## Signalized Intersections Appendix 2## Establishing a Clearance IntervalMethods 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: 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 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)]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 ## ReferencesFederal Highway Administration, Institute of Transportation Engineers. Pline, J. L. |