Take a couple of sine waves of differing wavelength and amplitude. Now combine them. The resulting waveform is going to be a lot like your average road in western Pennsylvania, which is any shape but flat. The only time you’re not ascending or descending is when you’re at a standstill. This is where I learned to ride a bike.
I had this in mind when I wrote this chapter of my book, where the protagonist rides around in a futuristic “kinetic bike” that is able to capture some of the energy normally lost when using the brake or when over-pedaling, and release it later when the landscape turns against the rider.
The other day a reader sent me a link to this. A bit pricey, but facinating. They claim it can go 62 miles on a single charge without the rider pedaling, but I’m sure those are flatland numbers. I’ll bet you’d be lucky to get 20 miles out of it here in sine wave country. Still, you can supplement that with your own contribution, and it looks like it still works as a fully functional bike even when the batteries are depleted.
Still no flying cars, though.