Airbags - 20 lbs. Anti-lock brake controller, 5-6 lbs. Safety cage - no additional weight if implemented with high-strength steel or other advanced materials.
"During 1975–84, the fuel economy of the entire light-vehicle fleet rose by 62% while vehicles became safer, far cleaner, and no less peppy. Detroit then kept on innovating, but once its success had crashed the world oil price in 1985–86, ever-better powertrains were used to make cars more muscular, not more frugal. The average new U.S. light vehicle in 2003 had 24% more weight, 93% more horsepower, and 29% faster 0–60-mph time than in 1981, but only 1% more miles per gallon. If 1981 performance had instead stayed constant, light vehicles would have become 33% more efficient, displacing more than year 2000 Persian Gulf imports." Winning the Oil Endgame - Rocky Mountain Institute