Residents Want a Signal at a Stop-Controlled T
A neighborhood association asks for a traffic signal at a T-intersection where the minor street is stop-controlled. The major street is a two-lane road posted 35 mph carrying about 900 vehicles per hour in the peak hour, both directions combined. The minor street carries about 90 vehicles per hour, including 55 left turns. In the last three years the intersection has had eight reported crashes, two with injuries, mostly angle crashes involving left-turning vehicles. A marked crosswalk crosses the major street near the T and is used by schoolchildren. The agency's signal budget is tight, and the request has political support.
1. What must be established before a signal can be justified, and why are the peak-hour volumes and crash counts alone not enough?
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A signal is not justified by a request, a crash count, or one peak hour. The engineer must first run the MUTCD warrant analysis using actual data:
- Turning-movement and pedestrian counts for at least the required hours (typically eight or more hours, plus a four-hour and peak-hour analysis), not a single peak number.
- Crash analysis by type and severity. Eight crashes sounds like a lot, but if they are rear-end crashes caused by the stop condition, a signal may make them worse. If they are angle crashes from left turns against high-speed through traffic, that pattern is more responsive to geometric or turn-lane fixes.
- Speed data - the 85th percentile, not just the posted 35 mph - because it drives both warrant checks and clearance intervals.
- Gap study and sight-distance check on the minor street.
A signal is a control device of last resort: it increases rear-end crashes and delay on the major street. Before recommending one, evaluate less restrictive fixes - a left-turn lane, improved stop-line location, signing, lighting, or a restricted-crossing treatment. Only then does the warrant analysis mean anything.
2. Assume a signal is installed. Compute a yellow change interval and red clearance interval for 35 mph, a 60-ft intersection width, and a 20-ft design vehicle. State your assumptions.
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Yellow change interval uses the kinematic equation:
Y = t + v / (2a + 2gG)
- Perception-reaction t = 1.0 s
- v = 35 mph = 51.3 ft/s
- a = 10 ft/s² comfortable deceleration
- G = 0 (level)
Y = 1.0 + 51.3/20 = 1.0 + 2.57 = 3.57 s, rounded to 3.5 s (or 4.0 s if the approach is downhill or the speed data show higher actual speeds). This falls within the usual 3-to-6 s range.
Red clearance interval clears the intersection:
R = (W + L) / v = (60 + 20) / 51.3 = 1.56 s, rounded to 1.5 to 2.0 s.
Assumptions to state: level grade, 1.0 s reaction, 10 ft/s² deceleration, design vehicle length 20 ft, and width measured from stop line to far edge of the conflicting travel path. If the 85th percentile speed is above 35 mph, recompute - using the posted speed instead of actual speed is a common and dangerous shortcut.
3. The crash pattern is left turns from the minor street. Explain the protected-versus-permissive decision and what you would recommend here.
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Permissive left turn: the left-turner receives a circular green or flashing yellow arrow and must yield to opposing through traffic. It reduces delay but depends on gap acceptance, and it fails where opposing volumes are high or sight distance is limited.
Protected left turn: an exclusive phase with a green arrow; opposing traffic is stopped. It eliminates the angle conflict but consumes time in the cycle and adds delay to the major street.
In this case: 55 left turns per hour against 900 vph of opposing traffic is a demanding gap-acceptance problem, and the crash history is angle crashes from those turns - exactly the conflict that permissive operation does not solve. Recommendation:
- Protected-only or protected/permissive with a flashing yellow arrow, with the permissive portion restricted to low-volume periods if the controller supports it.
- A left-turn lane so turning vehicles do not block the through lane.
- Verify pedestrian timing at the crosswalk, because a protected left phase can conflict with pedestrians if not sequenced correctly.
The trade-off is real: protected phasing will lengthen the cycle or take green from the major street. That cost should be quantified before adoption, not ignored.