Traffic is returning to eastbound lanes of Highway 417 after a police officer’s vehicle was hit while pulled over Friday.

OPP say a marked police vehicle was rear-ended by a Volkswagon around 12:15 p.m. Friday near Casselman, while the officer was dealing with another driver.

The crash, which drove the police cruiser into the other parked vehicle, left the Volkswagon driver with serious injuries and two people inside the parked vehicle with minor injuries.

Commuters, many headed out of town for the long weekend, said the delays were frustrating.

"Stopped/slowly moving. Poorly managed. No police presence. Extra 120 minutes already. Avoid!!" tweeted Jacqueline Roy.

One lane of the highway was re-opened shortly after 6 p.m.

The province’s SIU will be investigating, as it does when someone is seriously hurt or killed while dealing with police.

The OPP said this is a good reminder to slow down and move over a lane (if possible) when an emergency vehicle is stopped with its lights on.

With a report from CTV Ottawa's Natalie Duddridge