Swain cyclists help DOT in rumble strip study
When drivers veer off the road, they’re often alerted to the error by a vicious vibration caused by an ingrained groove laid into the asphalt. That groove is known as a rumble strip.
Reuben Moore, a division planning engineer with the North Carolina Department of Transportation, has felt the vibration of the rumble strips. Not in a motor vehicle, but on a bicycle.
“I got out on one road and tried’em out,” Moore said. “It was extremely uncomfortable. It basically vibrated the handlebars right out of my hands.”
Rumble strips are cut a half-inch deep and run about a foot across. Studies and statistics show them to be a lifesaver for motorists. Cyclists, however, have a different perspective.
“I think that they’re absolutely necessary for motorists, they’re great to keep you from running off the road and that kind of thing,” said Swain County cyclist Jim Parham. “For cyclists, it’s the exact opposite.”
Instead of gently jarring reminders to stay on the road, cyclists tend to view rumble strips as teeth lying in wait for someone to rattle across them.
“You’ve got teeny little tires,” noted Diane Cutler, owner of Bryson City Bicycles.
Moore, also a cyclist, wondered if the rumble strips might be better designed to accommodate bicycles. Historically speaking, traffic officials had not taken bicycles into account when considering how best to place rumble strips on roadways.
“Bicycles weren’t on our radar,” Moore said.
The NCDOT official pointed to a 1999 Arizona Department of Transportation study that recommended leaving 12-foot gaps between sections of rumble strips to allow cyclists a space through which to cross. Such a gap was an improvement, as the sections typically featured no breaks.
“In 2007 or so we were putting in rumble strips without any gaps at all, and we were getting some immediate feedback from cyclists that this was a screw-up,” Moore said.
But even with 12-foot gaps interspersing the rumble strips, cyclists have complained to transportation officials that there is still not enough room to maneuver through.
“If you’re going at a high speed, 12 feet is not enough,” Moore said.
So, in 2012 Moore proposed a study focusing on the space between rumble strips. He wanted to know if cyclists would fare better at higher speeds if that space was increased.
The study was approved and handed to Sarah O’Brien, bicycle and pedestrian program manager for the Institute for Transportation Research and Education at North Carolina State University.
“Bicyclists don’t like rumble strips,” she said. “They’re extremely uncomfortable and potentially dangerous and can cause a crash.”
O’Brien’s team invited cyclists to a stretch of N.C. 28 in Swain County over a recent March weekend. They replicated rumble strips, leaving varying distances in-between
“We’d be shuttled to the top of a nice hill,” explained Cutler, who participated in the study. “We did some practice runs just to get comfortable with the hill, to figure out what our speeds should be.”
The experiment involved about 20 cyclists. Researchers used radar to collect speed data. Participants were not informed of the particulars.
“It was all blind to us,” Cutler explained. “They’d set up a series of rumble strips, then a space, then rumble strips.”
The gaps actually varied from 12 to 24 feet, in two-foot intervals. Researchers looked at a cyclist’s speed, as well as the interval at which they felt comfortable crossing.
Moore said he estimates motor vehicles would likely be served just as well by rumble strips interspersed with gaps larger than 12 feet.
“That’s probably a subject of debate among our traffic engineers,” he said. “I’ve tried to drive a car though a 12-foot gap and I couldn’t do it. I think up to 20 feet would be fine.”
Also important to the rumble strip study was the grade of the hill. The faster the cyclists were going, the worse the effect of the rumble strips.
“That’s when they’re the most dangerous to cyclists,” said Parham, author of several books, including Road Bike the Smokies. “If you’re going over 20 mph they can cause you to go out of control and crash.”
Parham estimates he was going between 35 and 40 mph during his rumble strip experiment runs. He points out that such a speed might be considered typical for a cyclist on a downhill.
“It’s not like you’re out there being a daredevil,” Tarham said. “That’s just your cruising speed.”
In real-world conditions, Parham tends to avoid rumble strips as much as possible.
“I might actually move out into the traffic lane more than I normally would,” he said.
The cyclist also noted that the amount of shoulder available on the other side of rumble strips is another factor to consider when discussing safety issues. The shoulder area is often limited or otherwise unattractive to cyclists.
“Rocks and wires and glass and all kind of stuff over there,” Tarham said. “It’s not a good place to be riding a bicycle.”
That’s something Cutler said she hears from riders at the shop in Bryson City.
“Larger shoulders, I think that’s the number one thing I’d like to see on all our roads,” she said. “It’s scary to ride. Especially with how curvy and winding the roads are.”
Cutler also suggested that the rumble strips be placed along the edge of the driving lane, as opposed to down the middle of the shoulder. But she’s also glad that transportation officials are taking cyclists into consideration at all.
“I just think it’s great that they’re even thinking about multi-use,” she said. “We’ve been so car-centric in this nation.”
Once the data collected during the N.C. 28 experiment is assessed, Moore is hoping that the findings feed into the science of rumble strips. That information gleaned during the downhill rides in Swain County can be used by transportation officials across the state and beyond.
“I’m hoping this can be used as a toolbox feature,” Moore said.