Why do the police in Batman Beyond all have a 5 on their uniforms?
I'll have to dig through the commentaries to confirm this, but according to this post, someone claims:
My friend also suggested that it's a pun: a five in a circle meaning 5-O
Yea its 5-O I heard it on the commentary on one of DVDs.
The more prevalent theory is that it denotes the precinct they serve (though, why they're all out of the 5th precinct would still be a mystery).