Reported that around 20% of people who experienced traumatic car accidents developed a phobia of travelling in cars (explained by CC).
Di Gallo(1996)

This 20% tended to make avoidance responses involving staying at home rather than making car journeys to see friends (explained by OC).
One limitation of the two-way process model is that it doesn't explain the development of all phobias. Some people cannot remember an event which triggered their phobia development. This suggests different phobias may be the result of different processes. However, Ost...
Ost says that it is possible that such traumatic events actually did happen, but have been forgotten, shut out, by the phobic.
Biopsychologists say that evolution would give reasonable explanation for phobia development. Seligman said that we are more easily afraid of stimuli which threatened us in our evolutionary past. For example: its rare to be afraid of stairs despite them being more dangerous statistically than spiders.
Seligman ignores rational thinking: focuses on social and emotional fear.