(Associated Press) Thousands of bridges around the U.S. may be one freak accident or mistake away from collapse, even if the spans are deemed structurally sound.

The crossings are kept standing by engineering design, not supported with brute strength or redundant protections like their more modern counterparts. Bridge regulators call the more risky spans “fracture critical,” meaning that if a single, vital component of the bridge is compromised, it can crumple.

Those vulnerable crossings carry millions of drivers every day. In Boston, a six-lane highway 1A near Logan airport includes a “fracture critical” bridge over Bennington Street. In northern Chicago, an I-90 pass that goes over Ashland Avenue is in the same category. An I-880 bridge over 5th Avenue in Oakland, Calif., is also on the list.

Also in that category is the Interstate 5 bridge over the Skagit River north of Seattle, which collapsed into the water days ago after officials say an oversized truck load clipped the steel truss.

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