One of the hassles of commuting in Wellington is that generally your home is higher than your work, so you heat up your brake pads in the morning descending the hill, then expend calories getting back up the hill in the evening.
Of course, hybrid vehicles like the Prius save the braking energy as battery charge, so it can be used to accelerate later on. But what about Bikes?
The answer may lie in a simple design by a New York mechanical engineering student, who attached a 7kg flywheel to his bike which collects the braking energy, then makes it available to the rider later on.
So assuming a frictionless flywheel, you could capture the energy from whipping down the Brooklyn Hill, and use it to roll back up again!