Are SUVs killers?

Railway timetables - Monty Python does Agatha Christie

My hate-hate attitude towards SUVs hasn’t improved after reading a new US research paper, The pounds that kill, by two University of California (Berkeley) researchers. They show being in a vehicle struck by a 1,000 pound heavier one results in a 47% increase in the probability of a fatality in the smaller vehicle (the paper might be gated for some – here’s an ungated version that looks very similar).

The authors note that from 1975 to 1980, the average weight of US cars dropped from 4,060 pounds to 3,228 pounds in response to higher petrol prices. However average vehicle weight began to rise as petrol prices fell in the late 1980s and by 2005 it was back to the 1975 level. In 2008 the average car was about 530 pounds heavier than it was in 1988.

A key reason for increasing weight is the safety “arms race”. Drivers seek large vehicles because they’re thought to be much safer for occupants than smaller ones, however they are more hazardous for other travellers in smaller vehicles. As the average size of the fleet rises, there’s an incentive for all drivers concerned about safety to trade-up. The authors note the “safety benefits of vehicle weight are therefore internal, while the safety costs of vehicle weight are external”.

Safety is a key public policy issue because road accidents are as dangerous to life as lung cancer. Moreover, traffic accidents kill more Americans aged under 40 years of age than any other cause. While only as quarter as many people die on roads each year as die from lung cancer, the average age of a road accident victim is 39 years compared to 71 years for lung cancer. The aggregate years of life lost from both causes is similar.

Noting that no detailed attempt has been made to measure the external costs of vehicle weight, the authors sought to:

quantify the external costs of vehicle weight using a large micro data set on police-reported crashes for a set of 8 heterogeneous states…..The data set includes both fatal and nonfatal accidents…..The rich set of vehicle, person, and accident observables in the data set allow us to minimize concerns about omitted variables bias.

They estimate a 1,000 pound increase in striking vehicle weight raises the probability of a fatality in the struck vehicle by 47%. Moreover, they find that light trucks like SUVs, pickups and minivans “raise the probability of a fatality in the struck car – in addition to the effect of their already higher vehicle weight”. The authors suggest this additional effect could be due to the stiffer chassis and higher ride height of light trucks, or possibly to the behaviour of light truck drivers. Read the rest of this entry »