Traffic nightmare as loaded semi crashes into Perth freeway overpass

Shocking images have emerged of a truck carrying heavy machinery that slammed into a Perth bridge and became stuck.

The Mitchell Freeway came to a standstill and the city was gridlocked for hours as crews worked to pull the trailer and its load free.

An excavator being carried by the truck hit the 4.3 metre-tall Hay Street overpass at 80km/h at 8am this morning, with the force of the impact separating the truck from its trailer.

Fortunately the driver of the truck, which stopped 100m down the freeway, was not injured.

Traffic quickly came to a standstill as two lanes of the freeway and Hay Street were closed, leaving a 10km parking lot down the freeway.

It took crews three hours to remove the machinery from the overpass, and another hour to clear concrete and other debris from the road surface.

"The damage is to the bridge itself and some of the supporting structure there," John Venables from Main Roads WA said.

"So we'll be moving some of the loose concrete debris that's been broken there."

Engineers were called in to assess the damage, removing a steel internal support cable and loose concrete.

"There'll be lane closures going back in at 8 o'clock tonight to do some further repair work," Mr Venables said.

Main Roads says it will be seeking compensation from the driver of the truck.