

LA In Sync: Traffic Signals To Cut Travel Times - stfu
http://losangeles.cbslocal.com/2013/02/19/la-in-sync-traffic-signals-to-cut-travel-times/

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purplelobster
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.

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lutusp
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.

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purplelobster
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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hammock
>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.

Most cities do this. I live in chicago, that's how it works. Note that you can
also combine sensors simultaneously with timing in certain cases- for example
whether to allow an advance green for a left-turn lane or not (if there are
cars in it, advance green, otherwise, normal green for both directions of
traffic on that road. No change in when the next red comes)

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jrockway
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?

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

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chriskelley
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.

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melling
How many miles is your commute?

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chriskelley
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.

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waterlesscloud
Hmm. Do you actually travel in the city of Los Angeles on that trip?

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chriskelley
90% of that drive is through LA... I go from Pacific/Washington to
approximately Robertson/Wilshire. BH starts between Olympic and Wilshire, and
MDR ends just after Wash/Lincoln. Currently working on the south-east end of
BH.

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wj
A friend of mine just opened a surf shop at your intersection.

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chriskelley
VB Surf! Looking forward to checking it out. They went from empty to open in
like 5 days... I wish I could ship software that quickly!

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

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lutusp
> 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.

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nilsbunger
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!

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w4y2
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.

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

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ars
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.

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socalnate1
"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.

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jug6ernaut
When you say "other forward looking cities" I say "every city in existence".

Seriously, why isn't EVERYONE doing this?

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shiftpgdn
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.

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drone
I love it when they're actually in-sync. I'm not sure of the cause, but they
always seem to de-sync after a period of time. I remember we used to have a
set speed, I think it was 26mph or so, that if you held, you'd cut right
through downtown if someone didn't get in your way.

I'm with you on the surface streets, 45 at the Pierce Elevated (and 10 feeding
into it) is its own little hell.

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waterlesscloud
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.

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jnsaff2
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.

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gdubs
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.

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sbashyal
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.

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X-Istence
So kinda like this: <http://en.wikipedia.org/wiki/Green_wave> ?

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to3m
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](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.)

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

