The incident happened at about 11:30 a.m. when CHP Officer Marco Castillo was stopped on the right shoulder of northbound state Highway 85 just north of Dana Street, CHP spokeswoman Officer Grace Castillo said. The two are not related.
The "large" piece of asphalt thrown through the left rear window was believed to have come from the Dana Street overcrossing, according to Grace Castillo.
She said Marco Castillo was uninjured but went off-duty after the incident.
Police believe the act was intentional after a search of the area didn't turn up any other debris, she said.