Speed limits are not for how fast any particular vehicle and driver could negotiate the road. They are for all the other factors in road use. How many roads/driveways intersect the road, what are the sightlines, what other users (bikes, pedestrians) use the road. Does weather make a difference? How homogeneous are the vehicle types and driving ability of those users? People who speed usually vastly overestimate their abilities to react - and then blame the other guy for what would have been prevented had they been driving at the speed limit.