As a rule, transitive verbs in Italian use the auxiliary avere (ho camminato, avrò camminato) when conjugating in any of the compound tenses. However, camminare & viaggiare are intransitive and seem to be in the category as other verbs of motion: scendere, andare, cadere, etc. that use essere as a helping verb.
Why are these verbs exceptions to the rule?
Are there other examples of verbs that are intransitive yet use avere as an auxiliary verb?