Plan Review: Clearance Intervals, Pedestrian Timing, and Detection on a 45 mph Arterial
You are reviewing the 60 percent signal design for a new intersection on a four-lane divided arterial posted at 45 mph (66 ft/s) at a two-lane minor street. Design inputs: stop line to the far edge of the conflicting through lane 60 ft; design vehicle 20 ft; perception-reaction 1.0 s; deceleration 10 ft/s²; level grade. The pedestrian crossing parallel to the arterial is 60 ft curb to curb. The plans show a 3.5 s yellow, 1.0 s red clearance, 7 s WALK, 18 s flashing DON'T WALK, and a 5 s minimum green. Detection is one 6 ft by 50 ft loop per arterial through lane, spanning 40 ft to 90 ft from the stop line, with a 2.0 s passage time; the exclusive left-turn lane has no detector. The left turn is shown as permitted with a circular green and a LEFT TURN YIELD ON GREEN sign.
1. Do the planned change and clearance intervals meet standard practice for this approach? Show the calculation and state what you would require.
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Yellow change interval
y = t + v / (2a + 2gG) = 1.0 + 66 / (2 x 10 + 0) = 1.0 + 3.3 = 4.3 s
The plan shows 3.5 s, which is 0.8 s short. At 45 mph a 3.5 s yellow creates a large dilemma zone and is a known red-light-running and liability exposure.
Red clearance interval
r = (W + L) / v = (60 + 20) / 66 = 1.21 s, so 1.2 to 1.3 s minimum.
The plan shows 1.0 s, about 0.2 to 0.3 s short.
Total should be about 5.5 s; the plan delivers 4.5 s, roughly 1.0 s deficient.
Recommendation: require yellow of at least 4.3 s and red clearance of at least 1.3 s (round up to 4.5 s and 1.5 s if the agency rounds to the next half second, applied consistently). Do not fix the deficiency by loading all of the added time into red clearance; the yellow is the driver's warning and must stand on its own. Confirm the grade assumption against the profile before finalizing, since a downgrade requires a longer yellow.
2. Verify the pedestrian timing and state the constraint it places on the vehicle green for the parallel movement.
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Flashing DON'T WALK
FDW = crossing distance / walking speed = 60 / 3.5 = 17.1 s
The planned 18 s satisfies this. A 3.5 ft/s design walking speed is appropriate unless the site serves mostly adults, in which case 4.0 ft/s may be used, but 3.5 is the conservative choice for a new design.
WALK
The planned 7 s meets the usual 7 s minimum, which allows a pedestrian at the curb to see the indication and step off.
Interaction with the vehicle green
Total pedestrian time is WALK + FDW = 25 s. The yellow and red clearance also serve part of the crossing, so the vehicle green must be at least 25 - (4.3 + 1.3) = 19.4 s whenever the pedestrian movement is recalled or the phase runs coordinated. The 5 s minimum green is acceptable only under actuated operation with no pedestrian recall. If the phase is coordinated or ped recall is used, the split must be about 20 s or pedestrians will be released into a green that terminates before they finish.
Also confirm that no right turn on red conflicts with the parallel crosswalk, and add a No Turn on Red restriction if the geometry puts turning vehicles through it.
3. Evaluate the arterial detection layout. Is it adequate for a 45 mph approach, and what would you change?
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Zone length versus passage time. If the design intent is that a vehicle approaching at design speed is continuously detected for the full passage time, the zone should be roughly v x t + L = 66 x 2.0 + 20 = 152 ft. A 50 ft zone gives a vehicle only 50/66 = 0.76 s of presence, so the 2.0 s passage setting is doing most of the work and the timing is sensitive to detector drop-out. Either lengthen the zone or state and document the design intent for a short presence loop.
Setback. With the near edge 40 ft from the stop line, the lead vehicle in a queue sits outside the zone. That is acceptable only if the call latches; verify the detector is configured for presence or locking pulse, not momentary pulse.
Width. A 6 ft wide loop in a 12 ft lane can miss motorcycles and high-clearance vehicles. Center it and confirm with the detector manufacturer's motorcycle requirement.
Left-turn lane. With permitted-only operation, the missing left-turn detector is not a defect, because left-turners are carried by the through phase. However, a permitted left from an exclusive lane across two lanes of 45 mph opposing traffic is a strong candidate for a flashing yellow arrow, and the reviewer should require a note that a detector be added if a protected phase is ever programmed.