Signalized Intersections Appendix 2

Establishing a Clearance Interval

Methods for determining appropriate clearance interval length vary from agency to agency, and there is no consensus on which is the best method. The Institute for Transportation Engineers (ITE) recommends several procedures for determining clearance-interval duration in a 1994 informational report on signal change interval lengths. These methods include:

  1. A rule of thumb based on approach speed, such as this one presented in the ITE Traffic Engineering Handbook:

    • Yellow Change Time in seconds = operating speed in mph/10

    • Red clearance interval = 1 or 2 seconds

  2. Formulas for calculating interval lengths based on site, vehicle, and human factors characteristics, such as this equation (Traffic Engineering Handbook):

    • where:
      CP = non-dilemma change period (Change + Clearance Intervals)
      t = perception-reaction time (nominally 1 sec)
      V = approach speed, m/s [ft/s]
      g = percent grade (positive for upgrade, negative for downgrade)
      a = deceleration rate, m/s2 (typical 3.1 m/s2) [ft/s2 (typical 10 ft/s2)]
      W = width of intersection, curb to curb, m [ft]
      L = length of vehicle, m (typical 6 m) [ft (typical 20 ft)]

  3. A uniform clearance interval length

    • Various studies report that uniform value of 4 or 4.5 seconds for the yellow change interval length throughout a jurisdiction is sufficient to accommodate most approach speeds and deceleration rates. The ITE Report "Determining Vehicle Signal Change and Clearance Intervals" should be referenced for more discussion on this.

The Manual on Uniform Traffic Control Devices states that a yellow change interval should be approximately 3 to 6 seconds, and the Traffic Engineering Handbook states that a maximum of 5 seconds is typical for the yellow change interval. The red clearance interval, if used, should not exceed 6 seconds (MUTCD), but two seconds or less is typical (Traffic Engineering Handbook). The traffic laws in each state may vary from these suggested practices. ITE recommends that the yellow interval not exceed 5 seconds, so as not to encourage driver disrespect for signals.

References

Federal Highway Administration, Manual on Uniform Traffic Control Devices (MUTCD), Washington, D.C. 2003.

Institute of Transportation Engineers. Determining Vehicle Signal Change and Clearance Intervals. Washington, D.C. 1994.

Pline, J. L. Traffic Engineering Handbook. Institute of Transportation Engineers. 1999.