A POLICE sergeant and two constables were injured when they were knocked down by a car while investigating a traffic accident in Clearwater Bay Road near Cheng Lan Shu early this morning. The officers were taken to the United Christian Hospital where they were treated. Their injuries were described as not serious. The incident occurred at 12.15 am when a police vehicle arrived at the scene of a traffic accident. As the sergeant and two officers alighted from their vehicle to talk to the car driver, another car came along the road and knocked the officers down.