June 12, 2014 5:53 AM EDT

Every day, roughly 90,000 vehicles cross the Interstate 495 bridge over Delaware’s Christina River to bypass downtown Wilmington en route to Philadelphia, New York City and Baltimore. Yet officials had no idea those drivers were facing serious danger until May 29, when an engineer who happened to be working nearby noticed two of the span’s support pillars tilting.

It was a lucky catch, and Delaware authorities acted on it, closing the bridge, fast-tracking repairs and ordering inspections of other spans. But with thousands of bridges in similar states of disrepair across the U.S., counting on serendipity is little comfort. More than 1 in 10 of the nation’s 608,000 bridges are structurally deficient, according to the U.S. Department of Transportation, which means they’re considered safe but in need of significant repairs. Fixing all the nation’s bridges would cost an estimated $106 billion, yet the wallet for it is nearly empty. The department says the federal fund for road and bridge projects will be depleted by September. (Congress is considering proposals to keep the spigot open.)

In Delaware, at least, help is on the way. Officials believe a 50-ton mound of dirt that had been dumped next to the bridge caused the pillars to tilt. They’ve already cleared the pile and are stabilizing the structure in the hope of reopening it by Labor Day. But thousands of other vulnerable bridges are still waiting for attention.

AGING ICONS

BROOKLYN BRIDGE

New York City

Year opened: 1883

Cause for concern: Possibly the nation’s best-known bridge, the span connecting Brooklyn and Manhattan is considered “structurally deficient” in part because of outdated access ramps–a problem the city is spending some $500 million to fix.

GOLDEN GATE BRIDGE

San Francisco

Year opened: 1937

Cause for concern: The orange-hued Bay Area landmark’s older features, such as narrow lanes and a lack of traffic shoulders, earn it “functionally obsolete” status, a grade above structurally deficient.

LIBERTY BRIDGE

Pittsburgh

Year opened: 1928

Cause for concern: A recent inspection called the condition of the aging steel span over the Monongahela River “basically intolerable.” The state DOT has lowered the weight limit, barring some heavy trucks and construction vehicles.

WITH REPORTING BY EMILY MALTBY; SOURCES: DOT; DELDOT; GOLDEN GATE BRIDGE HIGHWAY AND TRANSPORTATION DISTRICT; NYC DOT; PENNDOT

This appears in the June 23, 2014 issue of TIME.

Write to Noah Rayman at noah.rayman@time.com.

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