We studied this in an engineering class (back in the mid 80s) where our professor proudly showed a picture of him popping a wheelie on his bicycle when he engaged the flywheel that had just previously slowed him down to a stop. Anyway, one study we examined for an energy storage system had a bus with a huge flywheel that took up a couple rows of seats in the back. I forgot the details on how it stored energy upon stopping and how it delivered it to the drivetrain, but I do remember it worked quite well. Except for one problem: the momentum created would not allow the bus to turn. At times it could literally turn no faster than maybe to do a slow lane change type maneuver. Thus, it was concluded this strategy was not feasible for almost any type of motor vehicle. Overall, weight was a factor too (have to have a considerable mass). Use of an electric motor/generator/battery system was definitely deemed a better choice.

Dave