Paul,
I look at it a little differently.
The correct speed of descent in any heavy vehicle is the speed/gear/use of any non-service brake that keeps your speed in EQUILIBRIUM. That is you are neither speeding up nor slowing down and do not need to use your service brakes to do this.
We were descending a 17% grade in the Green Mountains of VT several years ago. 2nd gear with exhaust brake on-- doing about 22 MPH. We easily could have taken the curves at 35-40 MPH (so grade, not road shape dictated speed). Dianne was worried that someone would run up on us. Around the next bend was a loaded logging truck, doing his equilibrium speed-- about 8 MPH. We blasted right by him and continued on down the mountain. Both of us were descending correctly for our rigs.
