I would like to see how this bears out, to be honest. I think in order to do this, first we need to define what we mean by a street fight. I personally think the focus should be on one-on-one unsanctioned altercations:
A) in which both participants are intent on doing the other bodily harm. (as in one guy isn't spending his time trying to get away or posturing, as in a shoving match. Also included would be police trying to make an arrest or doormen dragging someone outside.)
and
B) which last beyond the initial exchange of blows, until one participant is incapacitated, concedes defeat or the fight is brought to a close through the intervention of bystanders.

By those standards I have been involved in one and witnessed 4 others in the last decade and 4 of those went to the ground. The only one that didn't was a standing knockout. So my personal experience is not too far off the high percentages tossed around.