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OCR for page 65
HFG TUTORIALS Version 1.0
Tutorial 4: Determining Appropriate Clearance Intervals
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 Engi-
neers recommends several procedures for determining clearance interval duration in a 1994 in-
formational report (see ITE, 1994) 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 En-
gineering Handbook (Pline, 1999):
Yellow change time in seconds = operating speed in mi/h/10
Red clearance interval = 1 or 2 s
2. Formulas for calculating interval lengths based on site, vehicle, and human factors charac-
teristics, such as this equation (from Pline, 1999):
V W +L
CP = t + +
2a ± 64.4 g V
Where:
CP = non-dilemma change period (change + clearance intervals)
t = perception-reaction time (nominally 1 s)
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 s for the yellow change interval length throughout a jurisdiction is sufficient to accom-
modate most approach speeds and deceleration rates. Refer to Determining Vehicle Signal
Change and Clearance Intervals (ITE, 1994) for more discussion on this.
The Manual on Uniform Traffic Control Devices (FHWA, 2007) states that a yellow change in-
terval should be approximately 3 to 6 s, and the Traffic Engineering Handbook (Pline, 1999) states
that a maximum of 5 s is typical for the yellow change interval. The red clearance interval, if used,
should not exceed 6 s (FHWA, 2007), but 2 s or less is typical (Pline, 1999). The traffic laws in
each state may vary from these suggested practices. ITE recommends that the yellow interval not
exceed 5 s, so as not to encourage driver disrespect for signals.
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