NORTHAMPTON — An allegedly erratic driver traveling south on Interstate 91 was injured after being ejected from his vehicle when it clipped a second car at 6:20 p.m. Monday, according to state police.

The driver, who is not being identified, landed in a northbound lane of the highway, while his Cadillac Deville rolled over, Trooper Paul Sullivan said.

The Cadillac Deville had been observed operating erratically, cutting in and out of lanes, before hitting a Honda Accord as it approached Exit 18, Sullivan said.

The man was taken to Baystate Medical Center in Springfield, while the driver of the Honda Accord went to Cooley Dickinson Hospital in Northampton as a precaution, Sullivan said.

Extensive backups occurred for vehicles traveling north, as only the breakdown lane between Exits 18 and 19 remained open as the Northampton State Police barracks cleared the scene.

Southbound traffic was not greatly affected, police said.

Sullivan said the northbound lanes reopened around 9 p.m.

The accident remains under investigation, with the driver likely to face charges related to the erratic driving, he said.