Once you get terrain that's too far away from what bikes were designed for, you're basically SOL. Seems that if you had crazy low enough gears like 24T in the front, 32, 36, or even 42 in the rear, you could probably do it. Makes me wonder why anyone would build roads like that as that would cause plenty of trouble for vehicle traffic and would be super dangerous when slimy or wet.

30% is stupid steep. I've ridden stuff like that before, but don't do it anymore simply because it's no fun. Just because you can doesn't make it a good idea. I personally do not enjoy climbing grades over 15% (for sustained sections, much over 8-10% isn't that fun) and prefer grades around 8% coming down (steep enough to be fun, not so steep that all you do is burn your brakes up)