One area of development that Sinclair purposely avoided was battery technology. Electric vehicles powered by lead–acid batteries had once actually outnumbered internal combustion engine vehicles; in 1912 nearly 34,000 electric cars were registered in the U.S.[16] However, the efficiency of internal combustion engines greatly improved while battery technology advanced much more slowly, leading to petrol and diesel-driven vehicles dominating the market.[17] By 1978, out of 17.6 million registered vehicles on Britain's roads, only 45,000 were electric vehicles in day-to-day use and of those, 90% were milkfloats.[18] Sinclair chose to rely on existing lead–acid battery technology, avoiding the great expense of developing a more efficient type.