LA In Sync: Traffic Signals To Cut Travel Times

http://losangeles.cbslocal.com/2013/02/19/la-in-sync-traffic-signals-to-cut-travel-times/

by stfu • 13 years ago

37 45 13 years ago

losangeles.cbslocal.com

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

tunesmith• 13 years ago

You might be uniquely suited for the job, then. :-)

lutusp3• 13 years ago

> 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.

dhimes• 13 years ago

A friend of mine in grad school used to do it for some jurisdictions. The optimization software was his own, he sold the solutions. I really don't know much more about it than that, however.

socalnate1• 13 years ago

"L.A. is about to become the first major city in the world to synchronize all of its traffic signals."

For such an obvious way to improve traffic, safety and reduce pollution, it seems very odd that this hasn't been done before in other forward looking cities.

jug6ernaut• 13 years ago

When you say "other forward looking cities" I say "every city in existence".

Seriously, why isn't EVERYONE doing this?

shiftpgdn• 13 years ago

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 fantastic. You can make it through the city center at great speed. I frequently take surface streets as they can be faster than the congested freeways.

bicknergseng• 13 years ago

I did my college senior design on this.

To put things in perspective, the city of San Jose, CA is currently doing a similar upgrade to the city's traffic signals and controllers at a cost of ~25 million dollars. While it is expected to save money in the long run, that's a serious chunk of change. The project (as government contract jobs tend to do) has also gone several years past its due date.

codex• 13 years ago

Once most of the cars on the road are self-driving, even more opportunities for traffic optimization arise.

ars2• 13 years ago

That's going to take at least 20 to 30 years at the most optimistic, I don't think we should wait till then to optimize things.

purplelobster3• 13 years ago

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 buttons are just props or if they actually do anything.

There's way more traffic in the US though, so not sure if this would work since there are so many traffic lights, and almost always someone waiting in each direction. Something that frustrates me to no end though are traffic lights (especially for left turns) that were obviously programmed with the belief that not many cars would be waiting, but is in fact heavily used. This results in having to wait 2-3 light cycles just to get through the intersection.

dkl• 13 years ago

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.

masenf• 13 years ago

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.

lutusp3• 13 years ago

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.

purplelobster3• 13 years ago

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?

gamblor956• 13 years ago

The pedestrian buttons are not placebo buttons, they actually work. Generally, the pedestrian signal is set to activate in cycles of X minutes (where X is usually a multiple of 2, 3, or 5) but only if it the pedestrian button has been pressed. If the button is not pressed before the cycle begins, the pedestrian signal does not activate on that cycle but will activate on the next cycle. (For example, see dkl's comment.)

In synchronized systems, the pedestrian light works the same way--this is why some intersections will sometimes have red lights for every direction of traffic even though there are no pedestrians.

ars2• 13 years ago

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.

purplelobster3• 13 years ago

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.

mturmon• 13 years ago

This will be highly variable from place to place. I've observed them in many places in the US, but that's anecdotal too, and I was unable to find anything online.

On arterial roads in LA, there will be inductive sensors (identifiable by the circles of cut-and-resealed asphalt near the stop line) at every approach to every stoplight.

Some lights will not change unless you are at the correct place. If you creep too far over the stop line, or don't approach it close enough, you will never get a green light (until someone else comes).

Depending on day-of-week and time-of-day, the lights will be on a different tempo. They will respond to ped signals more slowly at rush hour, but more quickly around schools at dismissal time, for example.

Changing the timing of the lights is barely noticeable to drivers but can have a huge effect on road conditions. There is a freeway offramp near my home that feeds directly onto an arterial street. The geometry encouraged people to barely slow down (60 MPH offramp, 35 MPH street). A carload of people were killed on the road (lost control and slammed into a median tree at ~70 MPH -- the tree won).

There are now inductive loops as you traverse the offramp. They sense if your car is exceeding a given speed, and they trigger a red light at the end of the offramp if you are. Then, after that turns green, the system triggers another red light four blocks down the road to be sure you're not drag racing.

It has been highly effective at enforcing reasonable speeds.

stevek• 13 years ago

> There are now inductive loops as you traverse the offramp. They sense if your car is exceeding a given speed, and they trigger a red light at the end of the offramp if you are. Then, after that turns green, the system triggers another red light four blocks down the road to be sure you're not drag racing.

Oh that's a wonderful nudge! These sound much more better than speed cameras. No need for a bureaucratic system of enforcement for the simple things.

Just as long as they're used judiciously. I can imagine the rage if the guy in front keeps triggering the red for everybody.

fomojola• 13 years ago

Spent a bit of time playing with traffic and parking. One of the issues with the dominant traffic sensor tech (inductive loops in the ground) is that they can fail in cold weather: frost heaves in pavement sometimes crack the loop wires.

In the south, southwest and the pacific northwest the incidence of loops is much higher than in the northeast. There's also more driving, so the incentive to optimize that part of the experience is much higher than say, in the Boston area, where they really just want you to give up the car and use public transportation.

sbashyal• 13 years ago

I am surprised that it wasn't already done but it makes me happy that the traffic in LA can get a little better (I live north of LA).

The sync will probably also discourage speeding as the sync works well at the speed-limit.

chriskelley4• 13 years ago

I live in LA, and today I honestly noticed a significant reduction in my commute time (from 45min down to about 30min), but this is the first I've heard of this. I thought I was just hitting all the lights, but interesting to find out it may become a more common occurrence! Looking forward to gathering more experience with the system.

melling• 13 years ago

How many miles is your commute?

chriskelley4• 13 years ago

A little over 9 miles, all surface streets. Marina del Rey to Beverly Hills. For what it's worth, I canvased about five people today and asked them the untainted question "Did you notice anything about your commute today?" and all of them said it was notably faster than usual.

jrockway• 13 years ago

I read the comments on the site and have pretty much lost my will to live. What a cornucopia of ignorance. Who let these people on to the Internet?

sampsonjs• 13 years ago

Nothing you wouldn't see at the appropriately named Yahoo News. Perhaps you can still read similar comments there.

joshAg• 13 years ago

Never, but never, read the comments on a mainstream news site. This is for your own protection.

X-Istence• 13 years ago

So kinda like this: http://en.wikipedia.org/wiki/Green_wave ?

to3m• 13 years ago

Fascinating quote from the Wikipedia page: "In the UK, in 2009, it was revealed that the Department for Transport had previously discouraged green waves as they reduced fuel usage, and thus less revenue was raised from fuel taxes" :)

I wonder if any of the planners have been thinking the same way about this project.

EDIT: let me guess.

Burn 1 US gallon of petrol = 17.68 US pounds of CO2 (see http://www.eia.gov/tools/faqs/faq.cfm?id=307&t=11) = 8.02 kg. 1,000,000 tonnes is 1,000,000,000 kg, so 124,688,279 US gallons of petrol (so 471,996,258 litres, or 3m barrels, or something like 0.04% of all US annual consumption).

The California petrol tax per US gallon is $0.36 (http://www.boe.ca.gov/sptaxprog/spftdrates.htm), so this amounts to $44m. (Don't forget my calculations are quick and dirty.) So that's hardly nothing, but compared to the rest of the budget, a mere drop in the ocean, and probably worth doing if it speeds traffic flow and if reduced spew of noxious poisons and greenhouse gases is a policy goal.

(The same calculation run across the whole of the UK would probably produce more compelling numbers, suggesting that the green wave is a poor idea for the government - but reduction in greenhouse gases is a current policy.)

johnwyles• 13 years ago

I swear Austin has to be the worst inner-city traffic light syncing I have seen of any major US city.

jnsaff2• 13 years ago

This is step one.

Step two I imagine would be car navigation equipment connected to city traffic controller. Kind of like SDN in the networking world.

People tell where they need to go and the system fills the streets in the most efficient way (or other criteria) possible.

This could route traffic away from schools during certain times etc.

waterlesscloud2• 13 years ago

I'm a little surprised to learn the city of Los Angeles only has 4400 traffic signals. Though I've never tried to estimate the number.

I do remember reading only a small percentage of lights have a left turn arrow. Even major intersections can be missing them. Hence the LA unofficial practice of "two cars can turn left just as the caution expires". It's the only way you'd get through some intersections at all.

nilsbunger• 13 years ago

It's actually a fascinating problem. Naively, it seems you can't sync traffic in both directions of a 2-way street at the same time -- e.g. eastbound and westbound.

Two things I've thought of that would work for both directions:

1) if it's a square grid, you could pick some streets that are optimized westbound, and some eastbound, for example. And then carefully arrange those so the north-south streets also mesh with it.

2) if you can space the lights the right distance apart, you could create a "standing wave" where it is actually optimized both ways. But you'd have to space them farther apart and more regularly than in real cities. For example, you could set lights apart by 1 minute travel time at 30mph, if the signal cycle is 1 minute.

I'm curious what other solutions are to this, and how they do it in a place like LA in practice!

w4y2• 13 years ago

Yes you can, as long as the lights are spaced within certain multiples of a certain distance, and the speed limit in both ways are the same.

gdubs• 13 years ago

LA citizen here. I just got to Silverlake from Santa Monica in half the time as usual. It took 40 mins compared to the >1.25 hours it usually takes. Stretches of green lights as far as the eye can see. If this keeps up it will be incredible – grateful to the City of Los Angeles for making this happen.

LA In Sync: Traffic Signals To Cut Travel Times

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