From the April 2016 issue
Pop quiz: The yellow diamond-shaped sign on the side of the road says you should take the upcoming curve at 35 mph. At what speed can you actually travel through the bend? 45 mph? 55? Can you double it?
Depends on the car, right? It’s no surprise that the suggested speeds through curves fall well below what the average BMW can manage. Traffic engineers design for trucks, inclement road conditions, inept drivers, and ambulance-chasing lawyers, not P Zero–shod sports coupes. But it also depends on the curve, and how reasonable that suggested speed actually is. We looked into how recommended cornering speeds are set, and we found a mishmash of malleable procedures that are inconsistently applied. That’s why a 45-mph curve in Happyland, Oklahoma, often looks nothing like a 45-mph bend on SoCal’s Angeles Crest Highway. To no one’s surprise, research indicates that these curve advisory speeds are among the most disregarded signs on the road.
Even as the Federal Highway Administration (FHWA) has formalized its procedures in recent years, there’s still wide latitude for how state, county, and municipal agencies determine the curve advisory speeds on the roads they manage. The FHWA’s 35-page guidance document, published in 2011, offers sanctioned methods that fall into the three following fundamental categories, but engineers aren’t obligated to abide by them.
ROADWORK AHEAD
Finally, Congress Gets Something Right. (Sort Of.)
In a fleeting moment of governmental competence, America’s elected officials approved a five-year, $305 billion road-funding plan this past December. The sensible, bipartisan action is known as the Fixing America’s Surface Transportation (FAST) Act. No transportation bill in the past decade has covered more than two years.
The bulk of the FAST Act money—$207 billion—will be distributed to states for road and bridge improvements, where the stability of a five-year plan could encourage local authorities to take on more ambitious, longer-term infrastructure programs. But it doesn’t provide much headroom for growth. The Department of Transportation contends that the 11-percent increase in spending from 2016 through 2021, primarily intended to offset inflation, isn’t enough to reduce congestion or keep pace with technology. The Obama administration had originally proposed a 45-percent increase.
As usual, finding the money that legislators have decided to spend remains a problem. Rising fuel efficiency means gas-tax revenues are falling and the Highway Trust Fund, which will pay for most of the FAST Act, already runs a deficit of $16 billion per year. With oil currently at unimaginable lows, it’s difficult to envision a better time to raise the gas tax or at the very least to index it to inflation. The current federal tax of 18.4 cents per gallon has been unchanged since 1993. The good news is that the FAST Act contains a provision to increase road funding in the unlikely event that our nation’s legislators grow a spine between now and 2021. Any additional revenue brought in to the Highway Trust Fund will automatically be authorized for highway spending.
♦ Direct
While it’s not common, highway planners are free to eyeball curves, drive through them, and set a conservative speed based on their seat-of-the-pants impressions. The FHWA would prefer that traffic agencies trust the driving public’s instincts instead. On the list of approved procedures, engineers can measure free-flowing traffic as it travels through the curve without any signage. In 2003, the Manual of Uniform Traffic Control Devices—the traffic engineer’s instruction manual—recommended setting curve advisories using the speed that 85 percent of vehicles do not exceed. The more modern FHWA manual backpedals, encouraging engineers to use the average speed of trucks, which typically correlates with the 40th percentile of passenger-car speeds.
♦ Accelerometer
Another approach requires engineers to drive through the curve at 5-mph increments while tracking lateral acceleration. This technique was popularized in the 1950s with the ball-bank indicator, a sort of crude accelerometer, and it remains one of the favored methods. When a true accelerometer is used, the FHWA suggests a speed yielding between 0.26 and 0.30 g of lateral acceleration.
If that seems comically low in a world where even the most ungainly heavy-duty pickups manage more than double that grip, it’s because the manual accounts for the fact that drivers typically exceed the recommended speed by 7 to 10 mph. The FHWA acknowledges that ambivalence to these advisory speeds is problematic. As drivers learn that they can disregard curve-speed signs on their regular routes, they become desensitized to the yellow diamond-and-square duo wherever it appears. But in some jurisdictions, the numbers are grounded in reality. Despite the abundance of modern traffic data, much of the research on driver behavior and curve speeds dates to the 1960s, ’70s, and ’80s. If the research had kept pace with the technological advances of the car, maybe our signage would be relevant today.
Cars that can better a full 1.0 g on the skidpad are becoming common. This Corvette Z06 managed 1.19 g’s, about four times a curve’s recommended lateral acceleration limit.
R = Radius of curve, feet
V = Average truck speed
B = Banking of curve, percent
♦ Design
Traffic planners can also calculate recommended curve speeds based on the road’s geometric parameters. This complex-looking formula does so based on fairly simple variables such as the radius of the curve, the banking of the pavement surface, and the speed of traffic as it approaches the bend.
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