Whanganui man Ammar Saleh noticed a car parked in the breakdown lane on his way to New Plymouth on Saturday - and then on fire when he returned.
"When I drove back at around 4.30pm it was on fire," he said.
"It looked like it hadn't been burning long and I wondered how it caught on fire."
Saleh said no one was in or near the burning car.
Whanganui police and firefighters arrived at 4.45pm.
Whanganui Fire and Emergency senior station officer Jes Sorensen said the blaze was extinguished quickly.
"I believe the car had been involved in an accident earlier in the day.
'I can't comment on what caused the fire as I believe the police are investigating."
Police confirmed that SH3 was blocked for a short time until the fire was out. The car was then taken away.
"The car had been involved in a collision at around 6.45am on Saturday," a spokesperson said.
Whanganui police also attended a car crash in Castlecliff near the corner of Mosston Rd and Puriri St at 10.30am on Saturday.
The police spokesperson said one driver was taken to hospital with a shoulder injury.
"Police spoke to witnesses and will be making further investigations."