Why creeping up won't bring the green light sooner

You see it time and time again. A driver sitting at a red light creeps up toward the intersection a few feet every 15 seconds or so. Most of them, when asked why they do it, say it makes the light change faster.

These people must be the ones who also believe that pushing the elevator button repeatedly makes the elevator come faster too.

Neither the traffic light nor the elevator will service you sooner if you repeatedly move your car or push the button, because they don't work that way. They both have to finish what they are doing before they can get around to you. And once they finish the tasks at hand, they will come to you, without any further prodding. Let's look at how they really work:

Both traffic lights and elevators work in almost the same way. A series of detectors gives information to the controller, which then decides what to do, based on the information it has. But in all cases, the purpose of the detector is to allow the system to skip unused points of demand, or shorten the time on a location of low demand. (Also remember that many traffic lights are the cheaper kind, which have no detection equipment at all.)

When you push the button on the elevator:

  1. The button closes a relay (an electromagnetic switch).
  2. The relay then locks itself on, to remember that the button has been pushed.
  3. This also turns on the light next to the pushbutton.
  4. Pressing the button again does absolutely NOTHING, because the relay is already locked on.
  5. When the elevator comes to your floor, it hits a switch that releases the lock on the relay, allowing it to turn itself off again.
  6. Pressing the button while the elevator is present opens the door again, and recycles the wait timer, but that is the only time that pressing the button again does anything.
When you drive your car into a detection zone:
  1. The detector closes a relay.
  2. The relay STAYS closed until there are no cars left in the detection zone.
  3. Moving your car has no effect on the relay
    • Unless you drive forward so far that you have left the detection zone.
    • When ALL cars leave the detection zone, it shuts off the relay and forgets you were there.
  4. The detector does not count the cars that are in it.
  5. It simply gives a signal to the traffic signal controller that:
    • There ARE cars in the detection zone, or
    • There ARE NO cars in the detection zone.
  6. Creeping does not affect this one bit
    • Unless you creep out of the detection zone, which causes the signal to forget you.
Red clearance

The signal usually does not lock in the detection, so it does not bring the green to an empty approach after cars have turned on red and have left the approach.

Signals have an optional locking circuit like the elevator, which locks in the detection once it occurs. This is enabled where the detector cannot be placed at the stop line for some reason. High speeds are usually the reason.

So why does the traffic light controller not give you a green as soon as you get there?

Most likely, the answer is because the controller is busy doing something else. Like an elevator with only one car, most controllers can do only one thing at a time. Some controllers, called DUAL-RING TIMERS, are like elevators with two cars, and can do two things at a time. But in any case, you do not have a green because the controller is timing out an interval for traffic that is moving in conflict with the movement you want to make.

Think about it. The elevator is not going to slam the door on someone's arm on another floor so you can be served faster. Likewise, the traffic light is not going to create a dangerous situation by shortening a timed interval, just so you can have a green light earlier.

Here is a list of the usual reasons a traffic light has not yet changed your light to green when you think it should have. Usually it is because the signal controller is busy doing something else:

  • Is the controller timing out a pedestrian WALK interval? This interval is usually a fixed period of time, usually set according to the time needed to start all of the pedestrians crossing the street. This gives people enough time to start their crossing.
Pedestrian signal walk
  • Is the controller timing out a pedestrian flashing DON'T WALK interval? This interval is also usually a fixed period of time, usually set according to the length of the crosswalk. This gives any people who are still crossing enough time to get safely across. The pushbutton provided for pedestrians just asks for the pedestrian periods. It cannot change their timing once they start. They must complete their assigned times before the light can change.
Pedestrian signal dont-walk
  • Are vehicles still actuating a detector on the approach that DOES have a green light? The signal will not come to you until it is finished emptying the approach with the green (or until a maximum period, specified by a setting on the controller, is exceeded).
Moving traffic
  • Is the signal controller timing out a fixed-time green interval? Since there are no detectors on an approach with a fixed green, the signal controller is told by an internal setting how long to display a green for that street. It cannot change the light until that interval is over.
Timing devices
  • Does your approach even have a detector? Or does it have a fixed green interval? Many signals in cities have actuated turns, but have fixed ending points for the straight-ahead greens. For that matter, is the entire signal a fixed-time signal?
  • Is the signal controller waiting for the synchronization pulse from the master controller for the area? This is used to synchronize the traffic lights at many intersections together. This allows engineers to make traffic flow faster through the entire area, and reduce the number of stops made by drivers on major roads. Until the intersection controller receives the pulse that allows it to leave the major street green, it will not change the signals.
Master control link
  • Are you the only driver wanting to make a particular movement at an intersection? If yours is the only car waiting, you might have just missed the synchronization pulse (or the end of the green lights timed just before yours) by a few seconds. This allows another green that normally follows yours to go in place of your green. Then you have to wait for the next cycle before you get your green light. The signal cannot change the order of the green lights, but it can skip some of them, if there are no cars waiting to use them.
=Dual lead-lag left signal sequence

You are going this way:

Direction to right
  • If you are the only car waiting at an approach, had the signal already changed to yellow for the signals that are normally green just before your light normally turns green? Did this happen before your car reached the detector? If so, your green will be skipped by the signal, until the next cycle. The signal controller must already know what light will turn green next at the time the previous light turns yellow. Since no cars are yet found on your approach, your green light will be skipped. Notice in the diagram how traffic on the other street receives the green twice, before you receive yours (the signal cycle is greatly speeded up in these diagrams).
  • Is the master controller for the area changing from one area timing pattern to another? This can cause unusually long waits at red lights for side street traffic, as the local signal controller resynchronizes itself to the master, usually during the major street's green period. Once the master controller comes around again to the point where the signal should change, the light will change. This change in timing usually occurs about an hour before rush hour begins, and an hour after rush hour ends.
Synchronized signals

A typical timing pattern: The groups of black cars don't have to stop.

  • Are you really next? The signal displays green indications for different movements in a predetermined order. A single-ring controller can handle 2 to 6 sets of lights for different movements, and a dual-ring controller can handle up to 8, or even 12. But, other than skipping approaches without any cars, signal controllers can't change the order of the green lights. The order is programmed when the engineer designs the signal installation for the intersection. The diagram (right) is of the normal phase order for a dual-ring "quad left" (left turns for all 4 directions) signal with "leading turns" (left turns go before opposing straight ahead movements) and "phase skip" (which leaves out green lights for approaches with no traffic).
Quad lead left signal sequence

You are going this way:

Direction up
  • Remember that, in any dual-ring "quad left turn" sequence, only 2 pairs out of the 8 pairs of greens possible under full load can give you a green light. The other 6 pairs can't. The same moving image (right) shows all 8 pairs. See if you can find all 8.
  • Did one left turn run out of cars before the other one did? If so, one straight-ahead movement gets a green before the other. See if you can find the cases where this happens in the diagram.
  • Are you expecting the correct order of the green lights? Not all signals use the leading left turns. The first intersection diagram (above) shows a pattern where some left turns go first, and some go last. So the expected pattern of lights does not happen at these intersections.
Signal facing away from you
  • Likewise, are signal faces you cannot see turning green, instead of yours? Here is an example of this, where traffic going the opposite direction on your street has green lights.
  • Has one timing ring timed out, while the other has not? In this case, the approach you are watching for cross traffic to clear out on has cleared, but the other approach the dual-ring controller allowed to go at the same time has not. The signal can't move the green from one street to the other until both rings have timed out their intervals.
Two rings at once
  • Are the yellow or red clearance intervals for another movement still timing out? If so, it's almost your time (assuming you are next). But nothing you do can shorten a clearance period.
Yellow clearance
Red clearance

In fact, the ONLY time you should see the light change within a few seconds of when you enter the detection zone is if the controller has already timed out all of its intervals (except yellow and red clearance) and is "resting", waiting for an actuation to occur.

There are also some strange and unusual circumstances that can delay the light more than usual:

  1. A detector may be broken. When a detector breaks, it usually "fails safe", calling the signal all the time, rather than failing to call the signal at all. This makes the timing for that approach extend the green all the way to the maximum setting.
  2. A detector may be broken, and the repairman has set a fixed-green interval on one phase, until the repairs can be finished. Often this repair requires the street to be dug up, so the authorities might not get around to fixing it, until the money and a low-traffic period appear together.
  3. An illegally parked car could be activating a detector.
  4. Construction barricades can cause traffic to cross the wrong detector.
  5. Jammed traffic that fails to clear will continuously activate a detector.
  6. Weather conditions can actuate a detector falsely. Usually a loop detector is falsely activated by rain or snow for a few minutes, until it can retune itself (and it might fail to detect a motorcycle just after a downpour ends). A television detector is often fooled by window reflections and shadows, but usually for only one traffic cycle at a time. A weak detection is the only case where motion might help trigger the detector. It is a rare case, not a typical one.

When the traffic light has timed out all of the intervals needed for other flows of traffic, it will then come to yours. Always! And you can't do anything to make it come to you sooner.

Some signals do count cars...

...but they use the count for extending green, not for shortening red.

There are some traffic signals that count cars approaching on the red, but this feature is usually restricted to high-speed approaches, where it is not safe to put the detector at the stop line.

Cars approaching at high speed must be detected and handled before they get near to the intersection, so the detectors are placed far from the stop line.

But even these signals do not usually do anything with the count that brings a green light any faster. Most of them use the count for nothing other than to figure out how long to hold the initial green light, in order to get the stopped lines of cars (between the detector and the stop line) through the intersection. Any cars that have not yet been detected when the light turns green will then extend this initial green interval once the line starts moving, by actuating the detector. Very few signals are left that can shorten the gap interval on the street with the green in order to bring a green to another approach earlier, and none of those can have left turn arrows.

You can prevent a green light from coming... creeping.

If you drive up beyond the stop line, you will usually drive out of the detection zone. With no detection signal, the signal thinks you turned right on a red light, and forgets that you were ever there. Once this happens, another car must come up behind you before the signal will change. OOOOPS!