Look i think the scene is awesome, you don't, no biggy. But the purpose of it is to show Joker escaping in a blaze' way, he just don't give a crap. Joker in the comics always has elaborate ways to rob banks, kill people or escape jail. And you can't get any more elaborate than escaping jail in COP CAR with your head hanging out of the window like a dog. It's just showing that Joker is a master planner, that he WANTED to be caught so he could get to Lau. I also think it shows how crazy he is. He's just blown up a major police HQ probly killing a lot of cops in the process and he is just relaxing letting the wind hit his face as he escapes. It's like he's in a little world of his own, oblivious to the chaos he's caused. I think the silence represents that he is truly in his own little world, he doesn't care about anything around him.