Imst is a small place in Austria.
It's not very hard to hitchhike out. The on-ramps are the same for direction Bregenz and for direction Innsbruck. There's two highway entrances, one to the West (where there's not much space for cars to stop) and one to the East towards Telfl. There you're on a bridge but still a car with a brave driver will stop for you soon; there's enough space to stand before the highway sign.