The Laws of Prayer /The Amidah (83)

1. The interdiction to start on a journey before praying is because doing this is considered like occupying himself with his oen needs. It follows that even if at his destination there is also a quorum for praying, and he will manage to pray before the final time for praying, he should nevertheless not start on a journey before praying. (Mishne Berurah 65)

2. However, in a time of difficulty, one may start travelling before praying even if he will need to pray on the way. (ibid)