For every parade I’ve been to, it’s not just the parade route that is blocked off, but nearby streets as well. There are signs, barriers, police cars, etc. all stationed several streets over in all directions from the actual route itself. The police cars have their blue lights activated and some officers are directing traffic. I just have a hard time seeing this happening as an accident. The person would have noticed that he was entering a parade way before he actually encountered the marchers.
I have been in several parades. At some the DPW dump trucks and snow plows are used as barricades at all the access points and side streets. And at most, the police check my ID and inspect the vehicle (one town demands that I show that I know the road hand signals, left-right-stop and have me touch my nose and walk a line!)
They have video of the suspect vehicle crashing over street barriers and no doubt the cops were close by. It’s a parade, crowd/traffic control etc.