A backhoe being hauled by a flatbed, and apparently riding too high, hit the support under the Carroll Canyon overpass and tumbled onto the roadway about 12:30 p.m., according to the California Highway Patrol.
No injuries were reported, though witnesses said at least one vehicle struck the excavating machine after it fell onto the interstate.
Crews closed a stretch of southbound lanes to allow for repairs and cleanup. Motorists were detoured around the accident site. The freeway reopened at 2:40 p.m. after a big-rig tow truck righted the backhoe and loaded it back onto the flatbed.
The overpass, which has a 15-foot clearance over the interstate, apparently sustained no significant damage, Caltrans spokesman Edward Cartagena said. The span is under construction as part of a project that will add carpool lanes to I-15 in the area.
Last Tuesday afternoon, a tractor-trailer struck the same bridge, dislodging a metal I-beam above the lanes and forcing a two-hour closure that snarled traffic for miles to the north.
Truckers with vehicles or loads that may be too large to clear overpasses are required to obtain a permit from Caltrans and plan a route with tall enough bridges, Cartagena noted.