Regenerative braking is a mechanism that reduces vehicle speed by converting some of its kinetic energy into another useful form of energy. This captured energy is then stored for future use or fed back into a power system for use by other vehicles. It is estimated that on hybrid electric vehicles one can save 10 to 25%. I added in the story about how NYC Taxi Cabs becoming hybrid electric vehicles will use more regenerative braking: It has been really great to see an increase in the amount of hybrid electric car and particularly cab owners (especially NYC taxi owners).