im a bus driver, if someone made a complaint about me, the absolute minimum that would happen would be an interview and threatened with the sack, and thats with no other evidence at all, apart from the complaint.
I reported a bus driver for badly overtaking a cyclist. The company figured out who the driver was and, because it was the first report, reminded him of his responsibilities and put it on file. They also said he'd be targeted by undercover inspectors to check his driving.
I also reported a car that aggressively overtook me, to the police. They couldn't do anything directly, but did a search on the reg number and placed the incident on file. They said another report would trigger a visit.