Freeway onramps in Sonoma County are getting a splash of color as transportation officials begin switching on the metering lights that will soon require motorists to hit the brakes before merging.

Caltrans Tuesday morning activated the southbound onramp traffic signals along Highway 101, many of which have sat idle since they were installed more than a decade ago. The lights, some of which were switched on Monday, will be set to solid green for the first week so that drivers can get used to them, officials said.

“The staged implementation will begin with solid green, prompting the public that the ramps are going to be metered,” said James Cameron, deputy director of the Sonoma County Transportation Authority, who presented the metering plan to the agency’s board Monday.

Starting Sept. 16, the lights will cycle through red and green during the morning commute from 6:30 a.m. to 10 a.m. A week later, they will also be active for the afternoon commute from 3 p.m. to 7 p.m.

The northbound metering lights will follow a similar schedule starting Oct. 7.

Caltrans began installing metering lights in 2001 as part of the Highway 101 widening project. The Metropolitan Transportation Commission mandated that metering lights be activated, and 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 key message is that it is managing the system to maximize its efficiency,” said Suzanne Smith, executive director of the transportation authority.

Metering lights are already used on other Bay Area freeways and have drastically reduced congestion, officials say. 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 cut by 52 percent.

Lights were installed at most onramps from Windsor to Petaluma as crews added carpool lanes through Sonoma County. The first phase of activation will include the 25 southbound onramps from Arata Lane in Windsor to Pepper Road just north of Petaluma. The northbound onramps between Highway 116 in Cotati and Shiloh Road in Windsor will be metered.

To ensure traffic does not back up onto city streets, sensors under the road will detect when traffic backs up and will trigger a green light if the line gets too long.

“After studying areas of concern, we will make sure that freeway ramp metering does not affect local streets,” Cameron said.

Caltrans spokesman Allyn Amsk said the agency will monitor traffic conditions and adjust the ramp meters as appropriate. Drivers entering the freeway where onramp meters are activated should stop at the red light and wait until it turns green before merging, he said.

Cotati Councilman Mark Landman, who is also a director of the transportation authority, said the lights will take some getting used to, but ultimately they will help solve the county’s traffic woes.

“I’m looking forward to seeing how they work,” he said. “Anything that helps congestion on the highways is a good thing.”