On their way down the California shoreline from Oregon to the Mexican border, 10 intrepid hikers promoting a coastal trail will be forced to detour onto North County highways today, dodging traffic instead of tides.

It is the longest stretch of asphalt the hikers will encounter on their four-month expedition, which began June 3. That's because the 50 miles of coastline on Vandenberg Air Force Base and the Hollister and Bixby ranches are off-limits to the public, representing the largest gap in California's coastal trail, said Richard Nichols, executive director of Coastwalk, the nonprofit group running the trip.

There are only three small points of public entry along that stretch -- at Point Sal, and Surf and Jalama beaches.

"To have these huge sections blocked off doesn't seem fair to me," Mr. Nichols said. "The coast is our commons. The state constitution guarantees that we can get to the coastline. Why can't we?"

Coastwalk's mission is to build support for the creation of a public trail along the entire 1,150-mile-long coast of California. Right now, 60 percent of it is accessible by way of beaches and bluff-top trails; another 20 percent can be toured along country roads and city streets. But the rest is strictly closed to the public. Military bases exclude the general public for security reasons. At Hollister Ranch and other gated communities, the property owners oppose public trails; and cliffs and rocks make it difficult for people to walk on the beach.

"It's the last part that's going to be the hardest," Mr. Nichols said. "There's a constant attempt to develop whatever's left. It's too precious to build on. We think construction should stop, and everything left on the coast should be saved."

Vandenberg Air Force Base officials said Monday they were trying to find volunteers on the base who could escort the Coastwalkers through at least the southern part of the base this week, saving them 18 miles of highway trekking. The group's original request, arriving as it did last winter, was simply bad timing, said Maj. Stacee Bako, a base spokeswoman.

"We were right in the middle of the war," she said. "All requests of this nature were denied because we did not know what was going to happen with the course of the war. We did not know if it was going to continue longer than it actually did."

Officials at Hollister Ranch, a gated subdivision west of Gaviota, said Coastwalk's application did not fit in with their educational and scientific access program.

"Hollister Ranch certainly is not entertaining any public trails, and considers Coastwalk to be promoting an adverse interest to Hollister Ranch's interest," said Andy Mills, manager of external affairs. "Also, what factored into their decision was that there were very unfavorable tides that day."

Because of Hollister's refusal, Mr. Nichols said, Coastwalk did not approach Bixby Ranch, the 24,000-acre property that borders Point Conception between Vandenberg and Hollister. Even if the hikers had received permission to pass, he said, they would have been stopped at Hollister and forced to backtrack to Jalama Road. The detour around Bixby and Hollister to Gaviota State Park is 15 miles, following Jalama Road and Highway 1.

This summer marks the 20th anniversary of Coastwalk's summer hikes; but this is only the second full-length trip. The first was in 1996.

The Coastal Protection Act of 1972 guarantees the public's right to enjoy the beaches of California below the mean high-tide line. When the state Coastal Commission approves new projects on the coast, it typically requires a public trail. But Hollister Ranch was created before the state law was passed and residents there have vigorously defended their right to privacy. In recent years, Hollister residents funded a lobbyist in Washington, DC, and helped derail proposals for a National Park designation on the Gaviota coast -- even though the Park Service early on promised to exclude the ranch.

Hollister does allow groups of geologists and marine biologists to conduct research on the ranch and birdwatching groups can visit. This year to date, 500 schoolchildren have been allowed to visit the ranch's tide pools, said Brent Muchow, the general manager. Coastwalk, he said, had a recreational -- not a scientific -- purpose.

The ranch regulates the tide pools because they are untouched and pristine, Mr. Muchow said, adding, "We even discourage our owners from going there."

Mr. Muchow said Highway 1, four miles inland, would be the best place for a coastal trail between Lompoc and Gaviota State Park.

"It would give you the old California experience," he said. "If they're talking about putting the trail through Vandenberg, Bixby and Hollister, it's a highly impractical route. There are too many steep canyons and abrupt cliffs out here on the front."