A police investigation ground traffic to a halt on Canada's busiest highway on Friday after a body was found lying beside Highway 401 in Toronto.

A high school student in her dad's car noticed the victim lying under the off-ramp from the southbound Don Valley Parkway to the westbound 401 at about 10 a.m., provincial police said.

Investigators believe the victim, a young man, was involved in an accident overnight. Police towed away an abandoned and damaged pickup truck in the area at about 1 a.m.

They believe the deceased was at the wheel of the vehicle when it slammed into the guardrail. Investigators suspect the man, whose name hasn't been released, wasn't wearing his seatbelt and he was ejected. Police believe the truck then rolled on without anyone inside for about 500 metres.

"The vehicle continued down the highway, crossing five lanes of traffic then hitting the steel guardrail and coming to rest,'' OPP Const. Dave Woodford told reporters.

The police investigation resulted in numerous closures in the area, including to the Don Valley Parkway on-ramps from the westbound 401.

The closure lasted for about four hours, which caused commuter chaos.

But westbound traffic crawled across the top of the city because of a separate incident. Two left lanes of the westbound express lanes were closed for emergency bridge repairs.

Police said a piece of metal dislodged from an expansion gap in the road.

The problem damaged five trucks overnight, causing blown tires, severed fuel lines and ruptured diesel tanks. No injuries were reported.

Police, meanwhile, are asking anyone who may have witnessed the fatal collision to contact them.

With a report from CTV's Austin Delaney