Is it legal to videotape police officers on duty?

Yes, it is legal to videotape police officers during the performance of their duties.  If you feel that the officer has violated some rule or regulation, or someone's constitutional rights, you should definitely forward the video to that officer's superior or to the Internal Affairs Department for further investigation. 

There is nothing illegal about you video taping police officers performing their duties.  Indeed, you are permitted to walk around with a video camera and video the police activities.  You may also send the police videos that you take to the police departments to show whether the officers that you video taped are performing their jobs properly, including but not limited to whether  the police are violating the rights of drivers.  You should know, that most police departments have video cameras on the police cars and the videos show the police driving and then stopping drivers.  If you are arrested or given a citation, you may request to see a copy of the tape of the stop.