Merge Taper Design for a Crest-Limited Lane Closure
A contractor will resurface the eastbound lane of a two-lane, two-way urban arterial posted at 45 mph. The work requires closing one 12-ft lane from 7:00 a.m. to 5:00 p.m. for four consecutive days. The remaining lane is 12 ft wide, daily volume is about 3,200 vehicles with a peak-hour volume of 340 and 6% trucks. The lane closure begins just beyond the crest of a vertical curve, and the nearest intersecting street is 180 ft upstream of where the contractor wants to start the taper. The contractor's plan set uses the standard MUTCD taper-length equation and the advance warning spacing table, but the submitted drawing shows a single ROAD WORK AHEAD sign on the shoulder 100 ft ahead of the taper, one flagger standing at the taper's upstream end, and no buffer space between the taper and the work area. You are the inspector who must approve or reject the plan before the crew mobilizes at 6:30 a.m.
1. Compute the minimum merge taper length and state the buffer space requirement. Show which equation applies and why, and identify the two most common errors made at this speed.
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Equation choice. MUTCD taper length depends on speed: L = W x S for speeds of 40 mph or less, and L = W x S squared / 60 for speeds of 45 mph or more. At a 45 mph posted speed the second equation is mandatory.
L = 12 ft x (45 x 45) / 60 = 12 x 2,025 / 60 = 405 ft.
W is the lateral offset being closed, which is the full 12-ft lane, not the shoulder. The taper is measured from the first channelizing device to the device that fully closes the lane.
Two common errors.
- Using the low-speed equation: 12 x 45 = 540 ft. This is conservative but often will not fit between the crest and the work area, pushing crews to shorten the taper on site without documentation.
- Using a rule of thumb such as 100 ft per closed lane, which yields 100 ft here and provides roughly one-quarter of the required length.
Buffer space. Between the end of the taper and the work area, longitudinal buffer space should equal the stopping sight distance for 45 mph, which is 360 ft. Taper plus buffer is therefore 765 ft. If that cannot fit, you do not shorten both proportionally; you relocate the work area, close the road with a detour, or add positive protection.
2. The taper starts just beyond a vertical crest. Explain how this changes your decision, and whether meeting the 405-ft taper length resolves the problem.
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It does not resolve it. Taper length is a geometric requirement for the merging maneuver; it says nothing about whether a driver can see the closure in time to begin that maneuver.
Stopping sight distance at 45 mph is 360 ft. If the crest hides the first channelizing device until a driver is closer than that, the driver is already committed to a 45-mph approach before the taper becomes visible. A compliant 405-ft taper placed on the far side of a crest simply produces a longer queue of surprised drivers.
Judgment calls available, in order of preference:
- Move the closure downstream past the point where the crest no longer restricts sight distance, even if that means longer haul or a short detour for residents.
- Add upstream warning that is visible over the crest, such as a portable changeable message sign or an additional warning sign placed before the crest, plus a flagger positioned ahead of the crest with clear escape room.
- Use positive protection or a full closure if neither works and volumes justify it.
Document the sight-distance check with the station of the crest and the measured available distance. Do not sign off on the basis that the drawing shows the correct equation result.
3. Evaluate the proposed flagger position and sign spacing. List the corrections you would require before mobilization.
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Flagger. A flagger standing at the upstream end of the taper is standing in the area a merging driver is most likely to occupy, and is invisible to anyone still behind the crest. Flagger stations should be located where approaching road users have adequate sight distance to react, and the flagger should stand off the traveled way with a clear escape path. With this geometry the flagger either moves upstream of the crest or the closure gets a different control strategy.
Sign spacing. A single warning sign 100 ft ahead of the taper is far short of the distance a 45-mph driver needs. The advance warning sign distances are taken from the MUTCD table for the posted speed and for urban versus rural conditions; do not invent a value. The intervening distance is what lets drivers decelerate and merge in sequence rather than all at once.
Corrections to require:
- Recalculate the taper to 405 ft and show the buffer at 360 ft on the plan.
- Add the full advance warning sign sequence at table distances, with the first sign visible from before the crest.
- Relocate the flagger station out of the taper and out of the traveled way, and confirm two-way radio communication if a pilot car is used.
- Add a downstream taper and verify device spacing at half the speed in mph in feet for the tangent section.
- Provide a night inspection if any work extends past dusk, since the sheeting and device reflectivity now matters.