Size Does Matter: Vehicle Compatibility

Over the last 15 years or so, since the introduction of EuroNCAP, vehicles in general have got heavier due to the introduction of better crash structures - more metal = more weight.  For example, a 2012 VW Golf is approximately 20% heavier than its 1995 predecessor.  

It may not really come as a surprise that if you are in a bigger, heavier vehicle, you are more likely to come off better in a collision with another vehicle - it is simple physics after all.

Interestingly though, over the last decade there has been a noticeable shift in the type of vehicles people in the UK drive.  The diagram below shows that, since 2000, there has been a 22% reduction in the volume of medium saloon cars, while both 4x4/people-carriers and minis/superminis have increased 192% and 29% respectively and all other vehicle types have remained steady.  

Source: Reeve, I. & Lloyd, L., The link between casualties and demographic data – and the importance of vehicle design. Presentation to Road Safety GB Conference, November 2012. 

This demonstrates that the UK vehicle fleet is diverging in terms of vehicle size, meaning greater potential for collisions between vehicles of different sizes/masses.  These are known as incompatible collisions, and include the following issues:

• Bumpers/crash structures often do not line up so the crash structure of the smaller vehicle is not properly engaged by the collision - this is particularly evident in crashes involving small cars and 4x4s, and the effects can be even worse for side impacts.
• Due to the difference in mass, the acceleration forces are greater on the smaller vehicle (and therefore on its occupants).

Although this video is from the US Insurance Institute for Highway Safety (IIHS) and the models of vehicle are a few years old now, it explains the principles of incompatibility really well.

So, it may seem that the simple solution is for everyone to go out and get a bigger car?  Not so simple unfortunately, as there would almost certainly be an adverse effect on pedestrian and cyclist survivability.  However, manufacturers could start to make their vehicles more compatible in terms of bumper and crash structure heights.  My personal view is that small cars are fine around town where the conflict speeds are much lower and there are higher volumes of pedestrians and cyclists, but out of town I'd prefer a big car any day... just in case!