Part from the link above:
Not sure if that is true or notBecause Hitler Said So
With the invention of the automobile, countries had good reason to pick a side and stick to it, although not all did. By 1938, there was another reason: Wherever Hitler invaded, he forced the native populations to drive on the right. Parts of Austria, including Vienna, Hungary and Czechoslovakia, that had historically driven on the left now had to drive on the right.
Panzermahn