Yes, you can drive downhill.
I was a volunteer at Mount Diablo’s summit, which is 3,800ft high.
I would start my journey on a full charge. Before the gas engine kicked in, my Chevy Volt averaged 42 miles per gallon. When I reached the top of the mountain (16 miles from my home), I had only about one mile left of electric range. (As you can see there is a drastic reduction in range when driving uphill).
I put the Chevy Volt into “L” mode, which maximizes regenerative brakes, and began to coast down the mountain at the end of the night. The car had enough electricity to allow me to drive another ten miles by the time I reached the bottom.
In summary, I had drained the battery in 16 minutes, reached the summit, and then coasted home. The car was still charging, so there were only 10 miles left. The total range is approximately 42 miles, which is about the same as what I would achieve in city driving.