How traffic signals work: Some terms


Traffic engineers have developed terminology to aid in communication

The ‘approach’ is the set of lanes that are coming into a particular intersection, from a given direction. So, there might be an eastbound approach of traffic that is moving in the easterly direction.  A ‘cycle’ is the complete amount of time that it takes to go from a red light to a red light. We think of it as a clock. ‘Cycle length’  is the amount of time it takes to complete a cycle, measured in seconds.  A ‘phase’ is part of a cycle that is allocated to a particular movement, which receives the right-of-way. There might be multiple movements that receive right-of-way simultaneously, as long as they are not conflicting.  The northbound and southbound movements might both get the green light at the same time. They’re on the same phase, and they’re not conflicting.

“What do you do with right (left – in right hand drive countries) turns?” Do you give them a separate phase? Or do they share the phase? If they share the phase, then it becomes more complicated. There are many possible patterns, from which traffic engineers aim to select the ‘optimal,’ but that depends on the objectives and conditions.

There are ‘movements’. ‘Protected’ movements have right-of-way, and don’t have to yield to any other conflicting movements,  opposing vehicles, or to pedestrians. The ‘permitted’ movement is most common for right turns (in left-hand drive countries like Australia), for instance when making a turn without a green arrow, the driver has the permission to make that movement, so long as it is safe, but is not protected by a red light in the conflicting direction. Left turns are also permitted if there are no conflicting pedestrians or bicyclists.

‘Lost time’ occurs at the start of the phase because the first car has to accelerate from a dead stop, which takes some time: drivers first perceive the green signal, then check to make sure the intersection is clear, and then accelerate from a stop. So the speed at which that first (and second, and third) car goes through the intersection is slower than subsequent vehicles. There is also lost time at the end of the phase as some drivers are reluctant to go through on an amber (yellow) signal. There is also an ‘all red’ phase in some places to make sure the intersection is fully cleared of vehicles and pedestrians.