A Gwent Police officer shunted into another driver's car in a road rage row at the M4 bridge toll booth, a misconduct hearing was told.
PC Dean Harris is accused of grabbing the wing mirror of the Mercedes while hanging out of his car window.
The hearing was told he had accused the driver of hitting his wing mirror on approaching the tolls on the Prince of Wales Bridge on 9 November 2018.
The 47-year-old denies breaching standards of professional behaviour.
The four-day hearing at the force's headquarters in Cwmbran heard the Mercedes driver, known as Mr W, heard a "bang" before he saw PC Harris pull alongside him shouting aggressively.
Jonathan Waters, representing the force, said PC Harris was "incensed" by the alleged collision and leaned out of his window to grab the driver's mirror.
The hearing was told he repeatedly used his car horn to intimidate the victim and bumped the back of the Mercedes.
It is also claimed PC Harris, who is based in Blackwood near Caerphilly, got out of his car to confront Mr W, shouting continuously and aggressively towards him.
Giving evidence, Mr W said there were "three or four distinct lanes" of traffic as he drove through the tolls into Wales on the M4.
He had let one driver into his lane but saw a red Vauxhall, driven by off-duty PC Harris who was driving his parents home from holiday, on his passenger side.
He said the officer kept shouting that his mirror had been smashed.
Mr W said: "He was grabbing my mirror and he was rocking it back and fore and he was swearing.
"The man was so angry he was trying to damage my car and I moved forward so he couldn't do any more damage."
The driver said he then felt an impact as PC Harris was behind him and heard a "slight thud".
"It was going through my mind that he was doing his best to inflict damage to my car," he said.
The hearing continues.