Traffic Lights

[geonuc]

I saw in the Occupations thread that we have at least one traffic engineer amongst us, so I thought I'd toss this out 'cause it's been bugging me and I have this insane need to know everything.

What is involved with converting a lighted intersection from a strictly time basis (e.g., one minute for this direction, then one minute for that direction, regardless of traffic) to one that employs sensors and switches the lights as required (or in conjunction with a timing scheme)?

I ask because a city near me has many intersections that operate on the simple time basis. No sensors. I'm not talking about the lights along the main drag that are timed with each other to allow decent traffic flow, but rather the more isolated intersections where cars end up waiting ... and waiting ... and waiting, for no good traffic-engineering reason that I can see. Lots idling engines polluting the air. Lots of gas burning.

So, is it a big deal? Expensive? Complicated? Other issues, such as reliability? I know it depends on many things, but I'd like a feel for the problem.

[KLIK]

I knew I should have kept my mouth shut......

Signals that work fixed time are very cheap: you only need a clock and some simple switching.

There is then a sytem whereby you have microwave detectors on the signal heads facing approaching traffic, they use doppler to detect objects approaching them, normally above say 10mph to avoid pedestrians or swaying branches setting them off. These need more intelligent (used loosely!) controllers, microwave detectors for each approach and more wiring, hence more expensive.
Advantages are that they will change to green when they detect a vehicle approaching, cutting down pollution and delays, downside is if you approach too slowly they don't realise you are there and so don't change. also , in Britain anyway, people think they are cctv and vandalise them or they go out of adjustment so maintenance is a more regular feature.

3rd system involves no microwaves, but instead there are wire induction loops buried in the road surface approaching the lights. The loops have a small electrical current flowing through them which detect metal (pushbikes to HGVs) and tell the signals a vehicle is approaching.
Advantages are that the equipment is more robust being buried in the road, by having several loops on the same approach speed of vehicles can be judged, quantity of vehicles and also gaps between vehicles, this allows a certain amount of latitude on WHEN to change the lights; delay change for fast moving vehicles, or change quicker if there are gaps between approaching vehicles or give longer green periods for longer queues.
Downsides are; more complex controlling equipment, (more expensive and more power usage), loops in the road require more wire, but also slot cutting of road surface, ducting in a verge from the loops to the controller, and if anything does break it involves digging the road, all this then incurs more manhours to install/repair.

4th system is used in urban areas where signals are close together. it uses 3rd option above but with extra loops LEAVING the previous junction, this enables the traffic flow leaving 1 junction to send a message to the next junction saying 'X' number of vehicles approaching you; give 'Y' amount of green time.
Advantages are; an entire urban network can be linked in to each other with all the signals acting together to allow platoons of vehicles to travel with only minimal delays, as it counts cars and knows average journey times from signal to signal it changes to green as they are approaching (or earlier to move existing queues which a new platoon joins), and they don't give too much emphasis to a vehicle approaching on a side road. they also actively adapt to changing traffic conditions, moving platoons in opposite directions at diffent times of day.
Disadvantages: need a mainframe to give overall control and tell individual signals when to change, equipment more complex on site, also modems, phone lines etc, so most expensive.


You can also use combinations of the above; eg microwave detectors to give long/short pedestrian greens for slow/quick peds.

search words will be UTC-urban traffic control or SCOOT-split cycle offset optimisation technique.

run out of lunchtime so not proofread.
have a good weekend

[geonuc]

Thanks!!

I think we use the embedded loop technology a lot here in the US.

I want all that good stuff. I will propose to the council to raise everybody's property taxes to pay for it.

As a humorous aside, we have one intersection near where I live where the 'stop' line is well back of the intersection. I don't know why. But the embedded sensor is back at the line, where it should be. I believe it extends four or five feet beyond the line. This light will stay green in the opposing direction forever, unless it detects a car.

Anyway, here in Atlanta, stop lines are taken as mere suggestions. The standard practice is to stop with your car astride the crosswalk and maybe with the front bumper threatening the cross traffic. Got to get a head start when the light turns green!

So, I have several times seen cars stopped way ahead of the line (and sensor), with the driver fuming because the light won't change. And it won't until someone comes up behind them and triggers the sensor (the technology is smart enough to reset if a car passes over it and clears).

[sarongsong]

How about when fire engines/police cars are able to control traffic-signals?

[NEOWatcher]

Anyway, here in Atlanta, stop lines are taken as mere suggestions.

I take them that way too, but only if I can see the sensor loop.

Around here the loop seems to extend quite a ways toward the intersection, and there usually isn't enough not looped* to fit a whole car. So the problem you mention usually isn't encountered.

My issue is the other way around. The car doesn't move far enough into the intersection to trigger the loop.

I have an intersection on the way home that I have sat as long as 8 minutes for because the idiot at the front of the line has no idea why people behind him are honking thier horn.
Part of the issue is the design. It's a 2 lane road going into a 5 lane cross street with heavy truck traffic. Many of the trucks have a hard time making the turn, so the left turn lane is set waaay back. The loop is about 25 feet (if memory serves me), but overlaps the area behind the stripe by only a foot or two. So the only way to trigger the light is to be right up against the line.
Plus; the light is long enough already... (2 minutes before it allows the cycle) So; it's a long time to realize if you even tripped it.

*Sorry; I hate double negatives...but this was the easiest way to say it without losing my train of thought.

[NEOWatcher]

How about when fire engines/police cars are able to control traffic-signals?

Yeah; how about them?

What exactly are you saying here? You do make an interesting point, but the question seems to vague to pinpoint an answer.

In my college days, I drove buses that had the technology to trip traffic lights. The system was pretty much disabled by the time I drove it, but there were still some active remnents. We had a layover spot on an unscheduled continuous run where the departing bus would trip a timer that counted down for the next bus to wait for enough distance between.

I have also heard of radio control for emergency vehicles, and some that actually were triggered by the sound of a siren.

[geonuc]

NEO: Agreed, that intersection I speak of is unusual in that there's plenty of room to fit a car between the sensor and the crosswalk.

sarongsong: There are a lot of lights around here that can be triggered by emergency vehicles. You can even see the sensor up by the light.

[sarongsong]

...the question seems to vague...

Vague?:

...We had a layover spot on an unscheduled continuous run where the departing bus would trip a timer that counted down for the next bus to wait for enough distance between...

and

...I drove buses that had the technology to trip traffic lights...

Why would a bus be given this "technology"?

[NEOWatcher]

Vague?:

To me? yes. To others? maybe not.
Although; I probably would have questioned my use of the typo "to".

Why would a bus be given this "technology"?

All in the name of efficient public transportation. But; in this case, it started as a research project that I hear was rather innovative for its time.

[tommac]

I have had this conversation with someone that works for siemens. I think the math gets VERY complex, there are many predictable things as well as unpredictable things ... like rush hour vs an accident in an intersection. Some systems can be smart enough to reroute ( by making a certain route slower ) or rather slow flows of traffic towards a congestion spot. I think also that GPS is being used in europe. On a slightly different note the traffic GPS things could really help out if they could reroute as an optimization based on time ( including traffic ), however that is difficult because you may not know about the traffic until it is too late ( at least when it comes to highways ).

[hhEb09'1]

Traffic issues?

Hey, KLIK, what's the latest on the longer yellow lights? There were a few articles some months agon that supported the idea that longer yellow lights reduced traffic accidents. Any updates?

The idea was that accidents were mostly a result of velocity changes (turn, slow, speed up), and short yellow lights tended to make drivers hit the brakes to make the red liight or accelerate to get through the intersection.

[tdvance]

I suspect longer yellow lights would reduce accidents....at first. Once people get used to yellow lights being longer, they will run them later. And green-light anticipators would have dangerous false starts more often too--that effect would increase the accident rate after people no longer stop so fast on Yellow.

Better to have them run at the time they need to be given speed of traffic, or at worst only a tiny bit longer, and then leave it that way. Also a light that turns green say, 1/2 second after the cross-light turns red would give time for the last car in the intersection to move out--that might help--but again, if you make the delay too long, people will just keep going when it's red.

[hhEb09'1]

Better to have them run at the time they need to be given speed of traffic, or at worst only a tiny bit longer, and then leave it that way.

Yes, as I remember the study, yellow lights were consistently too short for the posted speed.

[sarongsong]

...like rush hour vs an accident in an intersection...the traffic GPS things could really help out if they could reroute as an optimization based on time...

This morning there were 2 different accidents at opposite ends of a major North/South freeway in my county, with live-cams/helicopter shots of the resulting congestion shown on TV. The announcers were advising alternate routes for the stuck drivers to take.

[NEOWatcher]

Yes, as I remember the study, yellow lights were consistently too short for the posted speed.

That could be true, but we also need to consider the time before the opposite green.
I think I remember the lights normally going immediately green one way when the other way went yellow to red. Now there is a short period of time when all is red. This gives those who don't quite make it, and those who jump the gun a bit more margin.

[Maksutov]

In New York City, a yellow light means floor it.

Red means you still have some time, so floor it.

Green means the car in front of you hasn't floored it yet so it's time to lean on the horn.

[geonuc]

With lights that are on a strictly time basis (1 min this way, 1 minute that way), how are they typically programmed? Software, firmware, local, remotely?

In other words, say you wanted to adjust the settings - would the city have to send out a technician?

I ask this because I see lights that are poorly adjusted to the dominate traffic patterns and wonder how big a deal it would be to improve the situation.

[Kaptain K]

You're thinking too 21st century. Think simple timer. Originally mechanical (some may still be). Later ones electronic. And yes, changing the timing usually requires a tech visit.

[Jim]

Quote:
...I drove buses that had the technology to trip traffic lights...

Why would a bus be given this "technology"?

Houston recently constructed a light rail line; the trains run at street level on tracks embedded in the road. They use this technology to control the traffic lights, 'cause the trains ain't stoppin'.

The MTA is considering "rapid bus" transit that would use the same technology.

(The light rail trains are about the size of four buses. They are silver with flashing lights, bells and whistles. Yet, people still pull out or turn in front of them. We've even had a bicycists and a pedestrian do it.

Big and silver with flashing lights, bells and whistles may be the next stealth technology.)

[neilzero]

How about we raise some revenue by selling boxes to motorests for about $500 each? The box has buttons for right, left and straight and the appropriate green typically appears about 10 seconds after you push a button, unless you are more than 90 meters from the traffic signal. Police and fire etc could use the same boxes, but with a priority code, and 150 meters range. Neil

[sarongsong]

In New York City, a yellow light means floor it...

San Francisco has a similar arrangement, reversing the conventional red/green meanings. Its steep hills (think Bullitt) further enliven the traffic scene!
Mexico is onto something, tho:

...When you see a yellow light begin to blink, it means yellow will soon turn to red...
---"Day Trips From San Antonio"
google.books

[NEOWatcher]

...Big and silver with flashing lights, bells and whistles may be the next stealth technology.)

That must be what ET thinks when he's observing the Earth.

[KLIK]

How about when fire engines/police cars are able to control traffic-signals?

Hi,
there are 2 (that I know of) ways of doing this; one is to have a transponder under the vehicle that triggers the loop with a priority signal, second is to just do a series of timed runs and then the average is fed into the computer to change the lights at set times when a button is pressed in the fire station (normally only for fire brigade as they are the only ones setting off from the same place each time), third (ok, 3) swipe card in the windscreen and a card reader attached to the lights (I call it a swipe card as I forget the proper name).
{and if you have a chip in your ID I suggest keeping it in lead lined wallet as crooks can read them as you walk by}

[KLIK]

Traffic issues?

Hey, KLIK, what's the latest on the longer yellow lights? There were a few articles some months agon that supported the idea that longer yellow lights reduced traffic accidents. Any updates?

The idea was that accidents were mostly a result of velocity changes (turn, slow, speed up), and short yellow lights tended to make drivers hit the brakes to make the red liight or accelerate to get through the intersection.

No idea I'm afraid, AFAIK that's a US study. In Britain it should always be 2-3 secs of amber with a variable all-red period (illegal to jump an amber)

[hhEb09'1]

there are 2 (that I know of) ways of doing this;

IINATE, but I think a fourth way is the most common in the USA: a visible light strobe is part of the emergency vehicle's warning lights, and its particular frequency trips a dectector on the signal lights.

[NEOWatcher]

IINATE, but I think a fourth way is the most common in the USA: a visible light strobe is part of the emergency vehicle's warning lights, and its particular frequency trips a dectector on the signal lights.

Well; you can stop "thinking" and take comfort in the fact that you actually "know".

Those things are all over around here... Here's an example

Well; to be fair, I see the blue lights all over the place, but I'm not sure how many have optical sensors or are even working.

[Laguna]

In Germany we just have these blue lights and the horn on top of the emergency vehicles.
Works pretty good. As if an emergency vehicle would wait for green...
Oh, with one exception. Traffic lights that are located directly besides the depot are switched to all red as the vehicles leave.

After that, horn and lights... Would not be enough time do switch the lights of an approaching traffic light to all red and clear the crossing of all vehicles in time anyway.
I guess you would have to do it way in advance when the emergeny vehicles are still far away from the crossing. And then they would have to plow through the stopped traffic anyway.

What we do have is the possibility for buses that are behind their schedule to take influence on the timing of the next traffic lights,
so that their green phase gets longer.

[tdvance]

We have some system here for getting firetrucks through the lights, but I'm not sure how it works--I think it's radio transponder based on what I read in the paper a year or so ago. It seems someone was able to get a duplicate transponder for his pickup to get through lights faster. People where complaining about the timing of the lights, switching to red at funny times. Police started watching and noticed, collectively, a white pickup going through the green direction every time. They got him and he got in real trouble, though the papers never followed up on what exactly happened to him.

[RalofTyr]

Traffic lights....ARG!!! I hate them.

I drive less than a mile to work. I encounter seven traffic lights. It takes me an average of ten minutes to get to work. On a bike, it takes me less because I can avoid those lights and duck into the residential streets.

So, the tore down a car dealership and put a hospital instead. On the small residential street, they put in a traffic signal. So know, instead of going down this major street, I have to wait because one car wants to turn left or a guy doesn't want to use the crosswalk a block away. The light is always red. Maybe it was green a few times for me over the few years I have been working there, but most of the time, I stop.

Then, they build a new parking structure for a hospital and put a light it. This is out of control. Sometimes I think the street light company is "Good" friends with a member of the city government.

This streets kinds of a bottle neck. You can't go anywhere but down Main Street. To the right is a residential area that has the streets blocked off. You can do it and bypass the lights and traffic, but you have to break the law. To the left, is nothing but hospitals. You have to drive down Main street.

On a street that gets a lot of flowing traffic, (it's not named Main Street for nothing), stop lights slow down the flow of traffic, some a few people can have the convince of turning left during rush hour.

[Trebuchet]

We have some system here for getting firetrucks through the lights, but I'm not sure how it works--I think it's radio transponder based on what I read in the paper a year or so ago. It seems someone was able to get a duplicate transponder for his pickup to get through lights faster. People where complaining about the timing of the lights, switching to red at funny times. Police started watching and noticed, collectively, a white pickup going through the green direction every time. They got him and he got in real trouble, though the papers never followed up on what exactly happened to him.

The ones around here use a white strobe light on the emergency vehicles, picked up by an optical sensor on the arm holding the light. They did a trial some years ago of putting the strobes on buses to improve transit efficiency but I don't think they ever did it for real.

The town where I went to college had a main drag with about 10 lights from end to end, one each block. They were apparently all run off a common timer. Westbound you could go all the way through without stopping. Eastbound you stopped at every....single....one....all....the....way...thro ugh. Most annoying.