"Do you know how fast you were going?"
Or, in my case a couple weeks back, "any reason you were going so fast?"
You don't have to answer. I didn't (though I may have babbled some irrelevancies while digging out the paperwork). It's actually quite a mental & social breakthrough to realize you don't have to answer people's questions.
As for being a victim or witness, still best to keep statements very short, as in "that man assaulted me, there's the stick he hit me with, and that woman walking away saw it, I want to press charges and I need to talk to a lawyer" or "I witnessed the attack, here's my contact information". You say what you must to establish legitimate victimhood ("she stole my cocaine" isn't), identify perpetrators and witnesses, and identify probable perishable evidence.
Then.
Shut.
Up.
The only thing to say at this point is "am I free to go?" - repeatedly, if necessary. They'll say & do darned near anything, but at some point they have to either arrest you or not stop you from leaving.