My attorney. They can speak to the police on my behalf, which would likely be necessary in the first scenario (though it would yield no benefit to me).
In the latter case, I would not call them at all.
What good would police contact do in either of those cases? In each instance they cannot help me.
If someone tried to run me down with a car, what good can the police do? Arrest the person before they try to kill me again, that's what. Keeping me from dying is definitely "helping me".
Unlikely. In the case you describe without additional evidence they would have no basis for an arrest (otherwise you could have anyone arrested arbitrarily by accusing them of same). The police would likely only involve themselves if they actually succeeded in running you down. Even if there were video surveillance available they aren't going to go get a warrant and review the tapes because some guy came in accusing someone. It's simply not how that works.
The police (in the USA) are reactive and respond after crimes are committed, not before (unless they are responding to social disorder requests, which is generally unrelated to crime). Even then they choose which crimes they deem worthy of responding to (the classic example being rape victims being dissuaded from proceeding with making a police report).
Most people have a mistaken mental model of how police actually function. Most police officers are about as non-proactive as you can get (except near the end of the month when the traffic ticket quotas are due).
In the latter case, I would not call them at all.
What good would police contact do in either of those cases? In each instance they cannot help me.