6.1.3 – Brake Early
Control your speed whether fully loaded or empty.
Large combination vehicles take longer to stop
when they are empty than when they are fully
loaded. When lightly loaded, the very stiff
suspension springs and strong brakes give poor
traction and make it very easy to lock up the
wheels. Your trailer can swing out and strike other
vehicles. Your tractor can jackknife very quickly.
You also must be very careful about driving
"bobtail" tractors (tractors without semitrailers).
Tests have shown that bobtails can be very hard to
stop smoothly. It takes them longer to stop than a
tractor-semitrailer loaded to maximum gross
weight.
looks like that info is specific to big rigs, only the 1-ton GMC had a shorter braking distance loaded than unloaded in this test.
http://special-reports.pickuptrucks.com/2010/08/2010-hd-brake-tests.html
I wonder what difference a stiffer suspension would make on a passenger trucks braking distance.