Advisory 40 on a Wet Reverse Curve
You are westbound at dusk on a two-lane Illinois state route posted 55 mph, driving a loaded pickup with worn all-season tires. Rain has been falling for twenty minutes and the asphalt is wet and greasy. A yellow diamond warning sign with a reverse-curve arrow appears, followed 500 feet later by a yellow advisory plaque reading 40 MPH, mounted on the same post as a black-on-yellow symbol sign showing a truck tipping onto its side with the legend TRUCK ROLLOVER. Four chevron signs mark the first bend. A grain truck 300 feet ahead of you brakes hard at the curve entrance, its brake lights smearing on the wet pavement, and its trailer leans noticeably toward the outside of the curve as it turns in. There is no other traffic, no construction, and no reduced-speed regulatory sign anywhere in the sequence.
1. What is the legal status of the 40 MPH advisory plaque compared with the 55 mph posted limit, and what speed should you carry into the first curve?
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The two numbers carry different legal weight.
- 55 mph is the posted statutory maximum and is enforceable as a limit.
- 40 mph on a yellow plaque is advisory - an engineering judgement about the speed at which a typical vehicle can negotiate the bend. It is not a separate speed limit, and 44 mph is not automatically "speeding."
- Illinois' basic speed law still requires a speed reasonable and prudent for conditions. Wet pavement, worn tires, a signed rollover history, and an advisory of 40 together argue that anything near 55 is unsafe even though it is under the maximum. After a crash, the plaque and the TRUCK ROLLOVER sign become evidence that the hazard was signed and ignored.
Practical target: be at or below 40 at the entrance, and lower tonight - 30 to 35 - because the surface is wet and the tires are worn. All braking happens on the straight approach, not in the bend.
2. The grain truck brakes hard 300 feet ahead of you. What should you read from that, and what do you do in the next few seconds?
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His brake lights are free information: a driver who knows the road is telling you the curve is tighter or slicker than it looks from 300 feet back, and the trailer lean says his rear tires are already near the limit of grip.
- Brake now, while you are still straight. Braking inside the curve divides the tires' traction between turning and slowing, which is how vehicles leave the outside of bends.
- Target 30 mph or less at the entrance. Enter wide of the center line but not over it - wet paint is the slickest strip on the road.
- Add following distance to five or six seconds. Wet asphalt can take nearly twice the dry stopping distance; if he stops or jackknifes you need that room.
- Do not pass. No sight distance, no room, and the pass would place you in the bend at the worst possible moment.
If you cannot get speed down before the entrance, use the shoulder before the curve - never the oncoming lane.
3. Three miles later you meet the same chevrons but no warning sign and no advisory plaque. Does that change the limit, and how do you set your speed?
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No plaque does not mean no hazard, and it does not raise the limit.
- The legal maximum remains 55 mph unless a regulatory sign changes it; the missing plaque changes nothing legally.
- The chevrons are the clue. They are installed where the alignment needs delineation, usually at the tightest section. Assume a curve at least as severe as the one just driven.
- Set speed by sight distance, not by the last number seen: you should be able to stop within the distance you can see to be clear. At night or in rain, that alone may put you at 40 to 45.
- Read the corroborating evidence - skid marks, gouged guardrail, a truck-symbol curve sign, fresh asphalt patches - as the engineers' notes on what happens here.
The posted maximum is a ceiling for ideal conditions, not a target.