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Traffic Signals

Left Turn Signals


Left Turn Arrows

Drivers attempting to make a left turn are sometimes frustrated by the inability to turn due to the amount of oncoming traffic. Traffic engineers study these situations to determine if there is a need for a left turn arrow.

Left turn arrows are great if you're the one turning left, however they generally make the wait longer for everyone at the intersection. Traffic engineers must balance between adding a left turn arrow for the minor vehicle movement (left turners) and cause more overall delay for the other more major movements or allow left turners to use available gaps in opposing traffic as well as the yellow phase of the signal to make the left turn. Safety and accident history are always considered in this evaluation.

Protected Only Left Turn Signals

Protected Only left turn signals allow vehicles to proceed during the display of the green left turn arrow only. No permissive green ball is displayed, therefore vehicles may not move during gaps in the opposing through traffic.

The considerations for installation of Protected Only left signals include:

Protected Only left turn arrows that precede the through movements are referred to as leading left turns. Left turn arrows that follow the through movement are referred to as lagging left turns.

At some intersections, the protected only left turn may change from leading for both directions, to lagging for both directions, to leading for one direction and lagging for the other, during different times of the day. These adjustments allow for the maximum flow of through traffic in coordination with the other traffic signals along the roadway. This reduces the number of through vehicle stops and decreases the overall vehicle delay at the intersection.

Protected / Permissive Left Turn Signals

Protected / Permissive left turn signals will display a green left turn arrow allowing for a protected left turn movement, followed by a yellow left turn arrow indicating that the protected left turn is ending. A green permissive ball is then displayed which allows waiting left turn vehicles to proceed with the left turn during acceptable safe gaps in the opposing through traffic.

The considerations for installation of Protected / Permissive left turn signals include:

At some intersections using protected / permissive left turn signals, the left turn arrow may not be displayed all times of the day or may be displayed only when 3 or more vehicles are waiting in the left turn lane. At these times, left turn volume is low and most left turning vehicles can proceed either during gaps in the opposing through traffic or during the yellow vehicle clearance period. Left turn arrows are a definite convenience for the lower volume left turning traffic, but since the allotted time for the arrow must be subtracted from the much heavier opposing through traffic, overall intersection delay is increased. Safety and crash history are always considered when determining this type of operation.

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Traffic Signals