Flywheels
You have to have pairs of contra-rotating wheels, and a heavy frame to stop individual twisting forces.
The problem comes in a catastrophic accident (either a vehicle collision, or a flywheel-support failure or bearing seizure) where you are liable to release a heavy object with huge energy-to-mass ratio and odd physics. This rapidly-spinning steel wheel will tend to bowl down the road and cause secondary accidents, and will bounce and skip in an alarming way.
They are good for heavy vehicles like trains and trams (but there electric regenerative braking is easier - run the motors as generators and push power back into the overhead wires).