Signal Yellow Time Design

The termination of a green light at a traffic signal is followed by what engineers refer to as a “change interval. The change interval consists of two separate components:
  1. Yellow light
  2. “All-Red” light
Each of these clearance elements fulfills a different purpose. The primary purpose of the yellow light is simply to alert traffic that the green has terminated and that they need to stop if possible or clear the intersection.  Traffic signal design requirements stipulate that the duration of a yellow light should be at least three seconds and no greater than six seconds. 

The profession uses a standardized equation to calculate appropriate yellow time values for a given roadway speed. The equation allows engineers to select an assumed deceleration rate from a range of values, resulting in a range (not a fixed value) of possible yellow times for a given roadway speed. The majority of Plano’s thoroughfares are designed and posted for speeds of 40 to 45 MPH. The equations for yellow times result in a calculated range of 3 - 3.85 seconds for 40 mph and 3.2 to 4.2 seconds for 45 mph. Plano uses values of 4.0 and 4.5 seconds for these two situations respectively. These values are shown in the chart below. 


The second element of the change interval, the “All Red” time, has the primary purpose of clearing vehicles (which entered an intersection during the yellow) out of the intersection before the start of the side street green movement. These values are also established by standardized equations, which produce values between .7 seconds and 1.8 seconds. Plano uses values that range between 1.0 - 2.2 seconds, depending on intersection width and speed limits.

To further enhance driver safety Plano’s traffic signal controllers use “dual safeguard software” that provides reliable and adequate yellow times. The software has a non-violatable minimum yellow time of three seconds which is used if data corruption or data loss ever results in programmed values of less than three seconds.  If this safeguard were to ever fail, the signal system has a conflict monitor which trips the signal into all-way flash mode if a yellow time below the fail-safe minimum is detected.

Over the course of time the traffic engineering profession has debated the merits of using longer than calculated yellow times as a way of enhancing safety at intersections. Each time this debate gets rebuffed by research that shows the detrimental effect of lengthening yellow time. The research determined that motorists quickly perceive the increased yellow time and then they begin to “run” the light. Additionally, the greater the duration of yellow-time, the lower the amount of green time that can be assigned to other congested movements. While this lowered amount of green time appears minor (when compared against the total signal cycle length), it adds up over the course of an hour and has the impact of increasing traffic congestion and delay. For example, if each of the east/west and the north/south movements at an intersection received a 1 second increase in yellow time, and the intersection operated at a 2 minute cycle length, the loss of green time (that could be assigned to other congested traffic movements) over an hour would be 60 seconds. At intersections operating at near capacity (as during morning and evening commute periods), this “lost time” could be better applied in reducing the level of traffic congestion on other movements at the intersection.

For these reasons, safety and congestion, the Plano Transportation Division discourages the use of artificially long yellow times.