Is a bigger car safer than a smaller car?

Question

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?

Answer 1

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.

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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!

Answer 8

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).

Answer 9

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.

Answer 10

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.