DIABLO — A judge’s ruling to block public access to a popular cut-through for cyclists on their way to Mount Diablo State Park could put people’s lives at risk, biking advocates say.

For decades, bicyclists have been turning down oak-lined Calle Arroyo at the entrance to Diablo Country Club to avoid Diablo Road in Danville, which is notorious for its narrow lanes, fast-moving traffic and blind curves. Two cyclists were seriously injured there last year by a hit-and-run driver, reopening a long-standing debate over safety along the scenic corridor.

But that may be the only quick option for cyclists now. A Contra Costa County Superior Court judge on Monday officially signed a ruling declaring the public has no right to access the road — even if it may be a difficult ruling to enforce.

Limiting access to the country club disproportionately impacts students in the area who use the route to get to school and for training on the high school mountain biking team, said Al Kalin, president of the Mount Diablo Cyclists, a bicycling advocacy organization.

“The judge’s ruling affects tens of thousands of cyclists,” Kalin said, “but specifically the local mountain bike high school teams who have for years rode down Calle Arroyo.”

The alternative, however, of having cyclists barreling down the barely two-lane, unmarked roads in Diablo is also perilous, contends Robert Tiernan, the lead plaintiff in the suit. He filed the suit earlier this year on behalf of several other homeowners along the road after seeing a growing number of cyclists riding in “loud packs,” overtaking cars and endangering seniors and small children. His parents, who are both in their 90’s and live on the same road, have had cyclists clip their car or yell at them, he said.

“After all that,” Tiernan said, “it was becoming too dangerous.”

In the end, the court case hinged not on questions about which road is safer — but who can control its access. And it was here the plaintiffs prevailed.

They argued that because the country club community was formed as a private community, there has never been any express or implied public access, said Dominic Signorotti, the plaintiffs’ attorney. An attorney for Bike East Bay, a cycling advocacy organization and defendant in the suit, argued the Diablo Community Services District, which governs the country club community, received public funds to make improvements on Calle Arroyo. That would imply the roads are meant for public use, or else that the district used the funds illegally.

The judge said that wasn’t enough.

“There is no evidence that any of this money is spent on Calle Arroyo. And even if it did, so what?” Judge Charles Treat wrote in his ruling. “If the district is illegally spending money, it ought to stop doing so. But that doesn’t mean the district can create an easement over its members’ properties by spending money.”

The comment, while not dismissing the suit, did open another question: Has the Diablo Community Service District, which governs the bucolic community, been spending taxpayers’ money on private roads?

“It’s sort of an open question,” said Dave Campbell, the advocacy director for Bike East Bay.

The district is looking into it and will adjust future spending accordingly, said Christie Crowl, an attorney for the district.

But that still leaves the question of how to enforce the ruling. The judge determined the district has no authority to prevent the general public from using Calle Arroyo, Crowl said. And, doing so could easily violate people’s constitutional rights, said Lt. Jason Haynes, a sheriff’s deputy in charge of the Diablo subdivision. As long as the person in question agrees to move along, there is no way to cite them for trespassing, he said.

“If it’s private property but publicly accessible, it will be very difficult to take any enforcement action,” he said. “We can’t proactively patrol and try to ID people within the community, or that would be a blatant disregard for people’s rights.”

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Late but alive, Tour de France races into COVID epidemic For now, however, Campbell is recommending cyclists avoid Calle Arroyo, if they can. The judge’s ruling applies only to the single street of Calle Arroyo, though the plaintiffs recently filed an amendment to expand the ruling’s reach to include a tiny path over private property that links to a public road leading to Mount Diablo State Park. If the judge rules in favor of the amendment, it will constrict access to the park even further and could have serious implications for cyclists’ safety — implications with legal precedent backing it, Campbell said.

The city of Danville is redoing an environmental review of a proposed development because a judge ruled it would create more traffic on Diablo Road and endanger cyclists. Campbell is hoping the same thinking will apply here if cyclists are forced onto Diablo Road.

“If you attempt to close that path, that is subject to (the California Environmental Quality Act) and you need to do an (environmental impact report),” he said, “because that affects the safety of people bicycling.”