4.3 Yellow Change and Red Clearance Intervals

4.3 Yellow Change and Red Clearance Intervals

No other value in a signal controller carries the same legal weight. A yellow that is one second short becomes the central fact of every crash claim at that intersection for the following decade, and the malfunction management unit independently verifies the interval in hardware. A Level II field technician must be able to compute the values, cite the MUTCD boundaries, and verify what the intersection is actually displaying.

---

What the MUTCD Requires

The 11th Edition places the substantive requirements in Section 4F.17, with parallel guidance in the flashing-operation chapter:

• Standard: the duration of the steady yellow change interval shall be determined using engineering practices in accordance with Section 4F.17.
• Guidance: a yellow change interval should have a minimum duration of 3 seconds and a maximum duration of 6 seconds; the longer intervals should be reserved for approaches with higher speeds.
• Guidance: except when clearing a one-lane, two-way facility or an exceptionally wide intersection, a red clearance interval should have a duration not exceeding 6 seconds.
• Option: a steady red clearance interval may be used after the steady yellow change interval — it is permitted, not universally mandated.
• Standard: displays intended to provide a "pre-yellow warning," such as flashing green indications or vehicular countdown displays, shall not be used at a signalized location.

That last rule catches technicians who have seen vehicular countdown timers in other countries. They are prohibited in the United States.

---

The Kinematic Yellow Equation

The change period equation credited to Kell and Fullerton and carried in the FHWA Traffic Signal Timing Manual separates the problem into a yellow term and a red clearance term.

Yellow Change

$y = t + \frac{1.47 V}{2a + 2Gg}$

Worked example — 45 mph, level approach:

$y = 1.0 + \frac{1.47 \times 45}{2(10) + 2(32.2)(0)} = 1.0 + \frac{66.15}{20} = 1.0 + 3.31 = 4.3 \text{ seconds}$

Same approach on a 4% downgrade:

$y = 1.0 + \frac{66.15}{20 + 2(32.2)(-0.04)} = 1.0 + \frac{66.15}{20 - 2.58} = 1.0 + \frac{66.15}{17.42} = 1.0 + 3.80 = 4.8 \text{ seconds}$

The half-second the grade adds is not optional padding — it is the distance a vehicle needs because gravity is working against the brakes.

Reference Values on Level Grade

Note that 25 mph computes to about 2.8 seconds and is raised to the 3-second MUTCD minimum. The minimum is a floor, not a calculation result.

---

The Red Clearance Equation

$r = \frac{W + L}{1.47 V}$

where W is the distance from the stop line to the far side of the conflicting traffic path, L is the length of the design vehicle (commonly 20 feet for a passenger car), and V is the approach speed in mph.

Worked example — 30 mph approach, 70-foot crossing distance:

$r = \frac{70 + 20}{1.47 \times 30} = \frac{90}{44.1} = 2.0 \text{ seconds}$

Agency practice varies on what W measures. Some agencies clear to the far edge of the last conflicting lane; others clear to the far curb line or the far side of the far crosswalk. The choice can change the answer by a second, so a technician changes red clearance only from an approved timing sheet, never from a field estimate.

Restrictive vs. Permissive Yellow

States differ on how the two terms combine:

• Under the permissive yellow rule, the yellow interval equals the first term alone (but not less than 3.0 seconds) and a red clearance interval is added separately.
• Under the restrictive yellow rule, the yellow may be set equal to the sum of both terms.

Which rule an agency follows is a policy decision embedded in its timing standards. A technician's job is to apply the agency's standard consistently, not to select the rule.

---

Intervals That Are Never Shortened

This is the single most frequently tested principle in the subject, because every one of these situations tempts a programmer to save time:

The only interval a pre-emption sequence may truncate is the pedestrian clearance, and even that is an agency policy decision with its own analysis, not a technician's field call.

---

Field Verification After a Timing Change

A changed value in a database is not a verified interval. After any yellow or red clearance change:

1. Read the value back from the controller's timing screen, not from the laptop that pushed it.
2. Time the display in the field with a stopwatch or the controller's own interval log across several cycles, for every phase changed.
3. Confirm the monitor still accepts it. The monitor's clearance function faults when the yellow change plus red clearance is short — the specification window is no fault above 2.8 seconds, fault below 2.6 seconds, so a combined interval trimmed near that boundary will trip the cabinet.
4. Record it. Update the timing sheet in the cabinet, the agency's master timing record, and the work order. An undocumented timing change is indistinguishable from an unauthorized one when a crash is later litigated.