TxDOT says a heavy truck somehow hit a sign bridge that goes over I-45 near Highway 6, knocking it into the southbound lanes.
The cat walk from the sign bridge fell and hit a passing car, TxDOT said.
There were no reports of injuries. TxDOT said it believes the suspect vehicle did not stay at the scene.
As of 11 a.m., most of the debris was moved to the shoulder, but it was still causing delays.
TxDOT says it could take multiple hours to completely reopen the southbound lanes. Only one lane was open as crews worked to clean up the debris.