Whoa! My point was simply that a car coasting down a grade physically will not, as your post #35 seems to say, "keep going faster" with no limit (other than reaching the bottom). That's because drag (especially aerodynamic drag force, which tends to be proportional to the square of speed) will limit speed to some equilibrium. That has nothing to do with what's acceptable or legal or safe on any particular road, or with differences between your vehicle vs. mine. Funny that 5% seems to be considered a steep grade. I can't go anywhere without first ascending an immediate short ~6% upgrade if I head west, or a longer ~8% one if I head east. Or visit a local friend with whom I often go on bike rides without climbing the ~16% grade near her house. Fortunately, it's short. The most B-mode-worthy hill I frequent is about 6 miles of maybe 7%, I think. Sometimes I can get down that thing without filling all 8 segments of the charge-level indicator or resorting to friction braking, but usually not.
Ok, that's technically true ... And that's, narrowly, technically true: the statement "speed will be limited to some equilibrium" is true, whether it's your vehicle or mine. (That's not to say their equilibrium speeds won't be different.) But, you know, all of that can be true, and still ... if that theoretical equilibrium speed happens to be way faster than you'd ever drive on that road ... then, for practical purposes, what fuzzy1 said in #35 is also true. And practical purposes are the kind many PriusChat readers have in mind. Knowing you have a theoretical terminal velocity is more of academic interest if it's too high to keep you on the road at the curves, or within the speed limit. Then it's more interesting to be able to find the velocity that the car's tools, like regen and engine braking, will be able to maintain for you without picking up speed, and that will be something lower.
Safe and legal has everything to do with the defensive driving behavior we are interested in. Even airliners in power nosedives, and spacecraft falling out of orbit, have 'terminal' atmospheric speeds. But those figures are all lethal, so not relevant to safe and sane human behavior. I have a choice of two similar grades on the way to dad's house. In B mode from the top, my battery fills and the ICE goes into screaming banshee mode, 4600-4900 RPM, before even a quarter of the way down. It might be possible to go all the way down without resorting to friction braking, but I'm not interesting in dodging interference from traffic obeying the speed limit, or in possibly adding to the scrape marks on the safety wall lining the banked curve at the bottom of one of those hills. That barrier wall looks reasonable now, but previous versions had very serious marks from trucks that rolled over and slide around the corner on their side. Which was an improvement over what happened to the truck drivers who rolled there before that toboggan-slide wall was put up. But all this is a distraction from the natural glide braking rate of a Prius without regenerative braking.
