In the 24 years since a landmark law to conserve fuel, big cars have shrunk to less-safe sizes and small cars have poured onto roads. As a result, 46,000 people have died in crashes they would have survived in bigger, heavier cars, according to USA Today analysis of crash data since 1975, when the Energy Policy and Conservation Act was passed. The law and the corporate average fuel economy (CAFE) standards it imposed have improved fuel efficiency. The average of passenger vehicles on U.S. roads is 20 miles per gallon vs. 14 mpg in 1975. But the cost has been...