Starting at 3 p.m. Friday, bicyclists in Marin County will have one less hill and a lot less car traffic to deal with. The traffic will still be there, and so will the hill. But an old railroad tunnel that cuts through it has been dug out and daylighted, connecting San Rafael to the Larkspur ferry with a smooth, flat cruise for riders and walkers.

The Cal Park Hill Tunnel, which starts at the industrial south end of San Rafael and empties out behind the Century Larkspur Landing movie theater, is only 1,106 feet long. But it is a precious 1,106 feet because it will cut 20 minutes off the commute. During rush hour, a cyclist will be able to beat a motorist from one side to the other, and that's while sticking to the tunnel's speed limit of 15 mph.

"This will be a major transportation corridor," says David Hoffman, director of planning for the Marin County Bicycle Coalition, which is lobbying to open a second, narrow rail route through the Alto Hill Tunnel, which separates Mill Valley from Corte Madera.

Use of the Cal Park Hill Tunnel is estimated to be 800,000 trips a year, which is a lot of bikes but probably not enough to get a $27 million project done. The bike route was something of an afterthought to SMART (Sonoma-Marin Area Rail Transit), which owns the land and plans to return rail travel to the other portion of a tunnel that opened in 1884 and closed in the 1960s.

The afterthought is now the forethought, because sales-tax funding for SMART has slowed. The train is six years away at best, and all that can be seen of the walled-off transit side of the tunnel is a muddy approach.

SMART and Marin County split the bill for the tunnel, which is LED-lighted and has a yellow stripe down the middle, a ventilation system for airflow and enhanced receptions for cell phones. There are eight surveillance cameras and four emergency call stations. At no point is it steeper than a 5 percent grade, to make it wheelchair accessible. They've even smoothed one wall to soften knuckle scrapes.

The bicycle commute from San Rafael to the ferry has been either a steep climb over the ridge or an unpleasant roundabout that goes past the dump, past the water treatment plant, over broken glass and debris, and through a merge with cars coming from the Richmond-San Rafael Bridge onto East Sir Francis Drake Boulevard.

The return commute is worse, as it climbs past the entrance to San Quentin State Prison to the crossroad where cyclists going uphill must pump through a left turn against the traffic flying off the bridge, without benefit of a stoplight or even a stop sign. That is one ride to avoid, and among those who avoid it is Erin Clarke, 27, office engineer for Jacobs Associates of San Francisco, which managed the project.

"Everyone is getting on and off the 580 Freeway, and it's just too fast," she says "It's really scary for bikers."

For six months, Clarke commuted by ferry, then biked to the San Rafael side, where she worked out of a trailer. The ride got so bad that Clarke had her boss meet her on the Larkspur side and then truck her around to her trailer.

But on Monday, Clarke ferried over, crossed the street to the path and rode it. Five minutes into the tunnel, she passed twin green border markers, delineating that she was exiting the city of Larkspur and entering the city of San Rafael. In another five minutes, she was at the north terminus 1.5 miles from downtown San Rafael.

That's 10 minutes ferry to terminus, and that is on a 13-year-old mountain bike, weighed down by her steel-toe Red Wing work boots.

The bike-lane approach from either end is longer than the tunnel itself. On the north end it angles off of Andersen Drive, takes a new pedestrian bridge over Bellam Boulevard, cuts under the Redwood Highway (Highway 101) and eventually empties onto Andersen.

Friday's opening ceremony will take place at the Larkspur end, and there is speculation that 100 to 200 people might be jockeying at the ribbon, hoping to be the first through. They won't be.

"I was the first bicyclist," Clarke says. "I made sure of it."