So put the adjective "walkable" in front. The question is the same. These things are unwalkable and therefore not able to be part of any route. Same with your

So put the adjective "walkable" in front. The question is the same. These things are unwalkable and therefore not able to be part of any route. Same with your car: the places without roads are undrivable and therefore not available for routing. But with a car, the speeds are so much greater than bipedal motion, shortest vs. fastest is a viable question. When the routing system avoids obstacles, it's mostly going give you the shortest and fastest route. brador makes a good point about grade; with enough grade on the shortest route, it might not be the fastest.