Signal Spotlight: Flashing Yellow Arrows Coming to an Intersection Near You

What is a flashing yellow arrow?

The figure on the left below shows a typical “doghouse” display used for permitted-protected left turn phasing in Connecticut. When the green ball is indicated, left turning vehicles must yield to oncoming traffic. The figure on the right shows the typical arrangement for a permitted-protected left turn using a flashing yellow arrow. When the left turn movement is permitted, the flashing yellow arrow is displayed.

Why are flashing yellow arrows used?

Engineers design traffic signals with permitted-protected left turn phasing because it can increase the traffic capacity at an intersection, but it can also create an issue known as the “yellow trap.” The yellow trap occurs when a left-turning driver enters the intersection with a green indication to wait for a gap in the oncoming traffic. When the signal turns yellow, the driver mistakenly believes oncoming traffic also has a yellow indication. The driver expects the opposing traffic to slow down and stop, so he or she makes the left turn and is t-boned by an oncoming vehicle.

A comprehensive research project was conducted in 2003 to study the use of various left turn displays for permitted-protected left turn phases and is discussed in NCHRP Report 493: Evaluation of Traffic Signal Displays for Protected/Permissive Left-Turn Control. According to the study findings, the flashing yellow arrow was the most effective and easily understood display for the permissive left turn movement.

A flashing yellow signal typically means “proceed with caution” and drivers intuitively interpret the flashing yellow arrow to mean the same. Those who misinterpret the flashing yellow arrow typically think it means “wait,” which is a safer failure than assuming one has the right of way.  

A followup study, summarized in the technical brief Safety Evaluation of Flashing Yellow Arrows at Signalized Intersections, FHWA-HRT-19-035 published by the Federal Highway Administration in 2020, concluded that replacing the traditional signal displays used for permissive or permissive-protected phasing with those including the flashing yellow arrow reduces left-turn crashes by 15-50 percent, depending on the type of intersection.

Are flashing yellow arrows required by the MUTCD?

The North Carolina NCUTCD issued interim approval for implementing the flashing yellow arrow in 2006, and Maryland, Florida, Oregon, and Arizona began testing in the field. It was then incorporated in Section 4D of the 2009 version of the MUTCD. While agencies are not required to provide flashing yellow arrow displays for permissive left turn movements, many states and local agencies have adopted use of the flashing yellow arrow as a standard practice. The Connecticut Department of Transportation will soon implement its first flashing yellow arrow displays as part of signal upgrades under state project number 0007-0250 on the Berlin Turnpike (Route 5) in Newington.

Do you have a traffic signal-related question? Would you like to share a signal project you’ve been working on? If so, email Theresa.schwartz@uconn.edu and your town may be featured in a future Signal Spotlight!