A problem that limits the speed of a road with traffic signals on it is stopping high speed traffic safely. The standard method of displaying a yellow light for six seconds does not work very well for the following reasons:

  1. Many large vehicles take longer than six seconds to stop or clear.
  2. Trucks must slow to 40 mi/hr for each signal (expensive), or run the red clearance and risk a ticket or a crash.
  3. The "dilemma zone" exists above 40 mi/hr. Any vehicle in it when the signal turns yellow cannot stop safely, and cannot clear the intersection before the red.
  4. Heavy vehicles, the most dangerous in an accident, are also the most likely to be involved in this type of accident.
  5. The dilemma zone complicates things when a driver decides to stop in front of a driver who decided to clear.
  6. Drivers at high speed, seeing a green signal, may fail to slow in time for vehicles starting from the stop line.

Several methods have been tried and found wanting:


2-aspect distant
3-aspect distant
Free flow
Red ahead
flashing yellow, "RED" lit
Slow traffic
Flashing yellow
Flashing orange

Here is one way to do it that requires six things to work. They are:

  1. Adoption by the Manual of Uniform Traffic Control Devices (for just the last two displays).
  2. Public education describing how the system works.
  3. Proper timing and placement by the engineers.
  4. All vehicles keep adequate following distance (the hard part).
  5. All left turns from the highway must be only protected.
  6. Separated turning lanes that bypass straight ahead detectors and distant signals.

The terms come from railroad usage:

The method here uses the following equipment:

  1. A detector placed 5 seconds (at the speed limit) back from the stop line.
  2. The special "distant" signal placed over each straight ahead lane, 50 feet closer to the stop line than the high-speed detector.
  3. A presence detection zone, covering the last 50 feet before the stop line.
  4. A controller capable of timing distant intervals and displaying them.

Here is the sequence of operation (showing the two-aspect signal method):

Stage 1

The home signal is green. The distant signal displays the free flow aspect, and high-speed traffic is running through the intersection. The controller is timing extension periods from the high-speed detector actuations to determine when to yield (relinquish the green light) to cross traffic or left turns. Left turns are not allowed to turn across traffic, so all left turns from the high-speed must have only protected phasing, must be diverted away from the intersection, or must be prohibited. If no calls are present from the side road, the signal controller will rest (idle) in this position.

The presence detector at the stop line is ignored for this signal timing. Its output is useful for timing only when cars have been stopped for the light. If the stop-line detector detects vehicles for a length of time, the controller assumes that congestion has occurred, and changes the distant to the slow traffic aspect (stage 5).

distant blue, home green

Free-flow distant signal
Green home signal

Stage 2

The controller decides to yield, either from finding simultaneous gaps in both directions, or by reaching a maximum time for waiting traffic set in the controller. It changes the distant signal to the red ahead aspect, ignores the detectors on the approach, and times out the distant interval. The distant interval is usually equal to the passage time from the high-speed detector to the stop line.

Drivers who have already passed the distant signal continue on, because they never saw the distant signal change. Drivers who saw the change prepare to stop, or accelerate to clear the intersection. As long as they are not too far away, it does not matter which they decide to do.

distant orange, home green

Red ahead distant signal
green home signal

Stage 3

The distant interval has timed out. The controller changes the green home signal to yellow. A 4-second yellow is timed. This gives drivers 9 seconds to clear the intersection or stop safely. Both detectors on each approach now start looking for traffic approaching the intersection on the main road. The presence detector at the stop line performs one of its functions here. It detects any cars that enter the highway between the high-speed detector and the stop line. It also continuously detects vehicles waiting at the stop line until they leave. If all of the cars waiting make right turns on red, the controller drops the call. Any car detected by the high-speed detector is remembered until the car has had a chance to get to the stop line detector, then it is forgotten.

distant orange, home yellow

Red ahead distant signal
yellow home signal

Stage 4

The home signal changes to red. All traffic has either stopped or cleared. After a safety all-red interval, the controller services the side road vehicles or left turns that are present. The detectors are still looking for any actuation. If one is received, the controller remembers it, to tell side street timing to use a maximum. If two high-speed roads cross at the intersection, the other highway can have a distant signal system too. In that case, the distant signal on each highway goes through the same sequence, but at different times for each highway. Also, if one or more left turn signals are present, each straight ahead phase on the same road can start or end at different times.

distant orange, home red

Red ahead distant signal
red home signal

Stage 5

The controller has now given the green signal to waiting traffic. Both detectors are active. As long as the presence detector detects vehicles continuously, the controller assumes that traffic waiting at the stop line has not cleared, and keeps the distant signal in the slow traffic aspect to warn approaching traffic. This period may not be the same in both directions, because traffic may clear out earlier on one leg. Also, if leading left turn phases are used, they might not end at the same time, causing the through green lights to start at different times. (Remember, Left turns either must have only protected turn phases, must be diverted, or must be prohibited.)

distant orange, home green

Slow traffic distant signal
green home signal

Stage 6

The presence detector at the stop line gaps out (detects no vehicles in it for a short period). This indicates that the starting vehicles have cleared the intersection. The distant changes to the free flow aspect, and high-speed traffic runs through the intersection again. The controller starts ignoring the presence detector at the stop line for end-of-phase timing. Again, the stop line detector is used to detect congestion, turning the distant to the slow traffic aspect (stage 5) if there is a back-up, and the cycle continues from there.

If the congestion never clears up, no free flow aspect appears. The controller advances directly from stage 5 to stage 2.

distant blue, green home

Free flow distant signal
green home signal