4.7.4 Vehicle timing—Vehicle Clearance

The vehicle clearance interval consists of the yellow change and red clearance intervals. The recommended practice for computing the vehicle clearance interval is the ITE formula (reference 56, equation 11-4), given in equation 2 (to use with metric inputs, use 1 m = 0.3048 ft):

Change period equals perception-reaction time of the motorist plus the quotient of the speed of the approaching vehicle in feet per second divided by the sum of 2 times the comfortable deceleration rate of the vehicle in feet per second squared plus 64.4 times the grade of the intersection approach (percent) (positive for upgrade, negative for downgrade), plus the quotient of the sum of the width of the intersection from curb to curb in feet and the length of the vehicle in feet, divided by the speed of the approaching vehicle in feet per second. (U.S. Customary)

(2)

where:

CP = change period (s)

t = perception-reaction time of the motorist (s); typically 1

V = speed of the approaching vehicle (ft/s)

a = comfortable deceleration rate of the vehicle (ft/s2); typically 10 ft/s2

W = width of the intersection, curb to curb (ft)

L = length of vehicle (ft); typically 20 ft

g = grade of the intersection approach (%); positive for upgrade, negative for downgrade

For change periods longer than 5 s, a red clearance interval is typically used. Some agencies use the value of the third term as a red clearance interval. The MUTCD does not require specific yellow or red intervals but provides guidance that the yellow change interval should be approximately 3 s to 6 s and that the red clearance interval should not exceed 6 s (section 4D.10).(1) Note that because high-volume signalized intersections tend to be large and frequently on higher speed facilities, their clearance intervals are typically on the high end of the range. These longer clearance intervals increase loss time at the intersection and thus reduce capacity.

The topic of yellow and red clearance intervals has been much debated in the traffic engineering profession. At some locations, the yellow clearance interval is either too short or set improperly due to changes in posted speed limits or 85th-percentile speeds. This is a common problem and frequently causes drivers to brake hard or to run through the intersection during the red phase. Because not all States follow the same law with regard to what is defined as "being in the intersection on the red phase," local practice for defining the yellow interval varies considerably. For this reason, red light photo enforcement should not be used during the period of red clearance required by the ITE formula.

Current thought is that longer clearance intervals will cause drivers to enter the intersection later and will breed disrespect for the traffic signal. Wortman and Fox conducted a study that showed that the time of entry of vehicles into the intersection increased due to a longer yellow interval.(59) Additional research is needed to examine the effect of lengthening the yellow interval on driver behavior.