User:Hoskir/Revision Request 4246

902.23.7.2 Yellow Change and Red Clearance Intervals

Standard. A steady yellow signal indication shall be displayed following every CIRCULAR GREEN or GREEN ARROW signal indication and following every flashing YELLOW ARROW signal indication displayed as a part of a steady mode operation. This requirement shall not apply when a CIRCULAR GREEN or a flashing YELLOW ARROW signal indication is followed immediately by a GREEN ARROW signal indication.

The exclusive function of the yellow change interval shall be to warn traffic of an impending change in the right of way assignment.

Support. The yellow change interval and the red clearance interval (all indications displaying red) are required to prepare the intersection for the transfer of right of way. These intervals permit vehicles that are either within the intersection or so close to it that they cannot comfortably stop to clear the intersection, and to permit those vehicles that can come to a comfortable stop to do so.

Change Interval Timing Worksheet

Standard. The total time for the yellow change interval and the red clearance interval is the change period. All change periods shall be updated using the criteria below.

Support. To determine the appropriate yellow change interval and red clearance interval, ITE has developed a Kinematic Model – Formula shown below. The duration of change and clearance intervals, as well as the appropriateness of red clearance intervals, is a topic with no clear consensus. The following formula is developed based on a kinematic model of stopping behavior to determine the duration of the yellow and red indications and is in common use throughout the country.

Change Period:

$CP=t+\frac{V}{2a+64.4g}+\frac{W+L}{V}$

Guidance:

CP = nondilemma change period (yellow plus all red), seconds

t = perception-reaction time, recommended as 1.0 second

V = approach speed (recommended as 85^th or 15^th percentile speed), ft/sec

g = percent grade (positive for upgrade, negative for downgrade), expressed as a decimal

a = deceleration rate, recommended as 10 ft/sec²

W = width of intersection from stop line to end of far side crosswalk (if present), ft.

L = length of vehicle, recommended as 20 ft.

Support. A spot-speed study on an approach to an intersection will produce a range (or distribution) of speeds. The 85^th percentile speed is used to determine the yellow change and red clearance interval. A spot-speed study is performed during free flow conditions (typically during off-peak periods) to obtain the highest 85^th percentile speed to be used in the equation. It is important, however, to also consider slower traffic going through the intersection at the 15^th percentile speed. Low speeds and wide intersections or large left turn radii are a combination that can require a longer change period (yellow plus all-red). For this reason, it might be necessary to calculate the equation using both the 85^th and 15^th percentile speeds. Engineering judgment is used to determine whether the 85^th or 15^th percentile speed be used.

Using this equation for approaches with steep downgrades yields such long intervals that they appear unreasonable to drivers as well as the engineer. The remedy is not to ignore the physics of the situation when an unusually long phase change period results from a steep grade or from high approach speeds. The remedy might come from other devices such as warning signs, advanced detection or other countermeasures.

Districts can use the Change Interval Timing Worksheet to calculate the yellow change and red clearance intervals. Keep a printout of the spreadsheet with signal correspondence for future reference.

The sum of the first two terms in the formula is the yellow change interval and the red clearance interval is the last term. The purpose of the yellow change interval is to warn approaching traffic of the imminent change in the assignment of right of way. The first two terms include a reaction time, a deceleration element and approach speed, which are all necessary to determine a yellow change interval that will either allow the driver to come to a stop or proceed through the intersection. The red clearance interval is used to provide additional time following the yellow change interval before conflicting traffic is released. The last term in the formula includes the intersection width, length of vehicle and approach speed, which are all necessary in determining the intersection clearing time.

Standard. The yellow change interval shall not be less than three seconds.

Guidance. The yellow change interval should not exceed six seconds. The red clearance interval should not exceed six seconds.

Option. Based on engineering judgment, adjustments to the yellow change interval and red clearance interval may be made provided for each interval:

1) Yellow and all-red are not less than the calculated values using t = 1 second and a = 10 ft/sec² AND

2) Yellow does not go below the appropriate minimum of 3 seconds AND

3) Yellow and all-red do not go above their appropriate maximums of 6 seconds OR

4) Special conditions exist (e.g. Single Point Urban Interchange, Continuous Flow Interchange, Dual Lefts, etc.).

Standard. If the conditions are such that the district recommends using values that result in a yellow change interval less than what is calculated by the formula (using the values of t = 1 second and a = 10 ft/sec²) and/or a red clearance interval less than what was calculated by using the formula, then a high level of discussion/collaboration with Central Office Traffic Division shall be made.

Support. EPG 902.23.11 contains provisions regarding the display of steady CIRCULAR YELLOW signal indications to approaches from which drivers are allowed to make permissive left turns.

Standard. The durations of yellow change intervals and red clearance intervals shall be consistent with the determined values within the technical capabilities of the controller unit. The duration of a yellow change interval shall not vary on a cycle-by-cycle basis within the same signal timing plan.

The duration of a red clearance interval shall not be decreased or omitted on a cycle-by-cycle basis within the same signal timing plan.

Standard. Except for warning beacons mounted on advance warning signs on the approach to a signalized location (see EPG 903.6.28), signal displays that are intended to provide a “pre-yellow warning” interval, such as flashing green signal indications, vehicular countdown displays, or other similar displays, shall not be used at a signalized location.

Support. The use of signal displays (other than warning beacons mounted on advance warning signs) that convey a “pre-yellow warning” have been found by research to increase the frequency of crashes.