You can ready up the second you get into an empty lobby, but I don't see any way to just simply choose single player.
That means that if their matchmaking system is down, you don't get to play. And funny enough, their matchmaking system was down all night last night.
My overall largest gripe though is that in PD2, I love how if you are accurate with headshots, helmets pop with a single tap and you can just mow through 30 cops in seconds. In PD3, its taking me 2 to 5 headshots after the first wave. On the easiest setting.