Roundabout To Improve Busy Sonoma County Junction At Highways 116/121

SONOMA COUNTY, CA — Work is set to start on a roundabout project where state Highways 116 and 121 intersect southwest of the city of Sonoma.

Caltrans and the Sonoma County Transportation Authority held a groundbreaking ceremony Thursday for the nearly $27 million project to replace a four-way stop intersection with a roundabout.

As traffic has increased at this junction of State Routes 116 and 121 and Bonneau Road, it has been accompanied by increased congestion and broadside collisions. To make it safer for the traveling public, Caltrans and the SCTA studied two alternatives and chose the roundabout over a signalized intersection.

The roundabout will improve traffic flow and safety, Caltrans said. The project will also add bicycle lanes, sidewalks and crosswalks for those who walk and bike in the area.

The project is powered by $19 million in funding through the State Highway Operation and Protection Program, $6 million from SCTA Measure M, and $500,000 through Sonoma County.

Local and elected officials were on hand to mark the groundbreaking.

"I am excited to join local leaders to break ground on the State Route 116/121 Roundabout Project, which will help ease traffic and greatly improve safety for motorists, cyclists and pedestrians," said U.S. Rep. Mike Thompson. "I am proud to have helped secure over $18 million in federal funding to break ground on this important project."

Sonoma County Supervisor Susan Gorin, who represents the First District, said the project was "a long time coming."

"I’ve been having conversations about this roundabout throughout my tenure as Supervisor," Gorin said. "I am so appreciative of Caltrans, Sonoma County Transportation Authority (SCTA), Sonoma County Public Infrastructure and everyone who has worked on this project that will improve traffic safety and reduce congestion in this corner of our Valley. I look forward to its completion in the not-so-distant future."

Unlike traditional intersections enforced by traffic signals, roundabouts compel motorists to slow down in all directions. Reduced speeds within roundabouts have proven to lower the rate and severity of collisions. Roundabouts also move vehicles in a circle, eliminating many conflict points usually present at a signalized intersection.

"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, district director for Caltrans in the Bay Area. "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."

Another plus: if a storm or other incident triggers a power outage, the operation of a roundabout will not be impacted because it never relies on electronic signalization.

The project is an example of Caltrans' Complete Streets Program, which adds multimodal upgrades — i.e. bike lanes and sidewalks — to new projects when feasible, according to Caltrans.

A Caltrans park-and-ride lot has been relocated a half mile to the east at the junction of State Routes 12 and 121 for the length of the project. The lot will be permanently relocated to the north of the new roundabout.

For more information about the State Route 116/121 Roundabout Project, go to:116-121 Intersection | Caltrans.