Restriping a Four-Lane Arterial After Overlay
An urban four-lane arterial (40 mph, 12-ft lanes, no shoulder, curb and gutter) is milled and overlaid. The city restripes and adds a left-turn lane at a signalized intersection. The plan shows a 200-ft bay taper into the 12-ft left-turn lane, a 24-in stop line set 2 ft ahead of a 10-ft transverse-line crosswalk, and skip lines of 10-ft marks with 30-ft gaps, with 4-in edge lines. The contractor applies 90-mil thermoplastic on a morning when pavement temperature is 42°F and rising and the surface is damp with dew. By afternoon, marking on the milled edge is flaking off in ribbons. A week later, white skip lines in the wheel path read 55 mcd/m²/lx on a handheld retroreflectometer while the adjacent edge line reads about 210. The city wants to accept the work and restripe the rest of the corridor with the same material and method.
1. Check the taper length and the general layout logic.
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Use the MUTCD taper formula rather than the plan’s round number. For a lane-width offset W and speed S (mph), the merging taper length is L = W×S for 45 mph and above, and L = W×S²/60 for 40 mph and below.
Here W = 12 ft and S = 40 mph, so L = 12 × 1,600 ÷ 60 = 320 ft. The plan’s 200-ft taper is 120 ft short of the minimum and would produce an abrupt, late merge at the bay entrance. That is a comment, not a rounding error.
Two related checks belong in the same review:
- Bay length must be set by the storage demand at the signal (arrival rate, cycle length, turn volume), not by whatever length fits between driveways.
- Taper type matters: a shifting taper, a shoulder taper, and a merging taper have different lengths (a shifting taper is roughly half the merging taper and a shoulder taper about a third). Confirm the designer used the right one for what the striping actually does.
Approve nothing until the taper is recomputed and the bay length is justified by the signal timing.
2. Evaluate the stop line, crosswalk, and skip-line layout.
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Split the elements and judge each against standard practice.
- Stop line width: 24 in sits inside the usual 12–24 in range, so width is acceptable.
- Stop line position: 2 ft ahead of the crosswalk is too tight. Guidance commonly calls for several feet - about 4 ft is a common value - between the stop line and the nearest crosswalk line so that stopped vehicles do not encroach on the walk and so pedestrians have a buffer. Two feet will read as an encroachment at every red light.
- Stop line extent: it should span only the approaching lanes, not run across the median or the opposing approach.
- Crosswalk condition: a transverse-line crosswalk is acceptable at a signalized intersection; confirm the lines are wide enough to be seen at night and in rain, and that the crosswalk is not placed so far back that it conflicts with the stop line and signal heads.
- Skip lines: 10-ft marks with 30-ft gaps is a standard pattern at 40 mph, and 4-in edge lines are within the normal 4–6 in range - though 6-in edge lines are common on arterials and improve wet-night visibility.
The judgement point is that a 24-in stop line 2 ft from the crosswalk “looks” compliant because the number is in range, while the placement is what the driver experiences.
3. What should the city do about the flaking thermoplastic and the 55 mcd/m²/lx reading?
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Two separate failures, two separate responses.
Application. Thermoplastic needs a dry substrate and adequate pavement temperature - a common specification floor is 50°F and rising, with the surface dry. At 42°F with dew present, the material skins before it bonds, and 90 mil over a milled, textured edge is thin. That is defective work under almost any specification: require removal of the failed material, surface preparation, and reapplication under compliant conditions, at the contractor’s cost, with the temperature and surface condition logged. Do not “seal coat” the flakes and move on.
Readings. A single 55 reading does not prove corridor-wide failure, and it should not be used to justify a citywide contract. Wheel-path wear is expected: the skip line in the travel path abrades first, while the edge line at 210 shows the material itself is intact. That pattern says “localized wear,” not “bad product.” Before any decision, confirm the measurement was valid - dry surface, calibrated instrument with a reference panel check that day, 30-m geometry per the standard test method, and multiple readings averaged per segment rather than one spot.
Then act on risk: re-measure on a dry day, compare against the agency’s minimum maintained level, and prioritize the wheel-path segments on the higher-speed approaches. Accept the work only for the areas that pass.