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.
|