as uart originally pointed to, i think it's your 12v. in my opinion, your car is old for the 12v to still be "good"... they said it's still ok... to me that means near dead for a prius 12v. one incident of having the radio on while not in ready or leaving a light on means your battery will then be dead... this car needs a good 12v, or else voltage drop with resistance gives the prius a headache.
I'm getting a "404" error on that link El Dobro, but let me guess you're trying to link to a pic of this : http://www.cyburbia.org/gallery/data/504/medium/19Homes-2.jpg Is that the one? Funny picture, and it's been all around the web, but sadly it has been proven to be photo-shopped fake.
Yep, that's it. The interesting thing is that it looks like my niece's driveway. I'll have to see if I can get her to take a pic.
Judging gradient is deceptive. If you stand at the bottom and look up (or at the top and look down) people usually over-estimate the slope by a factor of about two. Only if you have a cut away section where you can look at it from side-on do most people seem to be able to make good estimates.
Just a quick update: The 12v battery checks out fine using the hidden screen. It was good at 12v and didn't drop too much with a load. The car has not rolled since I posted here, though I am using the emergency brake on my "not as steep as I think it is" driveway. So thanks for all your thoughts and ideas and I'll wait until it does it again or, hopefully, doesn't! Thanks, Aaron
OMG! I was shocked to see your post. I hope she is alright. What year was the prius? Do you have any idea what went wrong? If so, please let me know so I can have it checked on my car. Aaron
Many readers bring up non-Prius experience to compare and contrast with Prius behavior. Prius is unique only in some things, not all things.
Ah well, it was only the mother in law...... Getta load of that drive of El Dobro. It's not really a drive more like a vertical jet take off pad!