As other people have pointed out, he might just be having them on. If he's a legit police officer, then he could arrest them if he could get them into a public area with more police officers around. He's a professional con-man, I wouldn't believe any emotion he has or anything he says. He flat-out stated he knows she goes around cosplaying as a cop, which means she's been tailed and they know what she's done. It's not possible for them to have followed her like that and not known what she's capable of, since she only dresses up as a cop to school other thugs and biker gangs, so he shouldn't be shocked by the level of driving skill she has. But he is. And I think that's a lie. He's leading them into a trap, I'll bet you anything.
Also, I agree with
@isoycrazy . I don't know if that's how this arc is going to end, but if it actually does end with everyone beating the shit out of each other and only Ranna is left, having never fought anyone at all, I would laugh my ass off. That would be the best way to resolve that prophecy. Though the mangaka hasn't done me wrong yet, so by all means, I want him to surprise me. But that would be fucking great if that's how this went.