Sonoma County motorists merging onto the freeway have zipped past idle metering lights for more than a decade without noticing. But when transportation officials switch them on next year, the on-ramp traffic signals will give many drivers pause.

Caltrans began installing metering lights in 2001 as part of the Highway 101 widening project. Earlier this year, the Metropolitan Transportation Commission mandated that metering lights be activated. The regional transportation body could suspend discretionary funding for noncompliance.

Officials say the lights, which allow cars onto the freeway at preset intervals, will help with traffic flow.

"The global reason why this is necessary is to use technology to get the best performance from our infrastructure," said James Cameron, deputy director for projects at the Sonoma County Transportation Authority.

Metering lights are already used on other Bay Area freeways and have drastically reduced congestion, Cameron said. The signals reduced travel time by 19 minutes on a 10-mile stretch of Highway 101 in San Mateo County. On Highway 85 in San Jose, travel time was reduced by 52 percent.

Lights were installed at most on-ramps from Windsor to Petaluma as crews added carpool lanes through Sonoma County. The widened on-ramps also have carpool lanes, which could be incorporated into the metering light rollout.

Officials will switch on the lights as early as March, Cameron said. Until then, staff have been studying traffic patterns to determine the timing of the meters and the hours of operation. The meters could be used during carpool hours, Cameron said.

One of the biggest concerns is making sure on-ramp traffic does not back up onto city streets, Cameron said. Sensors under the road will detect when traffic is piling up and will trigger a green light if the queue gets too long.

"It will not back up onto local streets," Cameron said.

Lighter highway traffic would be a huge relief to travelers, especially in the more densely populated area of the county from Santa Rosa south, said Rob Sprinkle, Santa Rosa traffic engineer. But drivers will need to get accustomed to stopping at a red light before merging onto the freeway, he said.

"You can't judge success on Day One," Sprinkle said. "There will be a learning curve. There's going to be a change of driving habits."

Not all jurisdictions are welcoming the metering light rollout. Officials in Windsor, the north end of the widened freeway, have questioned the need for the signals.

"I don't really see the need for it right now," said Richard Burtt, Windsor public works director. "Traffic is pretty light when you get up this far."

At the southern end in Petaluma, where freeway widening work is still ongoing, the lights won't be switched on until at least 2016, said Larry Zimmer, Petaluma engineering manager.

"It doesn't impact Petaluma," he said. "We don't really have any skin in the game."

After the lights are turned on in March, officials will monitor their impact and make adjustments to the metering rates as needed, Cameron said. A study to identify any problems will be conducted in May before schools are out for the summer and traffic is typically lighter.

Some drivers, like John Wood, who commutes daily on Highway 101 from Santa Rosa to Richmond, can't wait to see the lights switched on.