After making a precautionary landing on a two-lane highway and adding oil to the airplane’s engine, the pilot attempted to takeoff from the highway. The local police provided vehicle traffic control and blocked off the amount of highway the pilot estimated he would need for the takeoff. According to the pilot’s report, he was reaching takeoff speed when he realized he would not be airborne in time to clear a police vehicle. He altered his takeoff ground roll to the right in an attempt to avoid the vehicle. The right wing of the airplane impacted a highway sign. The airplane continued off the highway to the right and down an embankment. It nosed over and came to rest inverted, resulting in structural damage to the wings and the vertical stabilizer. The pilot reported that there were no mechanical anomalies with the airplane or its engine during the takeoff attempt.