Are big cars and SUVs safer than small cars or do they just *feel* safer?

Question

I've long observed that many drivers in the UK choose SUVs (which are often disparagingly called Chelsea Tractors) do so at least partially because they believe such cars to be safer. I doubt this claim.

More importantly, the debate about safety has become a significant component of the US debate about fuel economy standards with some arguing against the government standards on safety grounds. For example, this article argues:

To improve fuel economy, auto makers primarily reduce the size and power of vehicles. Unfortunately, this downsizing has tragic consequences. As far back as 1989, consumer advocate Ralph Nader admitted that larger cars are safer as there is more bulk to protect the occupant. Numerous studies have proved this point. For example:

• Researchers at Harvard University and the Brookings Institution found that, on average, for every 100 pounds shaved off new cars to meet CAFE standards, between 440 and 780 additional people were killed in auto accidents or a total of 2,200 to 3,900 lives lost per model year.

• National Highway Traffic Safety Administration (NHTSA) data indicate that 322 additional deaths per year occur as a direct result of reducing just 100 pounds from already downsized small cars, with half of the deaths attributed to small car collisions with light trucks/sport utility vehicles.

• Using data from the NHTSA and the Insurance Institute for Traffic Safety, USA Today calculated that size and weight reductions of passenger vehicles undertaken to meet current CAFE standards had resulted in more than 46,000 deaths.

Of course these claims have been questioned. Here is an academic analysis that argues against the above interpretation of US data:

And there are other factors to consider. Big vehicles are probably worse for other road users. And the ones with higher centres of gravity may be more likely to roll over than normal cars.

So what does the worldwide evidence say? Do big vehicles decrease injuries to their passengers? And, even if they do, do they make the roads safer overall?

Answer 1

Heavier cars are safer than light cars. Larger cars are safer than small cars. This applies to SUV's as well especially since rollover is not a factor in late model SUV's with standard stability control. Crash rates from both types of vehicles are similar, with the heavier/bigger vehicle having a major safety advantage.

From the IIHS Status Report, Special Issue: Car Size, Weight and Safety, Vol. 44, No. 4, April 14, 2009 exploring mini vehicles and mid-sized vehicles:

The heavier car will push the lighter car backward during the impact, which means the velocity change of the heavier car will be much less than that of the lighter car. If the lighter car weighs half as much as the heavier car, the forces on its occupants will be twice as great.

Also from that report:

...vehicle size, specifically the distance from the front of a vehicle to its occupant compartment. The longer this is, the lower the forces on the occupants, provided vehicle designers take advantage of the extra length.

The safety advantage isn't just in collisions with lighter cars, either Single vehicle (solo) crash death rates in in small cars like the Honda Fit are as high in single- as well as multiple-vehicle crashes. The death rate in mini cars (as the report describes the Fit and Toyota Yaris) during 2007 was 35 per million, compared with 11 per million for very large cars - three times as high.

The NHTSA performed a study examining all vehicles up to SUV's, vans, and pickup trucks titled Vehicle Weight, Fatality Risk and Crash Compatibility of Model Year 1991-99 Passenger Cars and Light Trucks This study showed that:

Weight reductions in passenger cars, lighter vans, pickup trucks, and sport utility vehicles (SUVs) increased the risk of fatal crash involvement.

NHTSA did say that the slightly heavier "Mid-Sized SUV's" lost some of their safety advantage because of their increased rollover risk. But don't forget, these were older SUV's without stability control. In pure collisions however, they retained an advantage over lighter mid-sized cars because of their higher weight.

Which leads us to rollover risk as it stands today,

SUVs now least likely for rollover crashes From an IIHS report reported:

Each year, from 2006 through 2009, drivers of newer SUVs suffered an average of 28 deaths per million vehicles, according to the Institute. That's about half the average driver death rate for cars, which was 56.

About the same IIHS report, in Motor Trend, from 2011:

The rollover risk in SUVs used to outweigh their size/weight advantage, but that’s no longer the case, thanks to [stability control],” said Anne McCartt, IIHS senior vice president for research, in a statement announcing the report’s findings.

Also from that report: the IIHS says minivans were safest (25 fatalities per million registered vehicle years), followed by SUVs (28 per million), then pickup trucks (52 per million), with the broad “cars” class rated most deadly (56 deaths per million.)

Stability control was required in 2006 and phased in completely by 2009. When stability control is ubiquitous in passenger cars then this statistic may change. Nevertheless, that the is situation today - SUV's all have stability control and not every car has it.

On rollovers, if you buckle up then the risk is greatly reduced for you, and the rollover risk is largely mitigated. There is also a large variation of risk between specific models.

2% of crashes involved a rollover, but that 2% were responsible for 35% of the fatalities:

In 2010 alone, more than 7,600 people died in rollover crashes. The majority of them (69%) were not wearing safety belts.

If you choose to buckle up rollovers are far less of a worry. And don't forget in any case that a rollover less likely to happen now in a SUV than it is to happen in a passenger car!

Answer 2

Not exactly an answer, but there are two physical points that are more-or-less inescapable:

1. In a vehicle--vehicle collision the more massive body is subjected to smaller accelerations than the lighter one. Mass is favored here. Some readers may remembers those crash test films from the 1970s that show this effect all too well (but see the link on "quality of engineering" below).

2. In vehicle--fixed, solid object collisions (and vehicle--vehicle impacts with roughly equal masses) the expected accelerations experienced by the passengers go roughly by the inverse of the length of the crumble zone available to absorb the energy. Size is favored here.

I'm not going to provide citations for these fact as anyone who has passed freshman physics should be able to work a Back-of-the-Envelope problem to demonstrate them.

IIHS video on these issues.

Note however, that these are only two of many factors which apply. I haven't discussed collision avoidance, handling, tendency to roll (a problem with some trucks, SUVs and vans), the impact of choice of materials, quality of engineering (youtube link), compensatory behavior, what types of collisions are most prevalent (i.e. if many accidents involve sliding sideways into a tree having a long car isn't going to help you), etc, etc.

I will note that improved materials science may (indeed to some extent has) allowed cars to get lighter without getting smaller.