Unless the other driver hits you, with witnesses willing to testify, the cops won't do anything. In my experience they won't do anything even if they witness what happened. They are only there to notify the next of kin and arrange for the wreck to be towed. Expecting them to enforce the laws of the road, when it is the least bit inconvenient for them, is like expecting them to do something about home invaders in your house. If you’re not willing to deal with it yourself it won’t happen, certainly not in time to do you any good. The only reason to involve them is when it’s required to file an insurance claim.