Press Button to Cross

May 14, 2008 5:05 PM

How does the button on the pedestrian crossing lights work? Does it actually send some sort of signal to the traffic lights, or is it just a psychological thing? It seems rather random timing-wise.
posted by divabat to science & nature (21 comments total)

It totally depends on the lights and the specific equipment and how it's configured. In my city, there are some crossings where you will never get a cross signal unless the button has been pressed - usually where the crossing is simply across a road with no intersection - and others where pressing the button causes the don't-cross signal to light up, but the cross signal lights up regularly anyway.
posted by i_am_joe's_spleen at 5:12 PM on May 14, 2008

Depends where.
posted by phrontist at 5:12 PM on May 14, 2008

The wonderful Museum of Hoaxes has a little write-up on this.
posted by pedmands at 5:13 PM on May 14, 2008

If you don't push the button, the light shaped like a walking person won't be lit. At some intersections, an entire phase of the traffic signal cycle may be skipped if the button is not pressed.
posted by majick at 5:14 PM on May 14, 2008

There is one crosswalk here (near the high school) where as *soon* as you press the crosswalk button, the traffic lights turn yellow. It's wonderful.
posted by Lucinda at 5:15 PM on May 14, 2008

Some are hooked up and some are "placebo" buttons, simply put there to make you feel like you have some control when it's actually timed/computer controlled. It all depends on your city/intersection. Elevator "Close Door" button are notorious for being placebo buttons, as they are often not hooked up to anything.

For instance, in New York a report said: "More than 2,500 of the 3,250 walk buttons that still exist function essentially as mechanical placebos, city figures show"

In 2002, a reporter for the Honolulu Advertiser reported that 35% of Honolulu's walk buttons were placebos.

So the answer is...it depends.
posted by rooftop secrets at 5:16 PM on May 14, 2008

The ones I've used don't seem random at all. You push a button, the signal comes on when the traffic is moving in such a way that your walking is unlikely to interrupt it. When the button isn't pushed, the signal doesn't come on, so obviously it's actually sending a signal. I'm guessing you've either got odd signalling in your area or you just need to observe a couple cycles of light changes a little more closely.

Typically, if I'm walking on the sidewalk with traffic and want to continue in the same direction, the pedestrian signal will switch on when the green light for the car traffic does, which is usually after the left hand turn light has turned red for the traffic flowing opposite my direction of travel. If I'm walking on the side opposed to traffic, it's pretty much the same deal, I just can't see the green light for the cars from where I'm standing. Typically there's only one pedestrian signal per auto green light.

The only exception to this I've found is at lights that are controlled by sensors and don't get much car traffic, where the auto light can already be green by the time I get there. If there are no cars waiting to go different directions when I push the button, the pedestrian signal immediately changes.
posted by LionIndex at 5:17 PM on May 14, 2008

It's interesting seeing the other responses--in San Diego, if the pedestrian signal is automatic, there's no button. Not that they don't exist, but I haven't found a placebo button yet.
posted by LionIndex at 5:19 PM on May 14, 2008

Well here in Brisbane there doesn't seem to be any logic to the crossing - one time we had to wait about 10 minutes to cross, and this was just across the road without any intersections. There was another cross behind us and it went twice.

I personally found that if you tap on the button three times quick, the lights change in your favour very quickly, compared to if you just press it once. Works about 80% of the time. Does this happen elsewhere?
posted by divabat at 5:28 PM on May 14, 2008

Using the word "placebo" implies that they were installed for that purpose. Just so there's no confusion, the article states that they were at one time perfectly functional, but were at some point deactivated.
posted by gjc at 5:34 PM on May 14, 2008

I personally found that if you tap on the button three times quick, the lights change in your favour very quickly, compared to if you just press it once.

Sounds like superstition confirmation bias to me. AFAIK that's not how traffic signals work, and never has been. Although I did have a problems in my area with tapping the buttons too quickly for the relay to latch - there is no indication on some models whether or not the push has been registered, so I used to bang them hard, just in case. But as it turns out, the hard bang just bounced the contact off the plate so quickly that the relay didn't have current for long enough to latch, so I didn't get my light, and banged them harder, as they appeared to be broken, just making it worse.
I got so pissed off with the city because almost every button in my area didn't work. Then I learned to push them normally. Now the relay latches, all the buttons now work, and I've never had to wait out multiple traffic cycles since.

The new buttons that are replacing those old ones in my area have a solid-state latch (so there is no way you can push them too quickly) AND feedback that the push has been registered, so both problems are solved.

Unlike the crude old relay-logic systems, newer solid-state systems are more than smart enough to register multiple taps, but any traffic engineer who gave increased priority based on number of taps would probably be shot. :-)
posted by -harlequin- at 5:47 PM on May 14, 2008

I've found that some of the buttons are programmed according to what time of day it is. There's one intersection on my commute that will never give the Walk signal before 5am, no matter how many times I press the button or how many light cycles I stand through.

Another situation: The traffic light changing and the Walk signal running it's course and pressing the button brings another Walk sequence as there is enough time in the traffic cycle to run another pedestrian cycle. This happens almost exclusively at intersections where one direction of traffic has much more traffic than the other direction (and thus a much longer green light).

This highlights the difference between intersections that have traffic lights based on timers and others based on sensors in the road. There might be twenty cars waiting in the "generally not as busy" direction and no cars going in the "generally much busier" direction, but there's still enough time for me to get a Walk signal. The lights don't give a fuck how many people are waiting, they have to run their course sometimes.
posted by dogwalker at 5:49 PM on May 14, 2008

most of them (the ones that work anyway) work the same was as the sensing loops do. they just tell the controller that there's someone waiting. This can shorten the cycle for the traffic lanes that you are trying to cross or not, depending on setup.

some intersections might be set up to go only on timing during certain times of day, and therefore ignore any input.

so I guess the point I'm trying to make is that it depends on a couple things: the setup of the box controlling that particular intersection and whether or not all of the equipment is working correctly.
posted by ArgentCorvid at 6:32 PM on May 14, 2008

If it's not a placebo crossing, pressing the button multiple times won't help. Pressing it merely raises a signal to trip on the next available schedule.

(I used to live near a wonderful crossing that triggered as soon as you pressed the button. I wrote to the local roads department thanking them for it. Shortly afterward, it stopped cooperating, and the roads dept wrote back to apologize for leaving the crossing in testing mode, and had returned it to its intended mode of operation ...)
posted by scruss at 7:00 PM on May 14, 2008 [1 favorite has favorites]

At busy intersections, it's more a matter of the button not being pressed, than being pressed. If no-one is there to press the button, then the computer controlling the intersection knows it is able to skip the pedestrian cycle that time around. If someone does press the button, then the lights alter to include the pedestrian crossing cycle.

The fact that pedestrian-crossing is part of the cycle is important. Cars turning left get a turn. Cars turning right get a turn. Cars going straight ahead get a turn. Pedestrians going one way get a turn. Pedestrians going the other way get a turn. The button isn't there to demand change the lights right now dammit there's a WALKER waiting here! The button is there to let the traffic computer know whether or not to skip the pedestrian turn, whenever it may come along, in the next few minutes.

Some intersections, typically those with simpler traffic, or less frequent pedestrian traffic, may be programmed to switch the lights immediately once someone presses the button. The intersection may have been letting cars through continuously for the last ten minutes, skipping pedestrian cycles because no-one was there to press the button. Then, someone comes along and presses the button, and it immediately launches into the pedestrian cycle. You'll probably have to still wait a few more minutes for the next pedestrian cycle, however. I often find this on single-purpose pedestrian crossings, across a road, away from corners, where the traffic just goes through continuously without being stopped unless a pedestrian is actually there.
posted by Jimbob at 7:24 PM on May 14, 2008

It can depend on time of day as well as all the other things mentioned. Many lights are stuck in one direction all night long, unless a car trips the inductive loop sensor, or a pedestrian hits the button.

I've also noticed many intersections where the pedestrian button has no effect on the timing of car traffic, but does effect pedestrian signal. These intersections show stop on all pedestrian signals, all the time, unless a pedestrian button is pushed -- completely infuriating, because pedestrians should be allowed to go on a green light by default, and only restricted in very special circumstances.
posted by Chuckles at 7:41 PM on May 14, 2008

The one's by Winthrop University control if the 'Walk' symbol lights up or not. And if there's not a 'Walk' symbol, the cars going that same direction have a much shorter green light.
posted by theichibun at 7:58 PM on May 14, 2008

There's a Straight Dope write up on this as well, but it's a bit cursory and pretty much reiterates what's been said above.
posted by Dr.Enormous at 8:17 PM on May 14, 2008

Where I live, the bicyclist's button seems to change the signal much faster than the pedestrian's button. However, at some intersections, the walk signal thus does not engage. But you can still cross.
posted by acoutu at 8:35 PM on May 14, 2008

Here in inner south/east Melbourne, I've noticed several intersections that totally skip the walk phase if no one presses the button.

I love watching impatient people battering the crap out of the button as if hammering it repeatedly will get it to change faster. Never known it to make any difference at all.
posted by goshling at 12:11 AM on May 15, 2008

In brisbane, it depends on the intersection.

Some lights run a pedestrian cycle regardless; many only run a pedestrian cycle on demand. Basically, it's intersection-dependent. I'm unsure of what the figures are precisely.
posted by ysabet at 5:32 PM on May 18, 2008