The tacks had not penetrated far enough to cause flats, but represented a significant hazard in that they could puncture a tube on a curve on a fast downhill slope and send a rider tumbling, Portola Valley resident and bicycle commuter Nate McKitterick said.

Mr. Matheson and the other riders continued up the hill, and when they paused at the top to wait for their partner, they checked their tires. Seven bikes had tacks in the tires, he said.

The incident began some minutes earlier on Kings Mountain Road. "Life was fine and dandy, and we got near the top and one of us pulled over with a flat tire," Woodside resident Bruce Matheson said.

On Saturday, Sept. 12, a group of about 15 cyclists on the Bakery Ride -- a morning ride to the coast and back that begins at the Woodside Bakery -- discovered tacks in their tires when they reached Skyline Boulevard.

Bicyclists have a nemesis roaming the upper reaches of Kings Mountain Road in unincorporated Woodside, according to some cyclists who travel that road regularly. The routine appears to be the same every time: some unknown person scatters carpet tacks on the roadway, giving flat tires to the cyclists who pass over them.

Officer Art Montiel of the CHP said its office received a call shortly after 9 a.m. on Sept. 12 about "a large amount of tacks or small nails all over the roadway on Kings Mountain Road," at a spot a mile or so east of Skyline Boulevard. The CHP asked county public works staff to sweep the roadway "as the tacks were small and hard to see," Mr. Montiel said.

All seven riders on the Bakery Ride got off their bikes and replaced the tubes in the affected tires, Mr. Matheson said. One rider called the Sheriff's Office, he said. On the return trip, they took a different route.

"Some people don't realize they have a (tack) in their tire right away, some (tires go) flat immediately," he said in an email. "Getting a flat tire going down KMR, with both oncoming traffic and an unforgiving road surface, is a frightening prospect."

Mr. McKitterick, who is also on the Portola Valley Planning Commission, was not one of the riders that morning but alerted the Almanac about this hazard. He said he first heard about tacks on the roads in 2012.

About six weeks ago, Menlo Park resident Andrew Valentine ran over a dark "extremely sharp" tack while on Skyline Boulevard just south of Kings Mountain Road, he told the Almanac. Four days later, a woman riding with the Menlo Bike Club went flat in the same spot, from a dark tack, Mr Valentine said.

If they had found tacks, the crew would have swept them up with the tools at hand, not a street sweeper, Mr. LoCoco said.

This was not the first call to the county public works department about tacks on the road in the vicinity of Kings Mountain Road. "We received three reports to my knowledge of tacks or nails ... in the past couple of months," said Joe LoCoco, the county's deputy director of road services. "Each time, we sent crews to investigate, but were unable to find any tacks or nails on the road. The last couple of times, we also spoke with several cyclists who hadn't noticed anything during their rides."

Tacks scattered on Woodside roads are hazard to bicyclists