Traffic to Shift onto Temporary Roundabout on the State Route 116/121 Roundabout Project in Sonoma County

Published: Nov 10, 2024

Transition from a four-way stop to a roundabout begins Tuesday, November 12

Sonoma County- After six months of construction on the State Route 116/121 Roundabout Project, Caltrans is ready to eliminate the current four-way stop intersection and replace it with a temporary roundabout by 5:00 a.m. on Tuesday, November 12, 2024.

The temporary roundabout will handle traffic in various configurations as construction continues on the project, which is scheduled to be completed by the end of 2025.

Single nighttime lane closures from 9 p.m. to 5 a.m. will be in effect on the nights of Friday November 8, and Monday November 11 in preparation. At least one traffic lane will remain open in each direction at all times. All work is weather dependent.

The temporary roundabout will generally follow the outline of the final roundabout. Still, it will lack several components of the completed project, including an additional travel lane, sidewalks, bike and pedestrian striping, and a park-and-ride.

Also, the bypass lane connecting northbound Route 121 to eastbound Route 121 will be reopened, significantly reducing congestion. However, this lane will temporarily close again in spring 2025 for a brief period to complete the sidewalk and pedestrian facilities of the final configuration.

Caltrans urges motorists to drive slowly and expect delays as drivers adjust to the new configuration.

Caltrans and the Sonoma County Transportation Authority (SCTA) are partners on the State Route 116/121 Roundabout Project. The $26 million project replaces a four-way stop intersection with a roundabout to improve traffic flow and safety while adding bicycle lanes, sidewalks, and crosswalks to enhance the safety and travel experience for people who walk and bike in the area.

Four-way stop configuration.

Zoomed-in look

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 address this, Caltrans and the SCTA studied two alternatives and chose the roundabout alternative over a signalized intersection.

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

^{finalized roundabout will improve traffic circulation, reduce congestion and collisions, and provide sidewalks and bicycle improvements}

Should a storm or other incident trigger a power outage, the operation of a roundabout will not be impacted because it never relies on electronic signalization.

This project is an example of Caltrans' Complete Streets Program, which adds multimodal upgrades - such as bike lanes and sidewalks - to new projects when feasible

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.

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

For real-time information, please visit Caltrans QuickMap: https://quickmap.dot.ca.gov or follow Caltrans on Twitter at: https://x.com/CaltransD4

For more information about transportation and other infrastructure projects funded with state and federal investments, visit RebuildingCA.ca.gov and Build.gov.ca.

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