About two miles outside Clayton, traveling east on Marsh Creek Road, a road sign with a squiggly arrow cautions: "Next 8 miles."
Shortly after, the asphalt twists, turns, rises and falls, cutting through valleys and hanging on hillsides. A blind corner gives way to an S-curve, which precedes a 90-degree turn, just a short distance before a sign reading "Rock Slide Area."
The speed limit is 35 miles per hour for most of this stretch, but at a dozen points more signs warn motorists to drive slower. Solid double-yellow lines are constant reminders that passing other vehicles is a really bad idea.
The countryside is scenic -- oak and pine trees, hills and swales -- but if you lift your eyes to take it in, you risk becoming part of it. The two-lane pavement doesn't quit squirming until past Deer Valley Road, where the speed limit jumps to 50.
Once upon a time, this byway was lightly traveled. That was before the East Contra Costa construction-and-population boom, when Marsh Creek became a shortcut linking Antioch, Brentwood and Tracy to points west, and traffic of all sorts began traveling the road.
It hasn't been the same since. More than 6,100 vehicles travel the road daily, according to a 2008 traffic count by Contra Costa's public works department.
"The first time I drove it, I was doing the speed limit when I came around a corner and had to swerve to miss some bikers who were in the middle of the road," Karri Landers said. "Then I got passed on a curve by a big truck. I white-knuckled it all the way home."
Landers has had a Marsh Creek address for only four months, since becoming property manager for Clayton Regency Mobile Home Park, but she already has formed an opinion.
"I'm from Texas, so I'm used to crazy drivers, but that road scares me. You see people coming toward you drifting across the center line when they round a corner and you wonder: 'Don't they see me?' "
Terry Gross, a UPS driver who travels the road five days a week, remembers at least five nasty collisions in the past two years, but he faults drivers more than the road.
"I've seen guys take a turn too fast, then spin off and knock over a telephone pole," he said. "Speeding is the No. 1 problem."
He said the banked curves are easily navigable if you obey the speed limit, but many drivers do not. He often pulls off to the shoulder to let tailgaters pass.
That echoes the sentiment of California Highway Patrol Officer Tom Maguire, who patrolled Marsh Creek before becoming a spokesman for the agency.
"The No. 1 reason people get involved in collisions is speed," he said, noting that most drivers he cited were commuters.
He said those who use the road regularly become complacent about its hazards, and some of those rushing to work grow frustrated by slower drivers. "We see some impatience. Everybody thinks they're more important than everybody else."
Curiously, he said, more of the accidents happen east of Deer Valley Road, where the pavement flattens out and passing is legal.
"It's not as winding there," Maguire said, "so that means people drive a little faster."
Susanna Thompson, who has lived along that stretch for eight years, said she hears police or ambulance sirens about three times a month. From her front porch, she often sees vehicles exceeding the 50 mph speed limit. It's worst during the morning and evening commutes.
"It's not the road that's dangerous," she said. "It's people driving like idiots. I told my mom, 'After 3 p.m., it's not safe for you to be on this road.' "
People seem to forget, she said, that it isn't a thoroughfare, it's a "country road." Each blind turn can hide an unwelcome surprise -- a slow-moving tractor, a fallen boulder, a deer crossing the road -- and every upward tick of the speedometer reduces reaction time.
The biggest hazards on Marsh Creek Road often are the drivers.
Contact Tom Barnidge at email@example.com.