I think the worst kind of road is the kind where it is relatively smooth for moderate distances but then occasionally pocked by axle-cracking potholes, strategically placed like sand traps on a golf course. This allows the driver to approach highway like speeds under non-highway like conditions. One gets fairly adept at anticipating  the bumps and tucking one's head and bracing for impact with the roof. This effect is particularly exaggerated in the far back seat of a 4 x 4 vehicle. Not sure entirely why but it probably has something to do with basic geometry. Anyone sitting between the axle has their motion somewhat moderated, while anyone sitting beyond the axle base is like sitting on a see-saw. This means you get slapped like a pancake if the bounce is hard enough. This particular road yesterday seemed to stretch in a remarkably straight line cutting a swath to the horizon and off somewhere into the interior of southern Tanzania.