The yellow number under a curve sign is not a speed limit. It is a measurement of what a car can take the curve at, and no statute makes anyone obey it. Advisory is the law’s word for what the sign is, and the word matters most to the people who go over the limit it does not set.
The sign has a name and a number. The Manual on Uniform Traffic Control Devices, which governs the shape, color and placement of nearly every sign on an American road, calls it the Advisory Speed plaque, W13-1P, and says it “may be used to supplement any warning sign to indicate the advisory speed for a condition.” It “shall only be used to supplement a warning sign”; it is never a sign on its own. The number “shall be a multiple of 5 mph,” and, except in an emergency or for a temporary condition, the plaque “shall not be installed until the advisory speed has been determined by an engineering study.” The manual lists the tools: an accelerometer that measures side friction, a design-speed equation, or the old one, a ball-bank indicator, a curved glass tube with a ball in it, driven through the curve at rising speeds until the ball swings to 16 degrees for an advisory speed of 20 miles an hour or less, 14 degrees for 25 to 30, or 12 degrees for 35 and higher. And it records what drivers do with the result: “Research has shown that drivers often exceed existing posted advisory curve speeds by 7 to 10 mph” (MUTCD, Section 2C.08).
What it is not is a limit. Texas writes its limits in a statute, 30 miles an hour in an urban district, 70 on a numbered highway outside one, and the rest, and says that a speed above them “is prima facie evidence that the speed is not reasonable and prudent and that the speed is unlawful” (Transportation Code Section 545.352). The yellow plaque is not on that list. A driver who takes a 35 ramp at 45 has not broken the number on the sign, because the sign does not make a rule. He has only been told something.
What he was told is the point. The statute that does govern him asks for “a speed greater than is reasonable and prudent under the circumstances then existing” from no one, and then names the circumstance: a driver “shall, consistent with Subsections (a) and (b), drive at an appropriate reduced speed if” he “is approaching and going around a curve” (Transportation Code Section 545.351). The plaque is an engineer’s answer, posted on the road, to the question of what appropriate means on that curve, and a driver who passed it knew the answer before he chose his speed. That is the duty Reasonable and Prudent explains, with the number filled in.
And the number is filled in for a car. The ball-bank criteria are matched to the highway design standard for curves, and that standard produces, by the University of Michigan’s count, “lateral accelerations as high as 0.17 g at the advisory speed,” a sideways pull a car never notices and a loaded van trailer, with a rollover threshold the same study puts as low as 0.25 g, has a third of its margin left to meet (UMTRI, 1999). The commercial driver’s manual says it in the words a truck driver was tested on: “Tests have shown that trucks with a high center of gravity can roll over at the posted speed limit for a curve,” so “Slow to a safe speed before you enter a curve,” and “Don’t ever exceed the posted speed limit for the curve” (CDL manual, Section 2.6.3). The sign is the ceiling for the truck, not the target, and the arithmetic of why is Threshold. On a long downgrade the state has tried a truck-only version, the weight-specific speed sign, and the engineers who built it conceded that its speeds “are advisory in nature because they are impossible to enforce: police cannot easily assess gross truck weights” (FHWA, 1989), which is Brake Fade. Every advisory number on the road is the same confession: the state can measure the curve, and it cannot stand on it.
The move: because the plaque is not a statute, going past it is not negligence in itself, which is what Negligence Per Se would require, and a driver’s lawyer will say so first. The answer is that the plaque is not the rule; it is the evidence of the rule. Section 545.351 is the statute, the curve is the circumstance, the plaque is the state’s own study of what prudence required there, and the engine’s recorder (the black box) is what the driver did with it. A truck that took a 35 ramp at 45 was not 10 over a limit. It was 10 over the number its own manual made a ceiling, with a load whose fraction its carrier and its loader set at the dock. And the road has its own file: the manual says each location “should be evaluated periodically or when conditions change” (MUTCD, Section 2C.08), which means the date of the last ball-bank run on a ramp that has rolled more than one truck is a record someone keeps, and Spoliation is the letter that asks for it. One caution. The plaque is evidence, not a verdict. A truck can go over under the number and a car can hold the road well above it; the sign tells the jury what the driver was told, and the rest of the case tells them what he should have done with it.
Manual on Uniform Traffic Control Devices, 2009 edition with Revisions 1, 2 and 3, Part 2, Chapter 2C, Section 2C.08, Advisory Speed Plaque (W13-1P) (Option 01, Standards 03 through 06, Supports 07 and 08, Guidance 10). Tex. Transp. Code § 545.352 (subsections (a) and (b)); § 545.351 (subsections (a) and (c)(2)). Commercial driver’s manual, Section 2.6, Controlling Speed, as posted by the Georgia Department of Driver Services (2.6.3). C. B. Winkler and R. D. Ervin, Rollover of Heavy Commercial Vehicles, UMTRI-99-19 (Aug. 1999) (the 0.17 g and the van-trailer threshold), with the text as posted in the copy posted by Canada’s Council of Ministers Responsible for Transportation and Highway Safety. FHWA, Reducing Runaway Truck Accidents Through Weight-Based Advisory Speeds, FHWA-IP-89-023 (Sept. 1989) (the advisory nature of weight-specific speeds). This is general information, not legal advice. Figures and text as published at publication.
