Is it legal to refuse to testify?!...?

I witnessed (sole witness) a car accident involving a nice well-mannered person and a nasty mean person who is also some big city official. I think the good guy was at fault. However, I told the cops that I am not going to say anything and they can all go to hell. Can they legally make me speak? Because if they do I am going to say it is the nasty guy's fault...