When one is trying to recover the energy of a moving vehicle or
object slowing down one has a problem.  The energy is proportional
to the square of the axle RPM so at first there is a lot of energy recovery and braking effect. As the vehicle slows down the braking  energy rapidly falls off. If a generator is directly connected to the wheel the braking effect of the generator and energy recovery will fall off directly proportional to the RPM. This is not what is required to stop as quick as possible. The conventional brakes must be applied I suspect unless some way is found to rapidly increase the load on the generator such as changing gear ratios and/or by electronic means.  The same thing can be said about spinning up a flywheel to store the braking energy. This device, invented in the early seventeen hundreds, might be used to speed up the flywheel or generator as the vehicle comes to a stop
then is reset to wait for the next stop.

http://www.sailtexas.com/long.html
http://www.antique-pocket-watch.com/sinclair-harding.html

It will be very interesting next year when KERS is implemented in Formula 1
to see how the various teams achieve storing braking energy in a flywheel.

Paul Lamar