There were no injuries from the minor collisions, police said.
Teena Franzen, the mother of a Sabula police officer, took video of the mayflies while riding in his patrol car Saturday night. The video was later posted to YouTube.
"I've lived in Sabula since '75, I've never seen that many before, ever," Franzen told WQAD-TV.
Sabula Police Department Reserve Officer Stephen Thayer said the mayflies were "piled knee-deep" in the roadway.
"We had Iowa [Department of Transportation] came in with a snow plow and actually plow them off and then sand it because it was still pretty slippery. And then Illinois plowed their side," Thayer said.
The Iowa DOT said the lights on the bridge will be kept off during the next few weeks to discourage the mayflies from gathering.