Horizontal curves are some of the most dangerous sections of road, whether they are located on interstates or rural two-lane roads. Curved stretches of road comprise a small percentage of all road miles, but they are associated with a quarter of fatal crashes.
According to the U.S. Department of Transportation, “The average crash rate for horizontal curves is about three times that of other highway segments.”1 About 75 percent of such crashes are speed-related and involve “a single vehicle leaving the roadway and striking…fixed objects or overturning.”1 As a result of such startling statistics, there is a plethora of possible tried and tested solutions with the aim of reducing the dangers of horizontal curves.
The most basic countermeasure is traditional signage. The Manual for Uniform Traffic Control Devices (MUTCD) designates a wide variety of curve signs as optional, recommended and required, depending on the difference between the speed limit and the advisory speed for the curve.2 Some of the most common signs are listed below:
|Name||MUTCD identification code|
|Advisory Speed Plaque||W13-1P|
|One Direction Large Arrow||W1-6|
Source: U.S. Department of Transportation, Federal Highway Administration
But there are two problems with the sign-only approach. First, such signs are so common their impact on driver behavior lessens over time. The second problem involves speed advisory plaques in particular. According to the National Academies of Sciences, Engineering, and Medicine, speed plaques paired with one of the above signs were no more effective than the curve signs alone and, in one study, correlated with acceleration as often as deceleration.3
In short, drivers know their vehicle can safely navigate a given horizontal curve at a higher speed than a sign advises, so they ignore the sign. But, given the prevalence of crashes and fatalities at horizontal curves, it's clear drivers are still trying to navigate those stretches of road at unsafe speeds. To address this problem, A Guide for Reducing Collisions on Horizontal Curves recommends advisory speeds be recalculated based on current technology and scientific calculations.3
One road treatment that has successfully reduced run-off-road (ROR) crashes on horizontal curves is the use of rumble strips on the road shoulder. Their presence has, according to one study, reduced ROR incidents by 20 to 30 percent on rural freeways.3 While this sounds promising, it should be noted that the study included both straight and curved road segments. Little is known about how effective shoulder rumble strips are on horizontal curves. Potential disadvantages to the use of rumble strips include their incompatibility with bicycle traffic and their negative impact on pavement life.
High Friction Surface Treatment
At a scientific level, ROR events on horizontal curves occur when the friction achieved from the tires’ contact with the road surface is less than that required to overcome the centrifugal force directing the car to travel in a tangent rather than in an arc matching the roadway. It stands to reason, then, that increasing the friction decreases the likelihood of road departure.
One roadway improvement that increases the adherence of tires to road surface is called High Friction Surface Treatment (HFST), the addition of a durable polymer to the pavement sealed by a binding agent.4 The treatment was applied to an interstate ramp in Milwaukee, Wisc., where the number of crashes was reduced from 79 per year over a three-year period to “two crashes in the first year after applying the friction treatment.”4
HFST was also applied to a curved interstate bridge in Cedar Rapids, Iowa, where one year after application it had reduced crashes from an average of over ten per year to just four in the 12 months after application4. Such dramatic crash reductions clearly demonstrate HFST's ability to improve traffic safety.
The costs associated with HFST, however, are high. Disruptive lane closures are required. The mixture of polymer and high-friction material must be very precise and could require the hiring of outside contractors familiar with the materials. And at $20 to $35 per square yard, HFST may be too pricey for some budgets.4