Killer Cars: An Extra 1,000 Pounds Increases Crash Fatalities by 47%

Photo: AFresh1

Ever since the SUV craze began in the late 1980s, we’ve all known that heavier vehicles are safer for those driving them, but more dangerous for others on the road. Which is why we all started driving them. Now, in a new working paper, a pair of Berkeley economists have quantified not only the fatality risks of heavier cars for other drivers, but also the costs associated with them. Here’s the abstract:

Heavier vehicles are safer for their own occupants but more hazardous for the occupants of other vehicles. In this paper we estimate the increased probability of fatalities from being hit by a heavier vehicle in a collision. We show that, controlling for own-vehicle weight, being hit by a vehicle that is 1,000 pounds heavier results in a 47% increase in the baseline fatality probability. Estimation results further suggest that the fatality risk is even higher if the striking vehicle is a light truck (SUV, pickup truck, or minivan). We calculate that the value of the external risk generated by the gain in fleet weight since 1989 is approximately 27 cents per gallon of gasoline. We further calculate that the total fatality externality is roughly equivalent to a gas tax of $1.08 per gallon. We consider two policy options for internalizing this external cost: a gas tax and an optimal weight varying mileage tax. Comparing these options, we find that the cost is similar for most vehicles.

Some highlights:
From 1975 to 1980, average vehicle weight in the U.S. dropped almost 1,000 pounds, from 4,060 lbs to 3,228 lbs. By 2005 those reductions had all been gained back, and the average car was as heavy as it was in 1975.
Previous research has suggested that a heavier car fleet is a safer one. This study concludes that tightening fuel economy standards (and thereby reducing the weight of cars on the road) will not increase fatalities, so long as the standards are “footprint based” or unified across cars and trucks. Here’s how:

We quantify the external costs of vehicle weight using a large micro data set on police-reported crashes for a set of 8 heterogeneous states. Unlike the data sets employed in the previous transportation literature or Jacobsen (2010), our data set includes both fatal and nonfatal accidents. Using unique vehicle identifiers (VINs), we determine the curb weight of each vehicle involved in an accident, thereby minimizing concerns about attenuation bias induced by measurement error. The rich set of vehicle, person, and accident observables in the data set allow us to minimize concerns about omitted variables bias. Using these data, we estimate the external effects of vehicle weight on fatalities and serious injuries conditional on a collision occurring.

Two key results emerge:
1. That vehicle weight is a critical determinant of fatalities in other vehicles involved in multi-vehicle collisions. The data implies  that a 1,000 pound increase in striking vehicle weight raises the probability of a fatality in the struck vehicle by 47%. The authors then calculate that the total external costs of vehicle weight from fatalities alone are about $93 billion per year.
2. Second, light trucks significantly raise the probability of a fatality in the struck car – in addition to the effect of their already higher vehicle weight.