Willis Lamm's
Traffic Signal Collection

Early Protected Left Turn Variations

During the early days of traffic engineering and traffic signals, there were occasions on larger streets where formal designated left turn lanes and individual left turn signals were provided. However for years the majority of protected left turns were indicated by a fourth section or pendant head added to a regular traffic signal. This additional section simply provided a green arrow during the protected turn phase. These installations were made back in the days before modern standards that called for multiple signals to face each direction of oncoming traffic and often a single signal head or 4-way signal controlled up to three lanes of approaching traffic.

The idea of protected left turns addressed a problem that was created by the signals themselves. Particularly on narrow streets, the presence of the signals tended to cause traffic to move in blocks. This was not a bad thing except when vehicles stacked up waiting for clearance to make left turns. Early left turn pockets, where they existed at all, were often small and could only hold two or three vehicles. (Left turn pockets usually caused a reduction in greatly needed parking places since the through lanes had to be spread wider apart.) Vehicles wanting to turn left could stack up to the point that they blocked through traffic.

The first protected left turns simply provided a few seconds of a green left arrow display before the green light to let cars that had not made it through the last green light cycle clear the intersection before through traffic started moving. These short intervals would usually clear up any backlog of cars.

Also these signals appeared back in the days when few intersections had separate pedestrian signals. Therefore the most common display was for the green arrow to appear while the red light remained lit, even in instances where no conflicting oncoming left turns were permitted. Since pedestrians moved according to the vehicle traffic signal indications, traffic engineers wanted them to stay put and not step out into left turning traffic since doing so would present a risk as well as defeat the purpose of the protected left turn indication.

Generally speaking there were two common protected left turn displays in situations where separate signals and separate phasing was not provided for left turns.

In the first instance the signal provided a left arrow indication, then it changed to a green ball indication. Traffic had a protected left turn during the arrow, then vehicles still could make a left turn on the green ball indication provided that they yielded to oncoming traffic and pedestrians.

Protected left turn.
Left turns still permitted subject to yielding.
In some situations engineers did not want traffic to turn left after the protected interval. Such decisions may have been made to protect pedestrians or something might have obstructed motorists' ability to see oncoming traffic. In these instances it was not uncommon for the green ball to be replaced with double arrows, one pointing straight and the other right. In these instances a sign was typically placed advising motorists, "LEFT TURN ON LEFT ARROW ONLY."
Protected left turn.
Left turns no longer permitted.
Even two color signals were occasionally equipped with protected left turn indications, as in this reproduction of a 1950s vintage Daytona Beach signal demonstrates.
Protected left turn.
Left turns still permitted subject to yielding.
  Modern Developments


The widespread use of separate left turn signals and the "dog house" configuration combination left turn - through traffic signals have all but eliminated the old style simple green arrow protected left turn signal and only a handful of these old style signals still remain in service today. The dog houses provide an amber arrow to indicate when the protected phase is ending. The amber arrow allows greater programming options since traffic would have a clear indication when the protected left turn was ending. Thus the protected interval could occur whenever it was desired in the cycle.

Another recent development is the "yellow trap" signal that displays a flashing yellow arrow. This configuration was designed to prevent vehicles from being trapped in intersections during phase changes made possible by computerized controllers that didn't follow a specific phasing sequence. (A detailed explanation is presented on the Yellow Trap page.)

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