US Bridges

M-21 over Flat River

Fair

Kent County, Michigan · In Lowell (118 W Main St) · Structure no. 000000000004855

Built
1941
84 years old, rebuilt 2010
Condition
Fair
inspected October 2024
Type
Box Beam or Girders - Single or Spread
Prestressed Concrete
Length
73 ft
2 main spans
Daily traffic
18,836
2007 count
Owner
State
State Highway Agency

About this bridge

M-21 over Flat River is a prestressed concrete box beam or girders - single or spread carrying M-21 over Flat River in Kent County, Michigan. It was built in 1941 and reconstructed in 2010, so it is 84 years old, older than the Michigan average of 48.3 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 18,836 vehicles a day, roughly 4% of them trucks, projected to reach 14,750 by 2030. The structure is 73 ft (22.1 m) long and 77 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

Open in Google Maps
CarriesM-21
CrossesFlat River
LocationIn Lowell (118 W Main St)
RouteState Highway 21
CountyKent County
StateMichigan
Coordinates42.93417, -85.33929
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.
Superstructure8 Very GoodNo problems noted.
Substructure7 GoodSome minor problems.
Channel and channel protection7 GoodSome minor problems.
Last inspectionOctober 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourCountermeasures installed to correct a previously existing problem
Structural evaluation7: Better than present minimum criteria
Deck geometry2: Basically intolerable, requiring high priority of replacement
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating107.1 US tons
Inventory rating85.8 US tons

Structure

Structure typeBox Beam or Girders - Single or Spread
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans2
Design loadGreater than HL 93
Structure length73 ft (22.1 m)
Longest span34 ft (10.4 m)
Deck width77 ft (23.4 m)
Roadway width48 ft (14.6 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic18,836 (2007)
Projected traffic14,750 by 2030
Trucks4%
Lanes on structure4
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed1.2 mi (2 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 M-21 over Flat River built?

It was built in 1941, making it 84 years old in 2025. It was reconstructed in 2010.

What condition is M-21 over Flat River 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 October 2024.

How much traffic does M-21 over Flat River carry?

About 18,836 vehicles per day (2007 count), of which roughly 4% are trucks.

How long is M-21 over Flat River?

The structure is 73 ft (22.1 m) long with a longest span of 34 ft (10.4 m) and a deck 77 ft wide.

Who owns M-21 over Flat River?

It is owned by the State Highway Agency.