No-Passing Zone Delineation on a Curve-and-Crest Section
A rural two-lane, two-way road posted at 55 mph consists of a 1,200-ft tangent, followed by a 900-ft-radius horizontal curve, followed by a short vertical crest. Existing markings are a broken yellow centerline in a 10-ft line / 30-ft gap pattern carried continuously through the curve, with 4-inch white edge lines. Field measurements of passing sight distance taken at 25-ft stations show 1,300 ft on the tangent, dropping to 780 ft near the curve entrance, and recovering to 1,150 ft beyond the crest. Two near-miss reports in 18 months describe drivers beginning a pass on the tangent and meeting an oncoming vehicle at the curve. County maintenance has proposed installing DO NOT PASS signs and leaving the centerline pattern unchanged, on the grounds that the signs are cheaper and that the existing centerline has been there for decades. You have been asked to review and respond in writing.
1. Determine where the barrier line should begin and end in each direction, and explain how passing sight distance drives that decision.
Read the worked answer
Criterion. On a two-lane, two-way road, the no-passing zone is where the available passing sight distance is less than the passing sight distance needed for the posted speed. At 55 mph the needed value is about 1,000 ft. The zone is defined by measurement, not by the presence of a curve - a curve is a hint that a zone may exist, not the basis for it.
Applying it here:
- The tangent at 1,300 ft is above 1,000 ft: passing permitted.
- The 780-ft station is below 1,000 ft: no passing.
- Beyond the crest at 1,150 ft: passing permitted again.
The barrier (double solid yellow) must begin where the measured distance first falls below 1,000 ft, travelling in each direction, and end where it recovers. Because the two directions have different sight lines through a curve, the transition points are not opposite each other - that is why the centerline is a two-directional layout with solid on one side and broken on the other through the transitions.
The practical trap. Do not start the barrier exactly at the station where the measurement fails. A driver needs to see the barrier before it applies. Extend the barrier a short distance back onto the tangent so that a driver already committed to a pass sees the change while there is still room to abort, and check the upstream approach for any crest or curve that would hide the transition. Document the stations, the measured values, and the extension in the file.
2. Respond to the county's proposal to use DO NOT PASS signs only. What do the signs do, what do they not do, and what else should be in the package?
Read the worked answer
What the sign does. The DO NOT PASS sign is supplementary. It reinforces a restriction that the pavement marking establishes. It works on the driver who is already looking for a passing opportunity and scanning the roadway ahead; it is read at a glance and is easy to miss on a road where drivers have passed at this location for years.
What it does not do. It does not define the zone, it does not stay visible at night or in rain the way a retroreflective line does, and it cannot prevent a pass that was already begun. If the centerline remains broken, the roadway is still telling drivers that passing is lawful. Conflicting messages get resolved in favor of the permissive one, especially on a road with decades of habit behind it.
Sign placement. DO NOT PASS signs are placed at the beginning of the no-passing zone and repeated as needed, and can be placed on the left side of the road so they face the driver whose passing maneuver is being restricted. Confirm the mounting offset and breakaway requirements for a rural roadside before choosing the post.
Additional package elements:
- Convert the centerline to the barrier configuration first. This is the primary message.
- Add the DO NOT PASS signs at the zone entrances, both directions.
- Consider a supplemental plaque with distance where the zone is long.
- Check the edge lines and the curve for delineation - if the near-misses involve drivers misjudging the curve itself, chevrons or delineators may be the more relevant fix.
- Re-measure after any resurfacing. Sight distance changes with pavement profile and vegetation, and a zone from decades past is not evidence of anything.