The pandemic has also seen record increases in stress, gun sales and shootings.

After a speeding driver cut her off abruptly on a Californian highway in May, Joanna Cloonan gestured rudely towards the car. A passenger grabbed a pistol and fired at her vehicle, killing her six-year-old son in the back seat.

A woman in Texas was shot in the back last week while shielding her seven-year-old daughter from gunfire toward their vehicle, and another driver in Kentucky is recovering from gunshot wounds sustained after an argument involving a parking space.

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Road rage incidents involving a gun have been responsible for record levels of injury and death in the United States since the start of the Covid-19 pandemic, according to a recent report from Everytown for Gun Safety, a nonprofit that advocates against gun violence.

Activists attend a rally to protest the verdict in the Kyle Rittenhouse trial in Chicago, Illinois. AFP

Law enforcement in Portland declared a riot Friday night as about 200 demonstrators protested the acquittal of a teen who killed two people and injured another in Wisconsin.

The protesters were breaking windows, throwing objects at police and talking about burning down a local government building in downtown Portland, the Multnomah County Sheriff’s Office said. The crowd had dispersed by about 11 p.m., KOIN TV reported.

The Portland Police Bureau said several people were given citations, but only one person who had an outstanding warrant from another matter was arrested.

The protesters gathered following the acquittal of Kyle Rittenhouse, 18, in Kenosha, Wisconsin. Rittenhouse killed two people and injured another during a protest against police brutality in Wisconsin last year.

Protesters hold signs during a protest outside the police precinct centre in Minneapolis on Saturday. AFP

Protests have been held in several other US cities into Saturday over the verdict, including New York, Los Angeles and Chicago.

Data shows that traffic skirmishes involving firearms have been on the rise since 2018, and the report said that "if current trends continue, 2021 is on track to be the deadliest year on record."

The pandemic, which introduced many new sources of stress to people's lives, has also seen record increases in gun sales and shootings, Everytown said.

'Entitlement' and 'narcissism'

Ryan Martin, an anger researcher at the University of Wisconsin-Green Bay, told AFP that "just the existence of a life-threatening illness puts people on edge, when the frustrations that they encounter would have been kind of mild two years ago."

Civil rights leader Rev. Jesse Jackson marches with activists protesting the verdict in the Kyle Rittenhouse trial. AFP

In a country where the right to bear arms is fiercely guarded, the omnipresence of guns magnifies the problem, according to Martin, a psychology professor.

Firearms are "a driving factor in multiple ways because it gives you a lethal mechanism to act out that anger," he said.

"Data also shows that having a gun in the car with you makes you more likely to become angry. It's called the weapons effect."

Individualistic American attitudes may also be partly to blame.

"The individualism that we see in the US probably exacerbates a lot of anger response. There is a sense of entitlement that comes with the way in which Americans tend to think about freedom," Martin said.

Martin and emotional management specialist Pauline Wallin both suggest that deep political divides also contribute to the violence.