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Interview with Professor Pete Thomas: Why size matters when it comes to safety

Tristan Honeywill

We took some questions from our Facebook friends and put them to a safety expert. Professor Pete Thomas is the Head of the Transport Safety Research Centre at Loughborough University and a specialist in the area of accident and injury causation. He shared some interesting insights into vehicle mass, risk groups and child seat safety.


Looking at the data, is it possible to say which is the most risky class of vehicle to drive? Are certain sized cars more likely to be involved in a crash with serious consequences?

It’s hard to answer that simply. There are many other factors that determine accident involvement rates. Different vehicle types are sold to different types of driver. You have rep cars that spend a lot of time on motorways, which are our safest roads. We have small urban vehicles that tend to be used at lower speeds in congested areas and which sell to different groups.

It’s still the case that highest risk group of drivers is the 18-24 year old males. They’re young, inexperienced and can have too much testosterone, although many are perfectly sensible drivers.

Car occupant deaths are around half of all road deaths. Pedestrians, cyclists and motorcyclists make up the remainder. Most pedestrian deaths are urban and most cars in towns are smaller.


Most of us assume that 4x4s are inherently safer. Is that borne out in the statistics?

“In terms of car occupant safety there are two big factors: correct seatbelt use and vehicle mass.”

Professor Pete Thomas is the Head of the Transport Safety Research Centre at Loughborough University

There are a lot of factors involved. You’ve got to look at who you’re trying to save and then look at the whole crash and the different types of crash. Most people wonder what happens if they’re in a small car hit by a big car. That’s pretty clear, but there are other factors.

If you’re thinking in terms of car occupant safety, given that there’s going to be a crash. There are two big factors in determining injury outcome. One of those is seatbelt use and the other is the vehicle mass.

Correct use of the seatbelts is still the most effective single thing and people forget about it. It’s important also to avoid putting rearward facing child seats right next to a deploying airbag on the front passenger seat. Many people don’t appreciate that airbags deploy at around 200mph. You don’t want to put a baby’s head anywhere it can interact with a deploying airbag, even if there is a child seat protecting them. Most people have got the message that this isn’t a good idea. We don’t see many deaths from this anymore.


How important a safety-factor is the vehicle’s mass?

If you’ve got a crash between a small and a heavy car, then the basic physics of the situation are very important. If you’re in a one-tonne car that hits a two-tonne car and both are doing 30mph, the basic laws of momentum say that the two-tonne car is going to have a much smaller velocity change than the other car. The occupants in the lighter car are going to have a much more severe impact.

When you look at the Euro NCAP results, you’ll see that they class them according to size of vehicle and encourage you to only compare cars of similar mass, within 120kg.

The other factor is stability. The higher centre of gravity of 4x4s does make it more likely that they’ll rollover. I don’t know how true that it is with the most modern vehicles as most should now have stability control systems that can prevent this. If they don’t, it could be factor.


How big a factor is incorrect fitting of child seats in the death and injury statistics? 

We still see many child restraints that are not fitted correctly. Particularly the ones held in place with the seatbelts instead of ISOFIX. You find that there can be slack in the seatbelt system. And the more slack there is, the more the seat moves, the more the child moves…

In a crash the child seat can push down into the cushion of the car seat. That gives you a little bit more uncontrolled forward movement. It’s only when the slack is taken up that the child seat starts to decelerate properly.

The same applies to the straps around the child. If there’s slack, then the child’s movement isn’t controlled soon enough. When you’ve got a forward facing system and are putting it into the car, it’s a good idea to put your knee on it and push down and pull the belt tight. Try to lock the belt there before you put your child in.


Are rear-facing seats safer for children of all ages? Why aren’t they promoted over here?

It’s a Swedish thing. They favour rearward facing child restraints for older age groups there. I can understand why. The most frequent direction is a frontal impact. If you’re a child facing forwards in a frontal impact, you’re going to be restrained by the straps around you. You can work out how many square centimetres that is quite simply with a tape measure. If you turn the seat to face rearwards though, then you’ve got the whole of the seat back.

In particular the rear of the head is supported and kept in line with the neck and the chest of the child. The differential forces acting on the head and neck are very little. In a very young baby, you want them facing backwards because babies’ necks are so weak. They can’t take impact forces.

Before they became commonplace and we had carry-cot restraints, you saw some dreadful injuries. Nowadays we all know what to buy for infants, but exactly the same arguments apply to older children.

The limit is just the inconvenience of fitting them and using them with a larger child. If you’ve got a larger car, you’ve probably got space for a rearward facing system. I think they go up to four years in Sweden, sometimes even older.

Rearward facing seats don’t seem to have taken off outside Sweden and it’s mostly convenience and size that’s the argument. However, it is good practice to keep them rearward facing as long as possible.


If you’ve got any questions, get in touch via email, Twitter or Facebook and we’ll do our best to get you answers. 

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