Cell phone video shows Jaime Fonseca slashing a sign and going toe-to-toe with people protesting Azerbaijan's aggression in Armenia.
A man walked away gripping his right forearm after Fonseca slashed him too, one of two people to suffer injuries at Fonseca's hand.
Fonseca's defense in court was that he was one of the first people stuck in traffic because the protest blocked Blackstone.
"I think it does, to a certain extent, start with the decision to block traffic," said public defender Jason Westerfeld. "That's what kind of kept Mr. Fonseca there. That's what kept him there unable to leave."
Fonseca got out of his car more than once before heading for the sign with a knife and a large stick.
Prosecutors say he was not provoked in any meaningful way.
"There were a number of other alternatives that he could have exercised that did not need to result in the stabbing of two victims," said depity district attorney Alison Wilson.
After a hung jury at trial, Fonseca agreed to plead "no contest" to one count of assault with a deadly weapon.
A judge said he understood why someone would get frustrated, but the 41-year-old was responsible for the incident turning violent and there's no excuse for that.
He noted no violence in Fonseca's history, though, and agreed to give him no more than a two-year prison sentence.
"I think it would send a significantly damaging message to the community were the Court to say 'If you get stuck in a traffic jam and you're frustrated, go ahead and take out those frustrations at the point of a knife,'" said Judge Jonathan Conklin.
Fonseca has already served about eight months in jail.
The judge could've sent him to prison for less than three months, but gave Fonseca probation instead, with the understanding that if he commits another crime, he'll go to prison for two full years instead of just the remaining 77 days.