The solvent they thin the road paint with is usually Toluene or Xylene. You can find some at Walmart, unless you live in California, then you may have to resort to using some odorless mineral spirits. It will remove it, but very sloooowwwllllyyy.
Good luck, that's tough stuff. There is a lot of pigment in striping paint, so keep at it when you start. It will come off, hopefully leaving the paint behind you want to keep.
Blair