It's always "reasonable." Rarely enough.
The first CAFE standards (1974-1985) doubled mileage standards to 27.5 mpg in increments over that time. The real need for larger vehicles and the lower standards for light trucks and certain other vehicles created the entire breed of SUVs and minivans.
The law passed in 2007 will increase combined average standards to 35 mpg by 2020.
