Sonoma Raceway is taking steps to confine drivers' stunts to the track.

To stop what they say is a growing safety concern, raceway officials are installing a 600-foot concrete barricade at the track's main entrance off Highway 121. That, they hope, will stop evening commuters from making dangerous U-turns and using the entrance as a rush-hour shortcut.

"It's a pain to do it, but we've reached the point where we feel we have to," said Steve Page, the raceway's president and general manager.

Sonoma Raceway is near the intersection of Highways 121 and 37, one of the county's most notorious traffic logjams. Afternoon traffic headed from Marin County to Vallejo can back up for miles as two lanes narrow to one. Some impatient drivers try to skip ahead by getting in the fast-moving left-hand turn lane for Highway 121. They turn left onto Highway 121, and then make a U-turn in the race track's driveway before heading back the way they came and turning left onto Highway 37.

That maneuver has caused at least one crash and many near misses for the 70 people who work at the raceway and the nearly 350 who work in a motorsports industrial park there.

Raceway employee Colin Monahan was leaving work one evening this spring when a driver making a U-turn ran into the back of his new truck.

He wasn't injured; neither was his truck. But the other car had to be towed away, he said.

"I was cautious to begin with," he said. "But now when I see those folks turning I let them go ahead and save their five minutes off their commute."

The situation has developed over the past three years, Page said, calling it a symptom of a larger problem: increasing traffic on Highway 37.

In the evening, the road fills with people who work in San Francisco or Marin County but live in Solano and Napa counties.

"Congestion has gotten so bad that people are just doing anything they can," said raceway spokesperson Jennifer Imbimbo. She added that drivers are often very frustrated by the time they resort to cutting traffic, making their behavior even more erratic.

Page said he understands drivers' motivation.

"I got stuck Monday coming home from the airport," he said, adding that it took him more than an hour to travel from Novato to Sonoma.

"I can appreciate drivers' mentality," he said.

But faced with a growing safety concern, he began talking to the CHP nearly a year ago. For about two months, officers patrolled the area and pulled people over. But the enforcement did little to curb the problem, he said. So now, he's working with Caltrans and the CHP to install the concrete barrier, which the raceway already owns.

"We see this as a definite traffic safety concern," said CHP spokesperson Anna Paulson. "We appreciate the raceway's efforts to hopefully curb this illegal activity."

The concrete barrier will begin at Highway 121 and continue up to the raceway's main entrance gate. It will force drivers to sign in at the gate before turning around and returning to the highway. It is set to go up Dec. 2, but the raceway is trying to get out the word in advance.

CHP officers will also be patrolling the area to prevent drivers from making illegal turns on the highway itself.

Whether or not the concrete barrier puts an end to U-turns on Highway 121, the underlying problem of Highway 37 congestion remains, Paulson and Page agreed.

"There's been talk of widening Highway 37 for years, and I try to stimulate that talk whenever I can," Page said.

Caltrans Spokesman Allyn Amsk acknowledged that traffic on the road was an issue.

In the short-term, his agency is reviewing whether changing traffic signal timing might ease congestion. But, he noted, a recent review found that the traffic signal was probably near capacity during commute hours.

Caltrans will be talking about long-term solutions with the Sonoma County Transportation Authority, Amsk said. But solutions will likely be expensive because the low-lying road passes by the environmentally sensitive wetlands of San Francisco Bay. And, Amsk added, any improvements would have to compete for funds with other Sonoma County transportation projects.

"We're aware of the problem and we're working on a solution," he said.