A man who police say was involved in a fiery crash on the Mitchell Freeway in Perth has been arrested after allegedly stealing the car of a good Samaritan who stopped to help.

The 30-year-old man is alleged to have been driving a four-wheel-drive before crashing into a light pole on the freeway, near James Street, early this morning.

The vehicle then burst into flames.

It is then alleged he escaped with an arm injury but stole a white Holden Rodeo belonging to a man who stopped to help.

The good Samaritan, named Chad, said when he stopped by the road the car was already on fire and there were small explosions.

"I pulled over. I just wanted to see if anyone was on fire or if anyone was dying in the car," he said.

"Someone that rolls a car and it explodes and then runs off and steals another car is probably not the most mentally sane person.

"It's not the best feeling.

"I'd still do it again but I won't be leaving my keys in the ignition. I'll lock the car."

Chad said he is a construction worker and needs the tools in the back of his car for work.

The utility has been recovered by police.

Police said drivers from the northern suburbs experienced long delays heading into the city and into the Graham Farmer Freeway tunnel.