Freewheel energy storage in buses

Gizmag has a good post on bolt-on flywheel system for city buses claiming to reduce energy consumption by 30%. I've recently read about flywheel systems for electrical energy storage and have also been talking to some local scientists about similar systems that were installed in Mexico's UNAM university as a electrical backup system back in the 70-80's. These systems seem like a good idea, tying to increase efficiency which is usually less expensive then energy generation, and in the case of flywheels, in a compact case.