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Flashing Yellow Arrow

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Traffic Engineering

Improving Safety for Left Turns at Traffic Signals

Approximately 25% of all crashes that occur at traffic signals in Massachusetts have been attributed to left turning vehicles. Due to the angle of impact of collision, a crash that involves a left turning vehicle is more likely to cause injuries or fatalities. One potential contributing factor to left turn crashes is Protected-Permissive Left-Turn phasing (PPLT). PPLT is designed to give left turning vehicles a Steady Green Left Arrow for a period of time when opposing traffic is stopped – the Protected portion of the phase – and is followed by a Steady Yellow Left Arrow. Following the Yellow Left Arrow, a Steady Green Circular indication is provided and left turning vehicles are required to yield to oncoming traffic in the Permissive portion of the phase. Most drivers recognize this type of phasing when they see the following traffic signal arrangement:

Protected-Permissive Signal Display
Image Source: MUTCD, 2009 Edition

Left turning crashes typically occur in this scenario during the Permissive phase; motor vehicle operators either forget or do not comprehend that a Steady Green Circular indication requires left-turns to yield to oncoming traffic. Over the past several years engineers have begun to add a supplemental sign, as shown in the figure, to reinforce this message, but these crashes continue to prevail.

Flashing Yellow Arrow Display

An alternative to the traditional PPLT signal face has been extensively tested and in 2009 was approved by the Federal Highway Administration. Now in operation throughout the country, this new signal face is referred to as a Flashing Yellow Arrow (FYA). Similar to the traditional PPLT phasing, the signal transitions from a Steady Green Left Arrow to a Steady Yellow Left Arrow. However, instead of providing a Steady Circular Green for the permissive phase, a Flashing Yellow Left Arrow is displayed to left turning vehicles when opposing traffic has a green:

Flashing Yellow Arrow Signal Display
Image Source: MUTCD, 2009 Edition 

What the Arrows Mean to Drivers

Steady Red Arrow. Drivers turning left must stop.

Steady Yellow Arrow. The traffic signal will be turning red. Drivers should not assume oncoming traffic has a red light, as their signal may still be green.

Flashing Yellow Arrow. Left turns are allowed, but drivers must first yield to oncoming traffic and pedestrians, then proceed with caution.

Steady Green Arrow. Drivers may turn left. Opposing traffic has a red light.

Safety Benefits

Extensive research and implementation of FYA for Protected-Permissive Left-Turn phasing has shown significant safety benefits. Studies have shown that, on average, left turn crashes are reduced by approximately 20%. Drivers typically find the FYA display to be self-explanatory and need no further instruction. However, drivers that do not understand the meaning of the Flashing Yellow Left Arrow tend to make a safe maneuver, i.e. stopping and yielding to oncoming traffic, whereas left turning drivers that do not understand the meaning of the traditional Steady Circular Green often do just the opposite.

MassDOT Retrofit Project

MassDOT is in the midst of a systemic retroft project that will convert all eligible traffic signals on the State Highway system that operate with PPLT phasing from the traditional display to the FYA. This work will take place at over 350 traffic signals, covering more than 140 cities and towns and every District in the Commonwealth. Construction on this project is expected to start in 2015 and last two to three years.


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