When riding over medium/long distances (ie not city riding) it is preferable to keep one's speed constant. however, it is inefficient to brake on downhills and increase exertion on uphills. A solution to this problem is a speed governor for the bicycle wheel. The speed governor comprises several weights
attached to springs attached at the centre of the wheel (symmetrically placed). As the rotational speed of the wheel increases (eg when going down hill) the weights move radially outward which in turn increases the rotational inertia of the wheel hence storing energy and slowing the wheel down. As the speed of the wheel decreases (eg going up hill) the weights move radially inward increasing the wheel speed.

[+] This idea is the simplest and most elegant of the inertia distribution systems proposed on the HB! With centrifugal force doing the inertia management, there are fewer fault vectors in the system.
I would imagine the weights would be about 1/2 KG each, with two per wheel. This would not impact energy consumption as much as how the effect the drag coefficient of the wheel (the wheel is very sensitive to aerodynamics). A bullet shaped weight design would fix that. I would also suggest using bungees over springs, as they are much easier to implement.

This kind of governor could be used as a flywheel for power storage. The advantage over a normal flywheel is the power could be 'tapped off' at standard AC frequency (about 50Hz), which may be important for some devices.

This is not a new invention, merely the application of a
very old governor design to the bike, and probably not
the first time. Furthermore, the statment that "it is
preferable to keep one's speed constant" is simply not
true. What one wants is to keep one's pedal RPMs in an
acceptable range (more than 60, up to 120 or more,
depending on strength and skill). This is why we have 10,
15, and more gears. Forcing me to use precious energy
pushing against a governor is highly wasteful, and I will
tire quickly.

Crap --- there is no better store of energy than gravitational potential and kinetic... What is required is a cycle path without places that require braking. i.e. no speed limits.