It is easy to reach the Stubai valley by car!
From Germany, Switzerland and Austria:
Just take the motorway A13-Brenner from Innsbruck and then take the exit Schönberg. (It's about 20 minutes from Innsbruck)
From Italy:
Just take the Brenner motorway to the A13, exit Schönberg and from there it takes about 15 minutes to Neustift.