Mast Arm Layout That Fails the Two-Face Rule
An agency is reviewing a 60-percent design for a new signal on a four-lane arterial posted at 45 mph. Curb-to-curb is 60 ft: a 12-ft left-turn lane and two 12-ft through lanes on the eastbound approach, and two 12-ft through lanes opposite. The eastbound mast-arm pole sits 10 ft behind the face of curb with a 40-ft arm. The plan calls out one 12-in through-signal face 28 ft from the pole and one 12-in left-turn face at the arm tip, plus a near-side supplemental head mounted on the pole 18 ft beyond the stop bar. The pole is 20 ft beyond the stop bar. The designer wants sign-off this week because the fabricator needs the final arm length.
1. Is the number and type of eastbound signal faces acceptable? Justify your answer before signing the plan sheet.
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No. Count the faces per movement and per lane group.
- Eastbound has three lane groups: through (two lanes) and left turn.
- The plan gives the through movement exactly one primary face (the head 28 ft from the pole). The left-turn movement has its own face, which is fine, but it does not serve through traffic.
- MUTCD Part 4 requires at least two primary signal faces for each approach (see 4D.11 in the 2009/2012 editions; confirm the paragraph in the edition your agency has adopted). A supplemental near-side head does not count toward that minimum - near-side heads are supplemental only and may be added to, not substituted for, the required primary faces.
- Lens size is correct: 12-in lenses are required where the approach speed exceeds 40 mph (4D.07), and 45 mph exceeds that. Do not let anyone value-engineer these to 8 in.
- Backplates are required at these speeds in the current MUTCD, so the detail should show them.
Action: reject the sheet for the through movement and require a second primary through face centered over the other through lane (or a re-layout that produces two through faces), then re-check arm length and bracket spacing.
2. Compute where the two arm-mounted faces land relative to the face of curb. Are they over the lanes they control, and does the arm length work?
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Set a datum at the eastbound face of curb and measure outward.
Lane lines (eastbound, from the curb): lane 1 = 0–12 ft, lane 2 = 12–24 ft, left-turn lane = 24–36 ft, centerline = 36 ft, then 36–60 ft is the opposing approach.
- Pole is 10 ft behind the face of curb, so distance from curb = distance from pole − 10 ft.
- Through face at 28 ft from pole = 18 ft from curb = center of lane 2 (12–24 ft). It lands correctly over the inner through lane.
- Arm tip at 40 ft from pole = 30 ft from curb = center of the left-turn lane (24–36 ft). It also lands correctly.
- Arm tip is 30 ft from curb versus a 36-ft centerline, so the arm does not project into the opposing lanes - no vertical clearance conflict over opposing traffic, and no head is exposed to being struck by opposing vehicles.
Conclusion: the arm length is geometrically sound for the two heads shown, and the pole location 20 ft beyond the stop bar is not a problem for these faces. The defect is the missing second through face, not the arm. The practical fix is a longer arm (roughly 50–55 ft) so a second through face can be centered over lane 1 (about 6 ft from curb) with an acceptable minimum bracket distance from the pole, or relocation of the pole and re-layout. Tell the fabricator the arm length is provisional until the corrected layout is approved.
3. The contractor proposes switching the supplemental near-side head to a primary face instead of lengthening the arm, arguing it is cheaper and equally visible. Evaluate.
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Reject it as a substitution, not as an addition.
- A near-side head mounted on the pole 18 ft beyond the stop bar is much closer than the 40-ft minimum distance from the stop line that applies to a primary face on an approach with a posted speed above 40 mph (MUTCD requires primary faces to be in the 40–180 ft range at these speeds; below 40 mph the range starts at 30 ft). At 18 ft it fails the placement range outright.
- Near-side heads are legitimate supplemental devices, typically added where a primary face can be blocked by a truck in an adjacent lane or where there is a long approach. They increase conspicuity but they are not a substitute for the two required primary faces and cannot be counted to satisfy the minimum.
- Cost argument does not survive scrutiny: relocating a face is cheap now, and a signal that fails the two-primary-face rule is a design liability on every future inspection and a potential crash-investigation finding.
Decision: keep the near-side head as supplemental (it is worth keeping at this approach), and require two primary through faces on the arm plus the left-turn face, with the arm length re-derived from that layout.