Groundbreaking on new roundabout at Big Bend

Highways 116 and 121 converge at Big Bend, just north of the Sonoma Valley Airport and surrounded by vineyards. The new roundabout should be ready in 2026.

photo credit: Caltrans
A rendering of what the new roundabout at Big Bend will look like.

Elected and local officials on Thursday broke ground in Sonoma County on a new roundabout traffic feature to replace a four-way stop intersection for state Highways 116 and 121.

Caltrans and the Sonoma County Transportation Authority held the groundbreaking ceremony for the project, which will cost nearly $27 million but hopefully improve safety and enhance use for pedestrians and bicyclists.

Highways 116 and 121 converge at Big Bend, just north of the Sonoma Valley Airport and surrounded by vineyards. The old four-way stop will be replaced with the roundabout to improve traffic flow and safety while adding bicycle lanes, sidewalks and crosswalks, according to Caltrans.

For anyone who's never driven a roundabout, traffic moves in a counterclockwise circle, with each vehicle yielding the right-of-way to any vehicles already circulating.

Roundabouts are considered safer than four-way stops for several reasons. First, they reduce the likelihood and severity of collisions because in theory, all vehicles entering the roundabout slow down first, if not stop completely. A study done by the Washington State Department of Transportation found that the risks of "t-boning" and head-on collisions decrease with roundabouts.

Caltrans said that traffic at the 116/121 intersection has become congested and broadside collisions have been increasing, prompting the need for a new approach.

"The roundabout requires slower speeds, and the absence of sharp corners will result in fewer and less severe crashes and improved traffic flow, which is exactly what this intersection needs," said Dina El-Tawansy, Caltrans Bay Area District Director. "By adding sidewalks, crosswalks, and bicycle lanes, we will encourage alternative means of travel, which has been a positive and growing trend in Sonoma County."

Roundabouts are also better for pedestrians, who generally only have to focus on one exiting vehicle at a time instead of three as they attempt to cross.

Caltrans also points out that if a storm or other incident triggers a power outage, the roundabout will still be moving traffic along because it doesn't rely on electric signals.
Sonoma County is spending $500,000 on the project, along with $19 million in funding through the State Highway Operation and Protection Program, and $6 million from county Measure M, a quarter-cent sales tax approved by voters in 2004 that goes toward transportation improvements.

"This is a long time coming - I've been having conversations about this roundabout throughout my tenure as supervisor," said Sonoma County Supervisor Susan Gorin, who represents the First District.

A Caltrans park-and-ride lot has been relocated a half mile to the east at the junction of State Route 12 and 121 for the duration of the project. The roundabout project includes permanently relocating the lot to the north of the new roundabout.