Police say the officer was blocking traffic for a car that was stalled on the south beltway feeder on Wayside Drive at about 3 a.m. on Thursday.
During that investigation, police say an off-duty Pasadena officer driving his truck rear-ended the officer's patrol car while the officer was inside.
The off-duty Pasadena officer was also taken to the hospital. Police are looking into whether or not the officer was impaired.