Flashing Yellow Arrows
Introduction
The City of Flagstaff is making the change and moving towards flashing yellow arrows! The Transportation Engineering Section has added flashing yellow left-turn arrows in 2015 to the intersections along the Beulah Blvd. corridor and in 2021 to the intersections of Fourth St. and Soliere Ave. and Sparrow Ave. The intersections along Beulah Blvd. include McConnell Dr., Woodlands Village Blvd., and Lake Mary Rd. Along the Beulah Blvd. corridor, the City has also made the left-turn phases lagging in order to increase efficiency.
Flashing Yellow Arrows
Flashing yellow arrows replaced the green ball on permissive left-turn movements. Permissive left-turn movements occur when a turning vehicle yields to conflicting traffic and pedestrians, and then proceed, when safe, on a green ball. The flashing yellow arrows work as a yield for turning movements.
Drivers may turn on a flashing yellow arrow but must first yield to oncoming traffic and pedestrians and then proceed through the intersection with caution. Drivers have previously mistaken the green ball indication, above a left-turn lane, to mean "GO", when it actually means yield. The flashing yellow arrow is a more intuitive color and symbol of yield, over the traditional green ball.
Left-Turn Trap
According to the NCHRP Report 493, the flashing yellow arrow display offered the highest level of safety. Also, left-turn traps are completely eliminated when the flashing yellow left-turn arrow is tied to the opposing through green indication.
A left-turn trap occurs when a driver is making a left on a permissive phase and then becomes trapped in the intersection while the light turns yellow. The driver sees that the adjacent through traffic also receives a yellow light and assumes the same for the opposing through. The driver then proceeds through the intersection, possibly causing a T-bone type crash.
The flashing yellow arrow offers better driver recognition and eliminates left-turn traps, all resulting in a safer intersection. New signs were placed near the signals on the mast arms informing drivers how to handle the flashing yellow arrow signal head.
Lagging Left-Turns
Previously, the intersections along Beulah ran the left-turn movements as a leading phase. A leading left-turn phase means that the left-turn phases happen simultaneously, followed by the through movements. In 2015, the left-turns were changed to a lagging phase, which means that the through movements will start with a green indication, followed by the left-turns.
This change is expected to increase efficiency of the intersection by allowing many of the left-turn vehicles to turn on the permissive phase, and then clear out the remaining queue during the protected phase.
Crash Analysis
At the intersection of Beulah and McConnell, City Staff has compared 5-years’ worth of crash data from before the Flashing Yellow Arrows were installed to after the install. The results indicate that the yearly average left-turn crashes saw a 50% decrease. While all crash types saw a 16% decrease. Bicycle and pedestrian crashes were unchanged.