US Bridges

State Route 121 over Arroyo Seco

Fair

Sonoma County, California · 04-SON-121-8.43 · Structure no. 20 0023

Built
1923
102 years old, rebuilt 1967
Condition
Fair
inspected September 2024
Type
Tee Beam
Concrete Continuous
Length
82 ft
2 main spans
Daily traffic
19,000
2016 count
Owner
State
State Highway Agency

About this bridge

State Route 121 over Arroyo Seco is a concrete continuous tee beam carrying State Route 121 over Arroyo Seco in Sonoma County, California. It was built in 1923 and reconstructed in 1967, so it is 102 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 5 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 82 ft (25 m) long and 25 ft wide. If it were closed, the detour would be 9.9 mi (16 km).

Location

Open in Google Maps
CarriesState Route 121
CrossesArroyo Seco
Location04-SON-121-8.43
RouteState Highway 121
CountySonoma County
StateCalifornia
Coordinates38.24613, -122.43527
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
Deck5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionSeptember 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour within limits of footing or piles
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry2: Basically intolerable, requiring high priority of replacement
Waterway adequacy3: Basically intolerable, requiring high priority of corrective action
Approach roadway alignment6: Equal to present minimum criteria
Operating rating68.6 US tons
Inventory rating41.4 US tons

Structure

Structure typeTee Beam
MaterialConcrete Continuous
DeckConcrete, cast-in-place
Main spans2
Design loadHS 15
Structure length82 ft (25 m)
Longest span40 ft (12.2 m)
Deck width25 ft (7.7 m)
Roadway width25 ft (7.5 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic19,000 (2016)
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 Arroyo Seco built?

It was built in 1923, making it 102 years old in 2025. It was reconstructed in 1967.

What condition is State Route 121 over 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 Arroyo Seco carry?

About 19,000 vehicles per day (2016 count), of which roughly 7% are trucks.

How long is State Route 121 over Arroyo Seco?

The structure is 82 ft (25 m) long with a longest span of 40 ft (12.2 m) and a deck 25 ft wide.

Who owns State Route 121 over Arroyo Seco?

It is owned by the State Highway Agency.