Do You Need a Bigger Car to be Safe?

One reason for choosing a big vehicle in spite of the gas savings hit is safety. Most people feel that a big vehicle will be overall safer to drive. Is that true? The Monash University Accident Research Centre tried to find out and the result was this report.

They looked at two measures of vehicle "bigness": the mass and a measure of the physical size such as volume or wheelbase length. The Monash study was one of those "study of studies" where they reviewed many other studies in the literature looking for common factors and data. The aim was to find how size and mass are related to occupant safety. There were problems getting definitive results due to the variability of the data. For example, some studies used interior cabin volume to measure size and others used wheelbase. But overall?

Overall they found that in multi-vehicle crashes (things like head on collisions, rear ending, sideswipes, anything with a least two cars involved) the bigger the vehicle mass, the safer the occupant. On the other hand, in single vehicle crashes (rollovers, hitting a tree, crash into a wall) the bigger the vehicle size the safer the occupant.

In multi-vehicle crashes it makes sense that bigger mass will help more. Physics says that the bigger car will always be least effected in a 2 way crash. It also makes sense that mass doesn't help much in single vehicle crashes. If you hit a wall or embankment, it doesn't matter how much mass you have in your car: the obstacle will always outgun you. On the other hand, if there is a lot of interior space, there is room for the vehicle to crumple, compress and slow down gradually without the driver's body being impacted by something.

How can we relate this to saving on gas? Well, careful design of interior cabin space can give us vehicle inner volumes that are as close as possible to the outer volume (no wasted space). Careful aerodynamics will let us make larger volume, light mass vehicles with little air drag. So in that respect, we can have safety and gas savings at the same time. But we need Detroit to play along and start designing vehicles accordingly.

With regard to mass, it is the direct enemy of fuel economy and they can't really be made to live together. But if all the vehicles on the road are lighter, being made of composites and modern light alloys, then what counts as a relatively "massive" vehicle in a two-way collision will also be lighter. We might see that over time, the "heavyweights" of the road become quite light compared to our heavyweights of today. That would open up the choice of driving a "heavy" car for safety and still saving on gas.