It's immensely heavy and expensive, and not very effective. Edit: okay, that's probably going to need a little more explanation. You've got a car travelling (

It's immensely heavy and expensive, and not very effective. Edit: okay, that's probably going to need a little more explanation. You've got a car travelling (frictionlessly in a vacuum, as per all good physics experiments) with a flywheel that can be connected to the driveshaft with a clutch. When you engage the flywheel it disengages the driveshaft from the engine. Your car is travelling at 50mph, so it has some amount of energy that you can calculate from its mass. The flywheel is at rest. When you engage the flywheel clutch, the car will slow down and the flywheel will speed up until both have (neglecting losses, remember) an equal amount of energy, and the car will be doing 25mph. Bring the car to a stop, with the clutch disengaged again. The car has no energy but the flywheel is still spinning. Now engage the clutch to move off. The car will accelerate to 12.5MPH, because it gets half the energy in the flywheel and once they have an equal amount no more energy can be transferred. You can get around this with clever gearboxes, a bit, and if you're building a multimillion pound racing car that's definitely the way forward.