[Bicycle Bob]

First, let's remember that to get as much deceleration as acceleration requires a generator as powerful as the motor. A couple thousand watts would be barely enough to noticeably affect the coasting. Full electric or hybrid setups have the kind of muscle for average braking forces, but still rely on friction for quick stops.

If you only want to store enough power for a minute, you need something that will accept very high rates of input/drawdown. The trouble with air storage is the energy lost if the air is not kept hot. It could be OK in a small system. A big one gets dangerous, anyway.

Rubber is another potential storage medium, but as with all mechanical systems, you tend to want a wide-range, variable speed gearbox between the wheels and the variable torque produced at the storage levels.

Capacitors are nice because they are plenty fast, efficient, and long-lived. I'd always use them as a buffer on a battery, to save wear and tear and increase overall efficiency.

In Bicycling Science, Whitt and Wilson list lead-acid batteries at 85 watt-hr/lb. Ni-cad is somewhat better, and can provide high rates of power per pound, although run time would usually be over five minutes, and a thousand cycles per pack might seem a bit high-maintenance. W&W also list "flywheel" at 14 w-hr/lb, "compressed gas and container at 10, rubber bands at 1, [steel] springs at .06, and capacitors at .006, although they have improved considerably since then.