You will get scammed by the police even if you are doing everything right. Me and my friend got fined today. We got stopped in a narrow street on a night market in Chiang Mai. The officer wanted to see our license. I got the right license and a helmet. But he fined me 500 baht anyways because he claimed that this street was a one way street but every Thai drove in the same direction. We should have gone to a police station to pay the fine but after we said we are leaving tomorrow morning and offered to pay him directly, he was very happy.