Protected Left Turn Arrows
Protected left turn signals include a red arrow along with the normal green and amber arrow. They allow left turning drivers to proceed only on the green arrow. This turning method is very inefficient and generally not used in Tucson. Adding inefficiencies to signal timing reduces overall capacity and increases congestion. With increased congestion comes the potential for an increase in certain types of accidents.
Permitted/Protected Left Turn Arrows
This is the most common turning method used in Tucson at locations having left turn arrows. During the permitted "green ball" part of the cycle, vehicles are allowed to turn when there are adequate gaps in opposing traffic.
This type of left turn phasing is designed to help minimize delay by eliminating the need for the red arrow and allowing vehicles to turn on the green ball after opposing traffic has cleared. By not having the red arrow, motorists do not have to sit and wait to turn left even when there is no opposing traffic, a situation that often occurs during periods of low traffic volumes. The signal still provides a green left turn arrow for those not able to turn during the permitted phase.
Leading vs. Lagging Arrows
The City exclusively used leading left turn arrows until the early 1980's. In 1984 we experimented with lagging left turn arrows on 22nd Street between Park and Wilmot. Studies done before and after the change comparing the two methods showed that during much of the day lagging arrows were superior. They allowed improved progression, reduced delay and lowered overall intersection accidents.
Leading arrows do have some advantage over lagging arrows in certain circumstances. That's why there are still a few in the city. They generally are used at intersections where one direction of traffic requires the installation of an arrow while the opposing direction does not.