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  1. From the point of view of mass, a smaller car appears to be a disadvantage if in an accident with a bigger car, because the change in momentum of the smaller car will probably be higher.
  2. A bigger car carries more material that can absorb energy via deformation upon collision (on the other hand, bigger cars have higher kinetic energy than smaller cars at the same speed).

Is such analysis correct? (i.e. that bigger cars are safer) If yes, should it serve as a safety guideline when purchasing a car?

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    Is this assuming that an accident is inevitable and that it could not be avoided by being in a smaller, lighter, more agile car which changes direction quicker, stops quicker and fits through smaller gaps? – Steve Matthews Sep 16 '16 at 9:15
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    5th Gear did a good comparison to debunk this, using a volvo 850 and a Renault Modus. The more modern design of the Modus, with crumple zones etc. made it the safer vehicle for occupants. Obviously, this is an extreme example given the relative ages of the designs, but it illustrates that weight isn't such a big factor. – Neil P Sep 16 '16 at 13:53
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    here's a fun example of why you really need to be looking for youtube.com/watch?v=C_r5UJrxcck – Matthew Whited Sep 16 '16 at 14:00
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    here's an example of tiny versus big ... youtube.com/watch?v=kLm3xzCWZdg – Matthew Whited Sep 16 '16 at 14:02
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    Higher kinetic energy is really more of a concern for the things the bigger car hits, not the bigger car itself, since a collision will transfer the energy. – TMN Sep 16 '16 at 15:08

10 Answers 10

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In aggregate yes, however that's not the same as saying all large vehicles are safer than all small ones. The Insurance Institute for Highway Safety aggregates and publishes data about fatality rates in new vehicles in the US. You can see a general trend toward larger being safer, but bigger being safer is not guaranteed. The IIHS also publishes data about changes over time (available at the source); the short version though is that newer small cars compare favorably with older large ones.

http://i.imgur.com/Yp1Z3gM.png

Driver deaths per million registered passenger vehicles 1-3 years old, 2014

                                    Registered vehicles     Deaths  Rate
Cars                    Mini                  1,131,535         62    55
                        Small                 7,251,650        288    40
                        Midsize               9,700,209        335    35
                        Large                 2,741,490        116    42
                        Very large            1,724,015         32    19
                        All cars             22,548,899        838    37
Pickups                 Small                   777,825         32    41
                        Large                 3,495,386        115    33
                        Very large            1,167,182         41    35
                        All pickups           5,440,393        207    38
SUVs                    Small                 3,662,803         76    21
                        Midsize               6,509,578        102    16
                        Large                 1,734,489         31    18
                        Very large              377,309         13    34
                        All SUVs             12,284,179        223    18
All passenger vehicles  All                  40,887,585      1,290    32

For anyone interested in more details of how it breaks down per vehicle, I've found a report the IIHS did in 2011 showing stats for dozens of the most popular vehicles. This can be read online, but is an electronic copy of something that was apparently originally a bound paper booklet. The middle column of the per vehicle model table is split between two pages and is much easier to read if you have a large display and can download the pdf and view two pages side by side.

This is a noisy dataset (for many of the less common vehicles only a handful of fatal accidents are involved); but the spread within a single vehicle type is often larger than the spread between the averages for overall vehicle types. It's also several years old (2005-8 model years vs 2011-14 for the graph/table above).

Even in such a short period of time you can see the overall improvements in vehicle safety, the 55 deaths/million vehicle years rate for mini cars from 2011-14 is about a third less than the equivalent from 2005-08 models, and comparable to mid/large size cars from the era.

Over longer periods the overall improvement is even more dramatic. I made this graph from a table of data in the IIHS report linked to at the top of my answer. Modern vehicles of all types are roughly 5x less likely to kill occupants in accidents than they were 40 years ago. For SUV's its more like a 10x improvement; although that's mostly driven by massive improvements in the late 70s and early 80s that brought them inline with other vehicle types. Since then they've behaved more or less like any other vehicle type. You can also see the effect of electronic stability control becoming standard in pickup trucks at the end of the last decade as their survivability numbers improve significantly due to the reduction in rollover rates. https://i.imgur.com/D7bSWeK.png

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  • "isn't not guaranteed" --> "is not guaranteed" – jpmc26 Sep 16 '16 at 17:40
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    I think it would be interesting to also look at traffic deaths per collision. That way you could rule out confounding variables (e.g. if small cars just get in accidents more often) and get straight to what you really want to know. When in a collision, is a small car safer? – Michael Molter Sep 16 '16 at 17:42
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    @MichaelMolter agreed that would be interesting, however I'm not aware of anyone who publicly aggregates the data in that fashion. – Dan Is Fiddling By Firelight Sep 16 '16 at 18:44
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    Assuming most of the deadly accidents occur at high velocities, I suppose pickup trucks have less time on highways overall, making them more deadly than they appear in IIHS data. – Sparkler Sep 17 '16 at 16:58
  • @Sparkler I'd also assume heavier cars will have a higher chance to kill the other party in any 2 party accident. There are a lot of variables this kind of dataset cannot take into account. – Peter Sep 18 '16 at 12:52
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This is a question that has been addressed by Consumer Reports and other publications that are easily accessible via a Google search.

To provide an answer to your question, larger cars are not necessarily safer, according the the US Dept of Energy and the Berkley Lab's research.

The Berkley lab article also notes that vehicles are being made lighter with no sacrifice in safety.

There are too many variables involved in auto fatalities to make a determination that larger vehicles are necessarily safer. Many times they are less safe, and in few situations they are more safe, especially in head-on crashes with small vehicles.

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    The first article states that SUVs are safer in head on collisions, and that the risk of rollover is substantially reduced for newer models. The second article says, again, that SUVs are safer in collisions. The second article is also considering the increased risk of death to the other participants in collisions. It's clear that bigger cars are safer for the driver of the vehicle, and more dangerous to the drivers of other vehicles. It's basic physics and supported by the articles you linked as well as Dan Neely's answer below. – Brandon Sep 16 '16 at 17:21
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    Actually, as a matter of fact, the conclusion of both of those articles is that bigger vehicles are not necessarily always safer. You are ignoring the conclusion of each article and cherry picking the reasons why larger vehicles can be safer, and ignoring what makes larger vehicles more dangerous. – Drawninpictures Sep 17 '16 at 22:46
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All other things being equal, a bigger (more massive) car would be safer – for its occupants. But all other things are rarely equal, either in the design of the car or in the dynamics of the crash. Hence crash testing and endless debates about passive and active safety…

So, if you're looking for a safe car, you really need to look at the data on the specific (model and year) car that you're interested in – and if you're looking at a used car, how it has been maintained. Then you need to do your best to evaluate the factors that might change the results in your particular case (or whomever will be driving the car). For example, if you drive a lot on ice or snow, a front- or all-wheel drive car might actually be safer than a rear-wheel drive vehicle that does slightly better in crash tests.

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  • Why is this the case? The majority of crashes are single car crashes (lost of control, going off the road, etc..) and a much larger collection of fatalities are single-car road departure accidents. The problem with larger cars are that they tend to trip more and they have more energy when they strike trees and guard rails. I don't think you can easily say that bigger is better. Bigger usually means larger engines which also mean faster which doesn't help in accidents as well. – Gray Sep 15 '17 at 19:30
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tl;dr: It depends on many factors. Size is not a magic spell of safety.

Let's break down the first paragraph from the same previously cited IIHS link:

By far the largest number of motor vehicle crash deaths are occupants of passenger vehicles including cars, minivans, pickups, SUVs and cargo/large passenger vans. The likelihood of crash death varies markedly among these vehicle types according to size.

First, let's assume completely identical construction with no variation in passive restraint technology. This imagines that all vehicles are using the same materials and metallurgy, for example.

Small/light vehicles have less structure and size to absorb crash energy, so crash forces on occupants will be higher.

This makes sense. If the components are identical, there's less volume in which to absorb the energy. However, remember that a less massive car has less energy to absorb in a single car impact.

Hm, things are already getting complex.

People in lighter vehicles are at a disadvantage in collisions with heavier vehicles.

This also makes sense, not just because vehicle collisions tend to be inelastic and there's a mass difference. Larger vehicles often carry their mass higher off the ground. In the extreme case of a jacked-up pickup, the truck could climb over another car in an impact. This isn't a feature.

Pickups and SUVs are proportionally more likely than cars to be in fatal single-vehicle crashes, especially rollovers.

This is a consequence of the higher center of gravity. A top-heavy vehicle needs to be more cautious in a dynamic situation. Is a car that is more likely to rollover in a turn or a hard-braking avoidance maneuver "safer" than one that is less likely to roll? That might mean that a larger vehicle is less able to avoid an accident. Is that "safer"?

However, pickups and SUVs generally are heavier than cars, so occupant deaths in SUVs and pickups are less likely to occur in multiple-vehicle crashes.

This sentence is a bit vague. Data is data but is vehicle weight the only deciding factor? It is hard to tell with summary data that appears in the rest of the article. It certainly hides factors such as vehicle avoidance and simple ability to stop (remember, bigger and heavier vehicles have longer stopping distances...).

However, let's look at a counter-example: Kevin Magnussen's impact at SPA in 2016. He lost control at a speed that was almost certainly higher than 180 mph. He hit the tire wall, backwards, seconds later and essentially shattered his car. Shortly after the impact, he climbed out of the car and limped away.

Note: this was in a car that must be less than 180 cm wide and 95 cm tall. I think we can safely consider this car to be "small."

So, no, small cars are not always at a disadvantage.

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I read an interesting article a while back that views the question slightly differently. Their take is basically that it is likely any additional safety for the occupants will be a pyrrhic gain, imposing a disproportionate safety risk to those around you for a small personal gain.

The “Arms Race” On American Roads: The Effect Of Sport Utility Vehicles And Pickup Trucks On Traffic Safety

Abstract

Drivers have been running an “arms race” on American roads by buying increasingly large vehicles such as sport utility vehicles and light trucks. But large vehicles pose an increased danger to occupants of smaller vehicles and to pedestrians, bicyclists, and motorcyclists. This paper measures both the internal effect of large vehicles on their own occupants’ safety and their external effect on others. The results show that light trucks are extremely deadly. For each 1 million light trucks that replace cars, between 34 and 93 additional car occupants, pedestrians, bicyclists, or motorcyclists are killed per year, and the value of the lives lost is between $242 and $652 million per year. The safety gain that families obtain for themselves from driving large vehicles comes at a very high cost: for each fatal crash that occupants of large vehicles avoid, at least 4.3 additional fatal crashes involving others occur.

That last sentence is compelling.

The safety gain that families obtain for themselves from driving large vehicles comes at a very high cost: for each fatal crash that occupants of large vehicles avoid, at least 4.3 additional fatal crashes involving others occur.

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Short answer: no.

When crashing a small and large car together, there might be a difference, but you'd be far better off, as others have said, examining the specific safety scores and record of each individual model.

If yes, should it serve as a safety guideline when purchasing a car?

Hypothetically:

If people based their purchasing on the size of the vehicle, then the average size of vehicles would increase until it was limited by some other factor. Everyone would be driving giant cars which were all approximately the same size, and size would no longer be a variable in the calculation of safety.

The same goes for working on the assumption that smaller cars are inherently safer.

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    RE your hypothetical: one could make a case that the popularity of SUVs exemplifies this, and that price (of the car and fuel) is a major limiting factor. – jpmc26 Sep 16 '16 at 17:39
  • This answer contains mistakes. First you say "no" but then go on to contradict that by saying there might be a difference. Then you claim that if all cars increased in size, size would no longer be relevant. That is not true: if there is a correlation between size and safety, and all cars get bigger over time, then the fatality rate would drop for everyone as a whole. – JBentley Sep 17 '16 at 7:11
  • @JBentley: He says the "short answer" is no. As in, the answer is more false than true, so a one-word answer should be in the negative. And the correlation between size and safety is explicitly because of the small cars. Two smalls cars in a head-on are inherently safer than two large SUVs, because the small cars should collide at a reduced speed. But with a large SUV vs. a tiny car, the SUV wins. Ergo, the guy with the biggest vehicle is safest in a car-to-car collision, but only because he's unlikely to collide with another large vehicle. – MichaelS Sep 17 '16 at 7:31
  • @MichaelS That's ignoring the second premise of the question i.e. that a larger mass provides more deformable structure. We can't simply assume that it comes down to a competition between the size of the two vehicles involved in the collision. – JBentley Sep 17 '16 at 7:44
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it is very dependant on design and construction - Euro NCAP rating is probably a good indicator for you. For example, the 2015 Honda Jazz (a Supermini) scored 5 stars whilst the 2016 Hilux (Pickup Truck) scored only three

Clearly size isn't everything!

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    You cannot compare the Euro NCAP rating for different vehicle classes: In the Euro NCAP the vehicles are test-crashed against vehicles with similar size/type. en.wikipedia.org/wiki/Euro_NCAP#Comparing_test_results. This way you can compare the safety of different vehicle of the same class. The result mentioned by you by no means indicates that in a crash of a Honda Jazz against a Hilux the occupants of a Honda Jazz will have a higher chance of survival than those of the Hilux – Martin Sep 16 '16 at 8:48
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    I agree, the mechanics of an accident involving a larger car and a smaller car are not favourable for the smaller car - but a Honda jazz is certainly safer to drive than the Lancia ypsilon, and the Hilux is less safe than others of its class. If money is not an option I'd definitely recommend getting a bigger car with a higher rating. My point was just that Bigger does not equal Safer, there are other factors to take into account as well. I'd argue that you'd probably be better off in the five star'd smaller car than in a bigger two or three starred one. – Miller86 Sep 16 '16 at 9:17
  • Standardized collision tests also provide data mainly on the standardized type of collision. It is intended to be indicative, but is far from guaranteed to be accurate, as an assessment of how a given vehicle will fare in a real-life accident similar to that being tested for. That's what you get when you want a single metric that can be compared between different vehicles. Same reason Volkswagen did what they did with their diesel cars: when the test has actual, real, serious implications, then there is a very real incentive to design to get as good a test result as you can. – user Sep 17 '16 at 13:38
  • I agree in principal that they are indicative, but would disagree that they're inaccurate because they're standardised. Fundamentally, these test collisions are as good as you can get them at testing some of the most common forms of accident - Head on, Rear and Side impact. They also test a human analogue. giving a good indication of the sorts of stresses you'd be under in a crash. In the case of the Hilux for example, you'd find in a head on collision that your head would hit the steering wheel - admittedly not hard enough to seriously injure you but still not ideal! – Miller86 Sep 20 '16 at 7:57
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In my opinion the safer car is the best handling one. I believe all these analyses are for deaths in case of a crash. A better handling car avoids many crashes but is lighter (in order to handle better) and has a higher death rate per crash. I always prefer the best handling car to avoid as many crashes as possible since cars are made safe for crashes with 60km/h (in case of frontal collision they both have to go with 30km/h). So the safest car for me is light, large with predictable handling (mostly "family-friendly" sports cars).

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Other answers have given real information, so no.

But hypothetically, yes:

  • A heavier car has more momentum, hence in case of collision with anything that can be deformed, the car will slow down over a longer distance, reducing the deceleration for the passengers
  • A longer car (in front) will have more deformable structure, again giving more time to decelerate.
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  • I witnessed a wreck where a Ford F150 was taking a u-turn and failed to see a Subaru Impreza closing in at 50ish mph. In that case, the 3000 lb Subaru had MUCH more momentum than the 5000 lb Ford, and traveled a good 40 yards after the point of impact before coming to a stop, whereas the Ford only slid 10 yards (coincidentally, it ended up exactly where it started in the left turn lane). Both cars were <5 years old, and all occupants were without serious injury. – MooseLucifer Sep 16 '16 at 16:00
  • Seems like these two factors cancel out for long, heavy cars. – djechlin Sep 16 '16 at 16:38
  • Most other answers have not concluded that the answer is "no". – JBentley Sep 17 '16 at 7:12
  • @MooseLucifer: Momentum is relative to the cars in the collision, not the pavement. From the point of view of the Impreza, the F150 had a momentum of 50 mph * 5000 lbs = 50693 kg m/s, while the reverse point of view puts the Impreza at only 50 mph * 3000 lbs = 30416 kg m/s. The "slowing down" is actually "acceleration". The post-crash deceleration is only relevant for avoiding secondary impacts. The answer assumes we're talking about two vehicles moving about the same speed relative to the pavement, but works fine for other speeds if you transform the concepts properly. – MichaelS Sep 17 '16 at 8:44
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It depends on the type of the collision, heavier vehicle will have more potential energy so going of the road it will go further than a small one - will have more rolls and will hit more objects will get deformed more by hitting a tree that it can't 'cut' or a wall (rocks).

In case of collision of two vehicles weight ratio comes into play and heavier one will crush smaller one under normal circumstances, plus in case of side collision when changing lanes heavier one will not bounce as much as smaller which will fly to the side of the road.

It all depends on the situation but bigger is more likely to be better.

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