One apparent advantage of the cable car is its relative energy efficiency. This is due to the economy of centrally located power stations, and the ability of descending cars to transfer energy to ascending cars. However, this advantage is totally negated by the relatively large energy consumption required to simply move the cable over and under the numerous guide rollers and around the many sheaves. Approximately 95% of the tractive effort in the San Francisco system is expended in simply moving the four cables at 9.5 miles per hour. Electric cars with regenerative braking do offer the advantages, without the problem of moving a cable. In the case of steep grades, however, cable traction has the major advantage of not depending on adhesion between wheels and rails. There is also the advantage that keeping the car gripped to the cable will also limit the downhill speed to that of the cable.