US Bridges

State Route 121 over Branch Arroyo Seco

Fair

Sonoma County, California · 04-SON-121-8.51 · Structure no. 20 0121

Built
1922
103 years old
Condition
Fair
inspected September 2024
Type
Slab
Concrete
Length
22 ft
1 main span
Daily traffic
19,000
2014 count
Owner
State
State Highway Agency

About this bridge

State Route 121 over Branch Arroyo Seco is a concrete slab carrying State Route 121 over Branch Arroyo Seco in Sonoma County, California. It was built in 1922, so it is 103 years old, older than the California average of 54.8 years.

The most recent inspection reported to the Federal Highway Administration rates it in fair condition with a lowest component rating of 6 out of 9. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

It carries about 19,000 vehicles a day, roughly 7% of them trucks, projected to reach 27,636 by 2046. The structure is 22 ft (6.7 m) long and 27 ft wide. If it were closed, the detour would be 9.9 mi (16 km).

Location

Open in Google Maps
CarriesState Route 121
CrossesBranch Arroyo Seco
Location04-SON-121-8.51
RouteState Highway 121
CountySonoma County
StateCalifornia
Coordinates38.24618, -122.4327
Functional classRural Minor Arterial

Condition and inspection

Fair

The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

ComponentRatingMeaning
Deck6 SatisfactoryStructural elements show some minor deterioration.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure8 Very GoodNo problems noted.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionSeptember 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation6: Equal to present minimum criteria
Deck geometry2: Basically intolerable, requiring high priority of replacement
Waterway adequacy3: Basically intolerable, requiring high priority of corrective action
Approach roadway alignment8: Equal to present desirable criteria
Operating rating44.6 US tons
Inventory rating26.8 US tons

Structure

Structure typeSlab
MaterialConcrete
DeckNot applicable
Main spans1
Design loadUnknown
Structure length22 ft (6.7 m)
Longest span20 ft (6.1 m)
Deck width27 ft (8.2 m)
Roadway width27 ft (8.2 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic19,000 (2014)
Projected traffic27,636 by 2046
Trucks7%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed9.9 mi (16 km)
TollOn a free road
National Highway SystemNo

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusNot eligible for the National Register of Historic Places
Work proposedNone recorded
Estimated improvement cost—
NavigationNo navigation control on waterway

Frequently asked questions

When was State Route 121 over Branch Arroyo Seco built?

It was built in 1922, making it 103 years old in 2025.

What condition is State Route 121 over Branch Arroyo Seco in?

The National Bridge Inventory rates it in fair condition. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound. The last reported inspection was in September 2024.

How much traffic does State Route 121 over Branch Arroyo Seco carry?

About 19,000 vehicles per day (2014 count), of which roughly 7% are trucks.

How long is State Route 121 over Branch Arroyo Seco?

The structure is 22 ft (6.7 m) long with a longest span of 20 ft (6.1 m) and a deck 27 ft wide.

Who owns State Route 121 over Branch Arroyo Seco?

It is owned by the State Highway Agency.