Curve Sign Installation on a 55 mph Rural Route
A two-lane rural state route carries 55 mph traffic past a 600-ft-radius horizontal curve with a narrow 2-ft shoulder. A county crew is replacing a faded W1-2 curve sign and adding a W13-1P “35 MPH” advisory plaque on the same post. The crew sets an 8-ft galvanized U-channel post in a concrete-filled 12-in hole, with the bottom of the assembly 5.5 ft above the pavement edge and the sign face 4 ft from the edge line. The crew chief skipped breakaway hardware, reasoning that “the post bends over anyway when it’s hit.” A private driveway enters 40 ft beyond the sign. The maintenance supervisor wants the assembly moved to the driveway edge so mower decks stop clipping it. The advisory speed was chosen because the crew chief drove the curve at 35 mph in a pickup and it “felt fine.” About eight loaded log trucks use the route daily.
1. Evaluate the support, anchoring, height, and lateral placement of the assembly.
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Work through the roadside-safety logic first, because it is the item most likely to injure someone.
- Support and anchor: A U-channel post embedded in a concrete-filled hole is a rigid obstacle. U-channel is only breakaway when installed with the correct breakaway anchor or in properly compacted soil per the manufacturer’s installation instructions. Concrete defeats that. On a 55 mph road, the sign is inside the clear zone, so the support must be crashworthy (MASH-tested or accepted as equivalent). The fix is to pull the concrete, install a breakaway anchor or an approved slip-base system, and keep the manufacturer’s instructions in the file.
- Height: 5.5 ft to the bottom is acceptable for a rural location (the common minimum is 5 ft above the edge of the traveled way). This element is fine; say so.
- Offset: 4 ft is far too close for 55 mph. Guidance commonly cited is roughly 12 ft in rural areas and 7 ft in urban, with narrower values only where constrained. Push the sign to the greatest practical offset and check whether the curve’s outside should also carry chevrons.
- Driveway interaction: Moving the sign to the driveway edge may solve mowing but can place it in the driveway user’s sight triangle or make it more strike-prone. Evaluate rather than default.
Do not “fix” the whole assembly with one change; the anchor, the offset, and the mowing complaint each need their own answer.
2. What is wrong with the way the advisory speed was established, and what would a defensible process look like?
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A single drive-through in a pickup is a driver-comfort opinion, not an engineering study. An advisory speed plaque is Guidance tied to a study, and the study has to be reproducible and documented.
A defensible process: (1) collect the curve’s geometry (radius, superelevation, tangent lengths, sight distance) and the crash history; (2) run a ball-bank or accelerometer test along the curve at increasing speeds, using the accepted criterion for passenger cars, and repeat for the design or representative truck because the route carries loaded log trucks daily; (3) consider whether trucks need a separate consideration such as a lower advisory, additional chevrons, larger signs, or a truck rollover warning; (4) check that the curve sign and the advisory speed are visible and legible from the decision point at 55 mph, before the driver must brake; (5) record the run, the equipment and calibration, the criterion used, and the recommendation, and file it so the next crew does not re-invent the number.
If a ball-bank run supports 45 mph rather than 35 mph, install 45. Over-restricting an advisory speed erodes credibility and compliance as surely as under-restricting it.
3. The supervisor wants the assembly moved to the driveway edge. How do you decide?
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Treat it as a design question with three tests, then document the outcome.
- Roadside safety: Any new location must still be outside the usable shoulder and, if inside the clear zone, mounted on a crashworthy breakaway support. Moving from 4 ft to a driveway edge may or may not increase offset - measure it.
- Sight distance and operation: The sign must not sit in the driveway’s sight triangle, must not be shadowed by the driveway’s vegetation or mailbox cluster, and must remain visible to the 55 mph approach for the full advance-warning distance. It also must not invite a driveway user to interpret it as controlling the driveway.
- Maintenance practicality: Mower damage is real, but the remedy is usually a wider mowing setback or a different mounting location that still satisfies 1 and 2 - not simply the closest convenient spot to the driveway.
If the chosen location still clips mowers, consider a breakaway assembly set further back, or relocate to a point with better offset. Then record the decision in the sign inventory, including a photo and the measured offset and height, so the next supervisor does not reopen the same argument.