Police said an altercation between two men stemmed from a traffic incident on Terminal Avenue near Thornton Street on Feb. 12.
The video shows one driver getting out of his vehicle and punching another driver, who was in his car, through a rolled-down window.
Several witnesses saw the altercation, and a police officer who was across the street on another matter was called.
One of the witnesses, Norman Rydings, said his attention was drawn to some yelling and screaming outside of his passenger door and he saw the man take a swing at the driver.
Rydings said he left his van to try to defuse the situation and determine why the incident happened.
A former hockey captain, Rydings said he just followed his instinct and stuck up for somebody who was in need.
"We got lucky with a cop just nearby, so it's funny how that is, when you need a cop, he's right there," he said.
Police have arrested a 26-year-old man, who cannot be identified until charges have been laid.
Sgt. Randy Fincham said the dash cam video will be part of the police investigation.
"It's unfortunate in a case like this where we're looking at a person now facing criminal charges in relation to a lapse in judgment or a momentary aggressive act toward another driver," he said.
"Fortunately we don't see a lot of events where somebody does get frustrated to the point, allegedly, that they do assault someone on the streets."