1a is better than 1b, and 2a is better than 2b. The b choices are not wrong but the prepositions are unnecessary and repetitive because "mount" already means "mount on".
In the same way, you would say "Jack rode his horse" rather than "Jack rode upon his horse". "Upon" is not wrong, but adds nothing to the sentence.
As for #3 and #4, in the U.S. we never say "tolled road". Logically and grammatically it makes sense but we just don't say it that way.