At the seed coords (when travelling westbound) you will be approaching a raised section of highway that spans roughly one kilometre. In order to figure out the coords for the actual cache container, you will be driving over a number of "bumps" for the next kilometre or so. You will need to carefully count the number of "bumps" you hear as you drive and then do some math. If I were you, I would pull over sooner rather than later to do your math hint hint...
Final container is hidden at N 45.51.ABC W064.15.DEF
To obtain ABC, take the number of bumps multiplied by 5, and then add 96.
To obtain DEF, take 360 divided by the number of bumps, and then add 187.
Note, you can also solve this puzzle when travelling Eastbound, however you will then need to backtrack in order to find the final container.