Your "not speeding" causes a lot more accidents than those who speed ~10 MPH over the limit.
That speed limit seems quite rational.
Speed limits are anything but "rational", except in rare cases. The purpose of a speed limit should be to the set the maximum speed upon which a road can be tranversed by a vehicle in good operating condition under good driving conditions. Most of the time, they are set by bureacrats or municipalities looking to steal money from people. As such, they convey little information and most drivers treat them as such by simply ignoring them.