Your moving car contains a lot of energy as kinetic energy - energy of motion.
If your car is not a hybrid, when you stop by stepping on the brake, your energy of motion is transformed by friction (in the brakes, against the road) into thermal energy. It can't be re-used. Estimate how many gallons of gasoline you could save each time you stopped your car from a speed of 100 km/hr if you could store that energy somehow and re-use it. [Hint: One gallon of gasoline contains about 1.2 x 109 Joules of energy. (1 Joule = 1 kg-m2/s2).] Be sure to clearly state your assumptions and how you came to the numbers you estimated, since grading on this problem will be mostly based on your reasoning, not on your answer.