http://journalistsresource.org/studies/environment/transportation/arms-race-u-s-highways-external-costs-heavier-cars-suvs
Car weight has increased a lot these days. Research said fatality of an occupant in a car in a trafic accident and the weight of the car is closely related. That means, if you ride a heavier car you are safer. This idea is used in marketing and so bigger and heavier cars are being introduced and sold. But they are not for the people colliding with them. The article says “A 1,000-pound increase in striking vehicle weight raises the probability of a fatality in the struck vehicle by 47%.” A driver buying heavier cars than others' and others do te same thing for their own safety. The article compares this situation to "arms race". It also says "The total external costs of excessive vehicle weight from fatalities in two-vehicle collisions are estimated to be $136 billion per year." Thi is external cost of heavy cars.