Given the character is being advised to join the police prior to the phrase, I would say it is exhorting him to some kind of action, related to it being timely to act; the exclamation marks imply and imperative tense imply this.
I would therefore say this equivalent would best convey the meaning in the context of the dialogue: 'Stand up and be counted!'
For reference, the meaning of this idiom (taken from the free dictionary) is: 'to state one's support (for someone or something). If you believe in more government help for farmers, write your representative—stand up and be counted. I'm generally in favor of what you propose, but not enough to stand up and be counted.'

Another loose alternative would be 'Carpe diem' but this doesn't convey the sense of bravery linked to the act of joining the police that I think is intended.

Joint the traffic police and wield your baton (as much as you wish)

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Note added at 1 hr (2015-12-23 12:07:41 GMT)
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oops, I meant 'Join the traffic police..', not 'joint'

"Why don't you/Go get yourself a badge and catch some crooks instead!"