Hitching Out
South, towards Modena
Instead of the roundabout, it's better to walk a bit South and try the other highway entry. You might be able to hitch a ride already on the SS12.
North, towards Bolzano, Austria, Germany
This might be tricky. There is a roundabout, but not too much space for cars to stop. It might be better to find a spot closer to the city centre. If you're into that sorta thing, for 15 euros you can take a train all the way to Kufstein, which is very close to the German border.
You can also try the "statale" going North, which is Via Brennero.
Arriving
<map lat='46.063226319271095' lng='11.106491088867188' zoom='13' view='0' float='right' height='300'/> If you are dropped at the gas station on the autostrada some kilometers North from Trento you can ask the staff if they can let you out. There is a tramway that takes you from Lavis to Trento for EUR 1,20.