It’s important to remember that there is a difference between the static load (stopped) and the dynamic load (in motion). While moving the various accelerations will magnify the effect of the weight on the roof. The dynamic load is a fraction of the static load because the forces will magnify as you hit bumps and your vehicle will be very top heavy as you go around corners, etc. Remember the ultimate strength of the roof/gutter rails is strong enough to protect you in the event of a rollover, so it’s pretty significant.