Safe Speed Forums

What are the relationships between vehicle weight and road safety?

As time passes cars are getting heavier. Partly this is the result of additional standard equipment, partly it's a result of improved secondary safety standards. There's also a tendency (I think) for people to choose heavier classes of vehicle.

Heavier vehicles tend to protect their occupants better by virtue of their mass - hitting a standrd weight object will cause a larger peak deceleration in a lighter vehicle.

But heavier vehicles do more damage to things they hit.

Braking distance is a funny one. Heavier vehicles don't automatically have longer stopping distances, although in practice heavier vehicles often have a higher centre of gravity and that tends to increase stopping distances.

So what are the overall road safety effects of the tendency of vehicles to get heavier?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I agree with your synopsis so far.

I guess the most obvious negative effect not mentioned to date here is pollution - which is loosely linked to safety (even if not primary road safety).

Another effect is the perceived invincibility of the heavyweight, possibly subconsciously leading to greater risk-taking (qv "7.01" ).

Not sure where this is going. 4X4s have a historicaly had a poor crash rating. Thats because they are actualy weaker compared to their weight.

I think it is more apropriate to consider loading.

A car with 5 people onboard can weigh as much as half a tone than one with just the driver. Add roof rack (up to another 100kg) and s boot full of luggage (another 150kg) and you have a fully loaded car.

Just look at the maximum laden weight of your car in the hand book. Its scary!

Then add another 500kg of caravan on the back and you have a mid sized saloon with the momentum of a Silvarado Truck.

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Speed camera policy Kills

Pedestrian safety is another factor. No direct correlation and there are examples where this doesn't fit. but, the larger/heavier the vehicle the less pedestrian-friendly they are.

On balance, I would probably say marginally positive, as they provide so much better crash protection for vehicle occupants.

If you look at 1960s vehicles, in many respects they now look worryingly flimsy.

In theory, heavier vehicles may do more harm to pedestrians, but pedestrian casualty figures have continued to improve - and also newer vehicles are designed to cause less injury to pedestrians in the event of a collision.

Maybe more worrying is the effect of heavier vehicles involved in collisions with lighter ones.

I think the environmentalist would be more worried about heavier vehicles than the safety fanatic, as the improvements in engine efficiency over the years have in effect been used in propelling much heavier vehicles slightly more economically.

Another interesting question would be: "What are the overall road safety effects of the tendency of vehicles to become more powerful (and in bhp/tonne terms, not just absolutely)?"

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Nor me. That's why it's in 'brainstorming'.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Certainly there are risks associated with some large vehicles - and the rear wheels of artics have been known to mangle pedestrians horribly but...

Taking the example of a 75kg pedestrian and a 1200Kg car the ratio of masses is 16:1. I would have thought that anything over about 10:1 was pretty much the same as infinity to 1.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yup - but the impact point is much higher with bigger vehicles and more likely to knock them under instead of over.

True, but this seems to be a different point to the one Paul originally raised - vehicles are getting heavier without getting (appreciably) larger.

They are getting subtly larger. A current Focus is bigger in every dimension than a 1970s Cortina. And, if you only increase overall dimensions by 10%, you increase volume (and therefore probably weight) by 33%.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

I stand corrected. I'd not reread that subtlety. also picking up on another point raised: Yes the Focus is larger than the Cortina, but the Focus's "snout" I think is more likely to scoop a pedestrian over it compared to a Cortina's which will ... dunno really, probably double them up - 1/3 above, 1/3 below and 1/3 glued to the grill. Accordingly I withdraw my original hypothesis.

oops sorry...

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Speed camera policy Kills

I was doing some work for Ford SVT at the time the focus was in development. One of the big issues at the time was pedestrial impact. The had to re-design the bonnet catch system to gove a "soft" impact for pedestrial safety. Thats why it has got a daft lock at the front.

There are also new European laws governing pedestrial impact which VW estimate will cose them over 1Billion Euros to implement. The say the research behind it it flawed (same old story)

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Speed camera policy Kills

No need to apologise

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

As you go from motorbike up to 7.5t the safety gets considerably worse due to morons driving vehicles without decent training.

Once you jump to LGV C (Old Class 2) there is a massive improvement in safety which increases the higher up you get.

But do they?

The Volvo 140/240 ranges were always considered big heavy cars which would plough through anything they hit. This was not the case due to the large soft crumple zones.



This is something I've been meaning to look into. I need to dig out sales figures by class of car.

We've also seen the introduction of the super super mini which is comparable to the MK1 Fiesta though much safer.

So far it seems there is a shift from large family cars to SUVs but..



Shows the small car market is stronger than ever.

Figures elsewhere show the large MPV sector shrinking.

Need to do more digging but it would seem we have more small cars, more mini-mpvs and more SUVs. Does this mean more large car/small car collisions though.

And don't forget we have always had transit sized vans on the road, you never saw anyone complain they were responsible for all the road deaths (as ITV were vitually claiming for 4x4s the other week).