the pedestrian buttons are what might be called "placebo buttons". I'm sure you can guess what that means.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#14

Post by lutusp »

peter_l_downs wrote: post #2

I've always wondered who gets to write the software that syncs traffic signals. Seems like it would be a really fun job.

> I've always wondered who gets to write the software that syncs traffic signals.

Traffic synchronization in a city grid of streets isn't very challenging -- it's a matter of setting up a timing scheme that always works the same way. City-wide synchronization isn't about sensing cars at each intersection, it's more like conducting an orchestra and simply making sure everyone is playing in the same tempo.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#15

Post by ars »

purplelobster wrote: post #6

Something that puzzled me when I came to the US was that traffic lights (including a majority of pedestrian lights) just cycles through a certain schedule. Where I come from, traffic lights have sensors in the ground that manages the traffic. Also, all pedestrian crossings have very responsive buttons that truly work. The crossing light won't come on unless someone presses the button. In the US, I can rarely tell if…

The US is a very large place.

Where you were I guess they didn't do that, but they most certainly do use sensors in many cities.

Since I haven't been everywhere yet I can't say "most", "some" or any other words like that. But sensors are used in the US.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#17

Post by masenf »

dkl wrote: post #11
purplelobster wrote: post #6

Something that puzzled me when I came to the US was that traffic lights (including a majority of pedestrian lights) just cycles through a certain schedule. Where I come from, traffic lights have sensors in the ground that manages the traffic. Also, all pedestrian crossings have very responsive buttons that truly work. The crossing light won't come on unless someone presses the button. In the US, I can rarely tell if…

Cities are more and more doing this (what you specified for pedestrian lights outside the US). My problem: there's a very large intersection that I walk through daily that changed to "only allow walk when the button is pushed" and 9 times out of 10 when I walk up to it and push the button when it normally would have been set to "walk", nothing happens. Then, the next traffic cycle I get the "walk" light. However, I'v…

I am not a traffic flow engineer, but I have experienced what you describe in regard to the walk button behavior in many places in western Washington, US. I always reckoned that they were using a simple model in which a button press would cause the "WALK" signal to always be fired at the corresponding light configuration at the _next_ cycle start. This is most likely a safety consideration because, as you noted, there may not be enough time to safely cross.

In some complex intersections I've noticed that the 'cycle start' doesn't necessarily line up with the corresponding configuration for the walk button which was pressed. This may result in having to wait nearly two complete cycles for the walk signal in the worst case (pressing the last configuration's walk button immediately after the cycle start). The resulting behavior could be considered a bug, or perhaps just more consideration payed to vehicle traffic. But it is certainly frustrating as the pedestrian.

At many intersections I cross now the signals are automatic with the traffic flow. Most of them also contain a countdown timer indicating the number of seconds until the light changes. It seems that this behavior would be a good tradeoff for pedestrian efficiency and safety. However it comes with a bit of trust from the gov, that you know you can't walk across 5 lanes in 2 seconds.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#18

Post by fnordfnordfnord »

shiftpgdn wrote: post #10
jug6ernaut wrote: post #4

Earlier quoted context omitted.

When you say "other forward looking cities" I say "every city in existence". Seriously, why isn't EVERYONE doing this?

We have had this in Houston, TX for nearly a decade. They work pretty great if everyone is driving at the speed they're designed for. Otherwise you have to deal with red light runners, people parked at green lights texting, metro bus drivers parking their buses on the road while they go on break, people speeding and getting to the lights too early and people driving too slow. That said at off hours they're absolutely…

Right, it is still totally open-loop.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#19

Post by purplelobster »

ars wrote: post #15
purplelobster wrote: post #6

Something that puzzled me when I came to the US was that traffic lights (including a majority of pedestrian lights) just cycles through a certain schedule. Where I come from, traffic lights have sensors in the ground that manages the traffic. Also, all pedestrian crossings have very responsive buttons that truly work. The crossing light won't come on unless someone presses the button. In the US, I can rarely tell if…

The US is a very large place. Where you were I guess they didn't do that, but they most certainly do use sensors in many cities. Since I haven't been everywhere yet I can't say "most", "some" or any other words like that. But sensors are used in the US.

Of the 15 or so different states I've been to, this is what I've observed. Of course I can't say that there is no black swan, but seems unlikely. At least there's a tendency towards no sensors, and again, with the traffic volumes it might not make sense to have them in most US cities.

Re: LA In Sync: Traffic Signals To Cut Travel Times

#20

Post by purplelobster »

lutusp wrote: post #13
purplelobster wrote: post #6

Something that puzzled me when I came to the US was that traffic lights (including a majority of pedestrian lights) just cycles through a certain schedule. Where I come from, traffic lights have sensors in the ground that manages the traffic. Also, all pedestrian crossings have very responsive buttons that truly work. The crossing light won't come on unless someone presses the button. In the US, I can rarely tell if…

As it happens, you have the choice to sense and respond to traffic, or have synchronized traffic signals in a complex grid like a city center. You can't really have both. To have synchronized signals in a city-wide two-dimensional grid of intersections, you must ignore the fact that some intersections have many cars and some have none. If you give up on the idea of synchronizing traffic, then you can have traffic sen…

It would seem that sensors would be most useful with lower traffic volumes, and synchronization better for rush hour. Perhaps using each system at different times of the day would be theoretically optimal.

Or why not incorporate sensor information into the synchronization, and continuously plan the best light cycles for any given situation?

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