Signal Spotlight: Optimizing Yellow Clearance Intervals: A Proven Safety Countermeasure

At a signalized intersection, the yellow clearance or yellow change interval is the length of time the yellow signal indication is displayed following a green signal indication. The yellow signal confirms to motorists that the green has ended and that a red will soon follow.

Since red-light running is a leading cause of severe crashes at signalized intersections, it is imperative that the yellow change interval be appropriately timed. Clearance intervals are a function of operating speed, the width of the intersection area, lengths of vehicles, and driver operational parameters such as reaction, braking, and decision-making time.

When a yellow change interval is too short, drivers may be unable to stop and unintentionally run the red light. If the interval is too long, drivers may treat the yellow as an extension of the green phase and intentionally run the red light.

Municipalities can improve signalized intersection safety and reduce red-light running by reviewing and updating their traffic signal timing policies and procedures concerning the yellow change interval on a regular basis. The MUTCD does not require specific yellow or red intervals but provides general guidance that the yellow change interval should be approximately 3 to 6 seconds.

Current CTDOT practice is to use the kinematic equation outlined in the ITE Traffic Engineering Handbook for the yellow change interval, and a modified version of the kinematic model for the red clearance interval. Yellow change intervals of three seconds to five seconds are typically used.

The yellow change interval for each phase is computed using the following formula:
Y = t+V/(2a+2Ag)

Where:
Y = Yellow change interval in seconds
t = reaction time (use 1 second)
V = 85% percentile approach speed in ft/sec or m/sec
a = deceleration rate of a vehicle (use 10 ft/sec2 or 3 m/sec2)
A = Acceleration due to gravity (32.2 ft/sec2 or 9.81 m/sec2)
g = percent grade in decimal form (+ for upgrade, – for downgrade)

While providing an appropriate yellow clearance interval improves safety, it should be noted that there is no additional benefit to making the interval longer than it needs to be. Longer clearance intervals increase lost time at the signal and will reduce the intersection’s capacity.

Resources:
Connecticut DOT Traffic Control Signal Design Manual
https://portal.ct.gov/DOT/Traffic-Engineering/Traffic-Control-Signal-Design-Manual

NCHRP Report 731 Guidelines for Timing Yellow and All-Red Clearance Intervals at Signalized Intersections
http://www.trb.org/Publications/Blurbs/168017.aspx

Guidelines for Determining Traffic Signal Change and Clearance Intervals: An ITE Recommended Practice
https://www.ite.org/technical-resources/topics/traffic-engineering/traffic-signal-change-and-clearance-intervals/

Recorded Webinar: Introducing ITE’s Guidelines for Determining Traffic Signal Change and Clearance Intervals
https://www.pathlms.com/ite/courses/16489

Making Intersections Safer: A Toolbox of Engineering Countermeasures to Reduce Red-Light Running
https://safety.fhwa.dot.gov/intersection/conventional/signalized/rlr/rlr_toolbox/

If you have traffic signal systems questions, please contact: Theresa Schwartz, P.E.,
P.T.O.E. – Traffic Signal Circuit Rider (860) 486-4535 or theresa.schwartz@uconn.edu.