Napa County has turned to “rumble dots” to try to keep motorists from drifting off an eight-mile stretch of Silverado Trail and improve safety for bicyclists riding in the shoulders.
New shoulder striping along the sides of the road is made of a thermoplastic material. At intervals of every couple of feet, a cookie-shaped mass of the material has been added on top of the stripe.
People driving on the rumble dots, either from sleepiness or carelessness, receive a slightly jarring reminder to head back to the main part of the road. That can spare motorists from driving off the road, injuring or killing either themselves or bicyclists.
Alfredo Pedroza, chairman of the Board of Supervisors, cast a vote for the Silverado Trail safety project that added the rumble dots. His father was killed on this stretch in 2012 while riding a bicycle in the shoulder. A motorist veered off the road and hit him.
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The rumble dots are designed to keep this from happening to somebody else.
“Obviously, it’s a personal issue for me and my family, given what we went through,” Pedroza said. “I’ve been paying attention to it. I think anytime we can enhance the safety on the roadways, it’s something we have to support.”
The county didn’t have safety statistics readily available for this stretch of Silverado Trail, though he said it has a higher-than-expected rate of collisions involving motorists driving off the road.
The Metropolitan Transportation Commission’s Vital Signs website shows four cyclists injured in three incidents and one cyclist killed in collision accidents on this Silverado Trail stretch from 2006 to 2014. Vital Signs shows many more incidents of vehicle-only accidents, but doesn’t list the cause.
A more traditional way to provide a safety jolt is the rumble strip, such as those found along the sides of Highway 12 in Jameson Canyon. This method consists of grinding divots into the pavement near the shoulder line to shake up straying motorists.
“It’s creating damage to the pavement surface by this grinding process,” county Deputy Public Works Director Rick Marshall said. “If we have a better way to do it, that’s a good thing.”
The term “rumble dots” is unofficial at this point, though it’s a take-off on the better-known rumble strip.
“It’s a relatively new thing,” Marshall said. “I don’t know if they’ve settled on everybody calling it that.”
In general, Napa County has few places with either rumble strips or rumble dots along its road shoulders. Whether the 8-mile Silverado Trail stretch is the start of something new remains to be seen.
“We don’t have any more currently planned, but we now have this in the toolbox,” Marshall said.
Patrick Band, executive director of the Napa County Bicycle Coalition, has both ridden his bicycle and driven along the Silverado Road stretch.
“What we’re interested in is keeping people safe as they’re riding,” Band said. “Whether that’s a protected bike lane, whether that’s a green lane like those recently installed in St. Helena or whether it’s rumble dots or rumble strips, they all make it safer for riders.”
But all methods aren’t necessarily equal.
“One of the downsides to rumble strips is they can be 12 inches wide,” Band said. “They can take up a fifth of a bike lane.”
Rumble dots, in contrast, are only along the line between the lane and shoulder. And, since they are at intervals, cyclists having to momentarily shift into the lane to avoid an obstacle in the shoulder don’t have to cross over a jarring bump.
As far as he knows, rumble dots haven’t been tried on county roads on the same scale as the Silverado Trail project, Band said. The rumble dots are a test to see how motorists and cyclists respond.
On June 4, the Board of Supervisors voted for the $155,725 Silverado Trail safety project, with $143,000 coming from a federal grant. The project added the rumble dots and also improved the Yountville Cross Road intersection.
Napa County also has rumble dots along a short stretch of Solano Avenue near Yountville. That was installed by a vendor as a trial project so county officials could learn about them.
So how do rumble dots stack up to tried-and-trued rumble strip? Federal Highway Administration reports say rumble strips are more effective, but that rumble dots haven’t been studied as much.
Also, rumble dots can only be used in warm weather climates, an FHWA report said. They can be damaged by snow plows — not a Napa County problem.