This is a little vague.....what did they see that isn't being told......how did they only see the car on fire going down the road and not the people who set the fire then put the car in gear with the accelarator depressed?.....this car did not travel far from the point of ignition....
Remote trigger, timer, speed monitor, millions of easy ways.
Detonator attached to small container of gasoline, hidden either under the seat or under the dashboard. Either remotely command-detonated, or timer-detonated. More likely command-detonated from someone in a following car
She drives along, then BOOM, fire in the car, she loses control, and burns