US Bridges

Middle RD over Estero De San Antonio

Fair

Marin County, California · 4.5 MI from S.R. 1 · Structure no. 27C0124

Built
1958
67 years old
Condition
Fair
inspected September 2024
Type
Slab
Concrete Continuous
Length
196 ft
6 main spans
Daily traffic
40
1973 count
Owner
County
County Highway Agency

About this bridge

Middle RD over Estero De San Antonio is a concrete continuous slab carrying Middle RD over Estero De San Antonio in Marin County, California. It was built in 1958, so it is 67 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 40 vehicles a day, projected to reach 111 by 2046. The structure is 196 ft (59.7 m) long and 28 ft wide. If it were closed, the detour would be 6.2 mi (10 km).

Location

Open in Google Maps
CarriesMiddle RD
CrossesEstero De San Antonio
Location4.5 MI from S.R. 1
RouteCounty Highway 0
CountyMarin County
StateCalifornia
Coordinates38.26393, -122.92732
Functional classRural Local

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.
Substructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Channel and channel protection7 GoodSome minor problems.
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 geometry6: Equal to present minimum criteria
Waterway adequacy7: Better than present minimum criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating60.4 US tons
Inventory rating36 US tons

Structure

Structure typeSlab
MaterialConcrete Continuous
DeckConcrete, cast-in-place
Main spans6
Design loadHS 20
Structure length196 ft (59.7 m)
Longest span32 ft (9.8 m)
Deck width28 ft (8.5 m)
Roadway width28 ft (8.4 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic40 (1973)
Projected traffic111 by 2046
Trucks—
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed6.2 mi (10 km)
TollOn a free road
National Highway SystemNo

Ownership and planned work

OwnerCounty Highway Agency
Maintained byCounty 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 Middle RD over Estero De San Antonio built?

It was built in 1958, making it 67 years old in 2025.

What condition is Middle RD over Estero De San Antonio 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 Middle RD over Estero De San Antonio carry?

About 40 vehicles per day (1973 count).

How long is Middle RD over Estero De San Antonio?

The structure is 196 ft (59.7 m) long with a longest span of 32 ft (9.8 m) and a deck 28 ft wide.

Who owns Middle RD over Estero De San Antonio?

It is owned by the County Highway Agency.