I agree that it's best to leave the mountain with enough charge to turn around and/or take an alternate route in such situations, but 40% should have been plenty. If stuck for hours in traffic and/or behind an accident, my advice would simply be to reduce or eliminate heater use if there's any question about having enough range. Crank up the seat heaters, and if that's not enough, grab a blanket or a winter jacket. That's far easier than having to call a tow truck!
When driving through the mountains in cold temperatures, you should always carry warm clothing and blankets in the car anyway, regardless of what type of car you're driving. What if you're in an accident and need to keep warm until emergency services can reach you, or what if your car malfunctions?
Another recommendation would be to check PlugShare, as there are some folks up here (including me) who will let you charge if you're in a bind. Anything but the tow truck!
In years of Tesla driving up here, we've never run out of charge, and we've only rarely needed to watch our heater use.