Going to Napa in the evening? Plan to leave earlier if you’re taking Highway 121.

A $6.5 million Caltrans construction project to smooth out a bit the curves east of the Sonoma County line is creating long backups in the evenings, and they’re going to last awhile longer.

Caltrans spokesman Marcus Wagner said the traffic delays caused by one-way traffic controls typically are five to 10 minutes, although Road Warrior readers have reported waits of up to 30 minutes.

Andrew Smith of Santa Rosa, who was on his way to a concert Friday night in Napa, said some drivers tired of the wait and did U-turns to head back toward Sonoma.

He said he didn’t see any signs along 121 warning of possible delays until he encountered the backup at the Sonoma-Napa county line. Despite stopping for a half-hour, Smith said he arrived at the concert in time, “so Caltrans is forgiven.”

But Peter Belport of Petaluma wasn’t so forgiving after being held up for 20 to 30 minutes recently as he returned from a Napa restaurant.

“It’s inconsiderate” and “insulting” to have such long delays on such a key highway, he said.

Wagner said the traffic delays occur at night when crews are working 7 p.m. to 5 a.m. Sunday through Friday. Daytime work during is off the highway, and should not cause delays, he said.

The night work will take a break Oct. 17 to 30, resuming Oct. 31 weather permitting, he said.

The 1.7-mile construction project, from the county line to Duhig Road, started in June 2009 and is scheduled to be completed next March. Crews are trying to speed up their work to beat the winter rains, Wagner said.

He said Caltrans has message signs on 24 hours a day to alert motorists of the night work, its duration and recommendation to take an alternate route.

But there are no good alternates. You can take Mark West Springs Road north of Santa Rosa over to Calistoga and then south to Napa. There’s Trinity Road near Glen Ellen, but that would be a bit daring at night. And there’s Highway 37 to the south, but there’s really no time savings there. So, it might be best just to leave early.