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
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:
- The button closes a relay (an electromagnetic switch).
- The relay then locks itself on, to remember that the button has been pushed.
- This also turns on the light next to the pushbutton.
- Pressing the button again does absolutely NOTHING, because the relay is already locked on.
- 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.
- 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:
- The detector closes a relay.
- The relay STAYS closed until there are no cars left in the detection zone.
- 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.
- The detector does not count the cars that are in it.
- 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.
- Creeping does not affect this one bit
- Unless you creep out of the detection zone, which causes the signal to forget you.
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.
- 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.
- 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).
- 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.
- 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
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.
- 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.
You are going this way:
- 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.
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).
You are going this way:
- 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.
- 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.
- 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.
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:
- 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
- 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.
- An illegally parked car could be activating a detector.
- Construction barricades can cause traffic to cross the wrong detector.
- Jammed traffic that fails to clear will continuously activate a detector.
- 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
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
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...
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!
- COMMON TRAFFIC FALLACIES
- UNUSUAL TRAFFIC HAZARDS