I think that in reality Forester is on to something... and it has deep roots. Many of us were taught how to ride a bike by our parents... and in those lessons came the words "stay on the sidewalk" and "watch out for cars."
No doubt deep in our brains those words may resonate from time to time... much like the "hot" lessons our parents also taught us.
Now in my case, I taught myself to ride a bike by "borrowing" a neighbor kids' bike... I learned some other lessons... such as not to ride near plate glass picture windows before understanding the actions of steering and the workings of the brakes... but all humor aside...
I really do think that there is something to the phobia... based on early lessons in life. JMHO