Impetus is an explorer perk reduces base cost of sanity when traveling by 25% (from 5 to 3 on normal difficulty). This should not be confused with the separate travel-cost of specific terrain, and is instead applied to the base-cost of every travel leg.
Because it affects travel leg sanity-cost and not terrain traveling costs, this perk most benefits players who favor shorter, more cautious travel legs. It has no benefit on Tourist Trip difficulty, as the base cost is already zero.