Part 1: The Curve That Should Have Been Obvious

Most drivers assume that if a curve has a warning sign and an advisory speed, the road agency must have got it right. That assumption feels reasonable because advisory speeds exist for a reason. As the FHWA explains in Procedures for Setting Advisory Speeds on Curves, horizontal curves are associated with a disproportionate number of severe crashes, and warning signs are intended to alert drivers to a change in geometry that “may not be apparent or expected.” The Federal Highways Administration (FHWA) Manual on Uniform Traffic Control Devices for Streets and Highways’ (MUTCD) horizontal alignment framework exists to make the road legible in advance, using signs such as Turn, Curve, Hairpin Curve, Reverse Curve, Winding Road, Chevron Alignment signs, and Advisory Speed plaques as a coordinated language for the driver.

The underlying idea is simple: if the message is timely, credible, and consistent, drivers can adjust path and speed before they are committed to the curve.

But this is where the story becomes more complicated. The FHWA handbook notes that several research projects have shown that drivers are often not responding to curve warning signs or complying with advisory speed plaques, and it goes further: one reason curves are overrepresented in speeding-related fatalities is that advisory speeds are sometimes not consistent and therefore not credible. That point matters more than it first appears. Drivers do not read a warning sign in isolation; they read it through memory and experience. If one curve is signed aggressively, another lightly, and a third not at all despite similar severity, the road stops speaking in a dependable voice. Eric S. Leaming captured the practical consequence bluntly in Curve Evaluations using Rieker’s CARS.

Once that happens, the issue is no longer whether a single plaque is defensible on paper. The issue is whether the broader system still deserves driver trust.

That is why advisory speeds are not a minor signing detail. They sit at the intersection of human expectation, geometric design, and operating speed. Bonneson and colleagues argued that any effective procedure must identify when a warning sign and advisory speed are needed and then select a speed that is consistent with driver expectation.

In other words, the sign must feel believable as well as technically justified. That is the central issue. Advisory speeds exist because curve safety depends on early, intelligible information. But when the standard is applied unevenly, even a well-intentioned sign can contribute to the confusion it is meant to prevent.

This is the first in a three-part series exploring how that inconsistency developed, why it persists, and what it would take to do better.