Car Advice

Does size matter in a collision…it isn’t as cut-and-dried as you think

ANCAP awards a crash rating but do we really think a smaller car is as safe as a large car if the two collide? So, does size matter in a collision?

This was sparked by a conversation with some friends the other day when I asked about whether crash ratings factored in their thinking when buying a new car. One said, yes, but most said, no, and one even asked what ANCAP was.

But all my mates agreed that if you’re driving a small car with a five-star rating and collide with a larger vehicle with a five-star rating then the crash rating means nothing and that size is everything. But is it as cut-and-dried as all that?

Well, no. It isn’t. Occupant safety in a collision comes down to a range of factors whether the vehicle is big or small, like the age of the vehicle, its active and passive safety, the engineering of its crumple zones and so on.

To get started, we looked at what the Monash University Accident Research Centre (MUARC)had to say…only it didn’t have much to say on the matter. Rather, its authors looked at the research around size and weight in a collision context and concluded that more work needed to be done before a definitive conclusion could be reached.

Here’s some of what it summarised, “The literature on the crashworthiness relationship between vehicle mass, size and safety is rather ambiguous. There was general consensus by most authors that bigger cars were inherently more safe than smaller ones in a collision. However, trying to define this relationship more precisely from the literature is problematic, in part, because of the number of confounding influences and definitional differences”.

Okay. So, still no clearer. What else did MUARC find? “It was concluded in general terms that mass is probably a more important safety feature than size for most car-to-car collisions, although there was a suggestion that size may predominate more in rollover crashes and single-vehicle accidents generally.”

And then there was this gem MUARC stumbled across. “The level of restraint has been shown to have a marked influence on the mass (size) and safety relationship. One study reported that an unbelted driver in a 2000kg car had the same amount of protection as a belted driver in a 1140kg car. It was further claimed that drivers of small cars gained more from being restrained than those of larger ones. It is too early yet to confirm if there is any disproportionate benefit of driver airbags by vehicle size”.

So, there clearly hasn’t been a lot done in determining whether you’re safer in a bigger than a smaller vehicle in a collision, only that’s not exactly true. PM did some more digging and found some good information. According to Michael Paine, Technical Manager at ANCAP, “Generally speaking big cars will do better in a collision with a smaller car. This is a result of the physics of the crash, where the heavier object will experience a lower deceleration and therefore lower forces on the occupants” Michael says. “But if your car hits a larger stationary object, then there’s a lot more weight in a larger car, which means more crash energy to deal with.”

So, this is making more sense, right. But there’s still more to it than simply saying bigger is better. And that’s largely because none of the safety data explores the effect of one vehicle on another vehicle. It merely explores the safety of the vehicle tested in relation to a stationary object. Sure, this is good for comparing apples with apples, as long as you’re comparing small car results with small car results…and considering performance in a stationary collision event.

Researchers at the Berkley Lab’s Environmental Energy Technologies Division looked at ‘combined risk’ and that’s really what we’re talking about when we’re asking whether bigger is better in a collision. Now, so you know, the research conducted looked mid-1990s vehicles. One of the researchers, Tom Wenzel, said, “”We focused on the risk not only to occupants of the vehicle model in question in all types of crashes, but also on the risk to the drivers of other vehicles involved in crashes with the model in question.”

In the report, the authors determined that SUVs weren’t any safer for their drivers than the ‘average’ medium or large car and not much safer than many compact and subcompact cars.

Be that as it may, bigger sometimes is “better” as previously explained by ANCAP’s Michael Paine. And, in 2009, the Insurance Institute of Highway Safety, the US version of ANCAP, studied the effects of crashing larger cars into smaller cars, and concluded, “When a car crashes into a solid barrier, the outcome depends in part on the size of the front end. If one car’s front end is long enough to crush twice as much as another car’s in a barrier crash at the same speed, its restrained occupants will experience half as much force as the people in the smaller car because it takes them twice as long to stop.

“When two cars going the same speed crash front to front, the outcome depends in part on the cars’ relative weights. 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.”

Yep, this is all pure physics. But we’re still not talking about occupant safety. See, the development of crumple zones and vehicle crash structures, airbags and even things like multi-collision braking can have a huge effect when we’re talking about a bigger vehicle hitting a smaller one.

But there’s still more to the discussion. For instance, what if you’re in an older, heavier type of 4×4 and you hit a lighter more modern vehicle with a crumple zone. Who will come off better? The passengers in the heavier 4×4 or those in the lighter vehicle with a more sophisticated crash structure?

Well, we don’t have to wonder because Fifth Gear, about 10 years ago, did just that. It took a Series II Land Rover Discovery and crashed it into a current-at-the-time Renault Espace (now, the last generation model). The test was a slight offset collision at 40mph. The occupants in the Espace were better off than those in the older Discovery. If you go by the claims that a heavier vehicle with a longer bonnet (the Discovery) will perform better than that of something lighter with a smaller bonnet (Renault Espace) then the Espace didn’t stand a chance.

And the result? Looking at the wrecked vehicles you could clearly see the Espace’s crumple zone had worked perfectly in absorbing the energy and impact while the vehicle’s passenger cell remained relatively intact with the front and rear airbags firing to further protect the occupants and the impact deflected along the vehicle’s sill, A-pillar and roof. In the Land Rover, however, the solid main chassis member didn’t budge sending the shock of the impact cannoning through the rest of the vehicle causing the vehicle to crush and deform as the Espace was forced around that rail and into the softer parts of the Discovery. Indeed, the Discovery’s dashboard and steering wheel ended up crushing the legs of the crash test dummy, the door creased and the passenger cell buckled.

That was head on, but what about if one had been crashed into the other in a T-bone scenario? Well, the IIHS tested for those sorts of things and found that older 4x4s with a ‘higher’ physical structure posed a greater safety risk to those in a smaller car because the vehicle’s main structure would act like a battering ram.

So, in our above example of the Discovery and Espace collision, it’s likely the Discovery would have had the potential to do more damage in a side-on collision because of the height of its main chassis rails, but the Discovery would still have suffered in the collision because of its lack of energy absorption which still would have transferred onto the rest of the vehicle. The Espace occupants would likely still have fared better due to the side airbags and strength of the passenger cell and its ability to absorb and deflect energy.

Recent tests by the IIHS have shown those driving modern 4x4s pose less risk to those in smaller cars, in side-on collisions because the physical structure is now lower and stronger passenger cells and side airbags have improved occupant safety.

So, while bigger is sometimes better there’s more to it than that, like the age of the vehicle, its passenger cell construction and strength, its energy-absorbing crumple zones, how the collision occurs, and so on. So, never look at a heavily crumpled car and think it performed poorly, look at the passenger cell and how the impact affected the occupants, because it’s likely the vehicle crumpled to absorb energy and protect the occupants. So, is big better in a vehicle collision? No, not always, but there’s still always a but and in this case it would be if an older, heavier 4×4 collided head-on with, say, a medium car. The 4×4 would likely ride up and over the bonnet of the medium car with its stiffer chassis members battering the smaller vehicle.

But where modern vehicles are concerned the differences become much smaller. This is a complicated topic and not so cut-and-dried as my mates would have thought.


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Isaac Bober

Isaac Bober