[GKR9900]

The Ministry of Road Transport and Highways (MoRTH) is observing road safety week from January 11th to 17th, 2023, to promote awareness about road safety to the masses. Various activities are being organised throughout the country involving citizens and officials to promote road safety. Being a medico by profession and an automotive enthusiast by passion, I wanted to contribute something from my side that could help in this cause. I am better known for my medical educator role among my circles. So I decided to put that into effect. This is my humble effort at educating about one of the most common scenarios that occur on our roads on a daily basis – a car crash, from the viewpoint of a medico trying to incorporate principles of physics and biology, or should I say physiology! What happens to the human body in a car crash? Why does it happen? What are the steps that we can take to modify fatal outcomes? I try to answer these questions in the video below:

https://youtu.be/wmLFfFIjs6M

For the uninitiated, let us discuss about that in the following section.

An estimated 400,000 road traffic accidents and 150,000 deaths occur on our roads every year. This equates to about 15-20 deaths an hour. Those who are critically injured are even more. Although we might think that two-wheeler accidents make up the bulk of this statistics, car accidents are not so far behind. They are the second most common cause for injuries and fatalities. And the reason for almost 75% of these accidents is over-speeding. Knowing certain basics of a car crash could help us make sound decisions that can potentially save our lives.

A large percentage of car crashes are head on collisions with a stationary or moving object. For the sake of simplicity, this is what we are going to focus here and I will try to keep the technical and medical terminologies to a minimum for better understanding.

Physics behind a car crash

When we mash that accelerator pedal and build up speed in a car, we are creating kinetic energy. When we slow down, we apply brakes which helps in dissipating this kinetic energy as heat. In a normal braking scenario, the energy is dissipated gradually over a period of time in a manageable way. What happens when we brake hard? Here, the time available to come to a stop is less but energy has to be dissipated nevertheless. Deceleration occurs rapidly as the energy needs to be dissipated faster, leading to the violent shedding of speed. You might remember that last instance when a hard braking threw you forward. Nothing personal here, just energy trying to dissipate itself. Keeping that in mind, imagine a scenario where the entire kinetic energy of a moving car and its occupants need to be dissipated in a matter of milliseconds and almost nil distance. This is what happens in a head on collision. When there is not enough time available to dissipate the entire kinetic energy as heat via brakes, it finds other ways to dissipate itself, one of which is a massive amount of mechanical energy. Some transfer of kinetic energy into heat and sound also takes place. The transfer of this mechanical energy from a metal cage (the car) into a fleshy capsule (the human body) is what happens during a crash. The energy released can be so huge that it can rip through steel, not to mention fragile human tissue. That is why I said in the beginning- this is where physics meets biology; tremendous amounts of physical forces exerting their effects on biological systems.

So what are these effects?

To answer that, we need to look at a crash from a different perspective. In a single car crash, there are 3 collisions which take place.

Collision no.1 : Metal hits metal
Here the car collides with an object and comes to a stop. Based on the nature of the collision, the speed and size of the involved vehicle, it can come in many forms. Kinetic energy of the car dissipates by breaking the car’s structure, the object that comes into contact and the rest is transmitted onto the occupants.

Collision no.2 : Body hits metal
This is felt directly where the occupant collides with an object inside the car like a steering wheel or dashboard. When a car is mobile, the occupants inside are moving with the same velocity as that of the car. When the car suddenly stops, unless some restraining mechanisms are in place, the occupants continue their motion until they hit an object. This is in accordance with Newtons 2nd law of motion, an object in motion remains in motion until acted upon by an external force.

Collision no.3 : Body hits body
Here, the internal organs bump into the inner lining of the body cavities or bump against each other. Our internal organs are housed inside cavities, whose walls are made of bones, muscle and skin. Similar to the occupants inside the car being subjected to kinetic forces, the organs inside the body experience the same. When the occupant hits an object inside and comes to a stop, the internal organs are still moving along with the original velocity, bumping into each other. This can cause a variety of damages.


So what kind of damages are we talking about? We are fragile creatures. And there are a lot of things that can break when crash energy enters into our body. We will try to understand some of the most common and potentially fatal injuries that can occur.

Biology behind a car crash

Every tissue in our body has got an inherent strength to it. Collisions during a car crash release shock waves throughout the body. These shockwaves exert varying amounts of tensile, compressive, shear and torsional stress on the occupants. When the forces generated by these waves exceed the native strength of the tissues, injuries occur involving everything from cells to organ systems.

Head injuries are one of the most dreaded injuries that can occur in a car crash. Our brain is a soft and pliable structure that is safeguarded by multiple layers of protection. We have the skull, the skin and muscles over the head that helps in this regard. The brain is cushioned and suspended inside the skull in a fluid known as cerebrospinal fluid (CSF). To start with, blunt force trauma can cause skull fractures and hematomas on the outside of the brain. If the forces are stronger, it can lead to dreaded traumatic brain injury (TBI). There are multiple ways in which TBI can manifest.

When the head suddenly decelerates as in the case of a car crash, the brain suspended in the CSF continues moving forward and hits the front side of the cranial cavity, then rebounds to hit the backside. This is known as a coup-contre coup injury.


This can cause shearing of the blood vessels and nerves inside the brain. Shear stress can tear blood vessels causing bleeding into the brain tissue. The problem here is that the bony skull is a closed container which is already packed tightly with the brain, nerves, vessels and other structures. When blood leaks out into this closed cavity, as there is no further space to expand, pressure inside begins to rise. This can compress the fragile brain tissue causing damages. Loss of blood also results in inadequate perfusion of the brain tissue leading to its death. Shear stress can damage the nerves arising from the brain, leading to something known as a diffuse axonal injury which results in neurological deficits. The shockwaves in a crash can cause changes at a cellular level also, with various chemicals and ions leaking out of the damaged nerve cells. These multiple insults cause brain tissue to die off, in a process known as necrosis, leading to loss of precious grey matter. Depending on the extent of damage, the occupant can have a myriad of presentations from memory loss to coma and even death.

The neck connects the head to the body. Sudden deceleration can cause whiplash injuries as a result of neck hyperextension and flexion, although they are more common in rear end crashes. This can damage the neck structures like muscles, ligaments, blood vessels and cervical spine.


The thorax is another major cavity that houses important organs. During sudden deceleration, heart and lungs can hit the inside of the ribcage or breastbone causing contusions. This can also cause rib fractures. A fractured rib can puncture the lung causing air to leak into the thin space between the lung and its outer lining, a condition called as pneumothorax. Just like TBI in the head, a dreaded complication that can arise in the thorax is traumatic aortic injury (TAI). Aorta is the major blood vessel that supplies blood from the heart to various areas of the body. Its initial part (ascending aorta and arch) is mobile and the rest is fixed (descending aorta). During sudden deceleration, the mobile part continues moving forward tearing itself away from the fixed part, leading to TAI. If the forces are strong enough, it can even lead to rupture of the aorta, which is as fateful as it sounds.


Your abdomen contains a lot of delicate and important organs, which unlike the brain, heart and lungs are not protected by a bony cage. This puts them directly in the line of fire of the shockwaves arising out of a car crash. Injuries ranging from contusions to lacerations can occur depending upon the strength of the crash. Imagine your liver or spleen getting sheared in half. Not a pretty picture right?

During a crash, it is natural for the occupants to adopt a defensive posture. The limbs that stick out of our body – the arms and legs, are among the first structures that get struck. The bones inside can break, they can tear away neighbouring vessels and nerves causing a whole lot of problems.
I know, we might be getting a bit too ‘medical’ here. This is just the tip of the iceberg. Depending on the nature of the collision, the velocity of impact, position of the occupants and a whole lot of other variables, there can be umpteen number of injuries that can occur in a car crash. Not to mention the post-traumatic stress disorders that some can be crippled with.

And you may not even need to be subjected to a crash to realise the horror. I still remember that one case I came across during my residency training 10 years ago, where a 20 something year old bike crash victim came into the ER, unconscious and mangled up. Some cloth was wound around his right forearm. I took off the cloth to assess his injuries and I still can’t shake of that feeling. The force of the crash had shattered his forearm revealing the anatomy of his bones, arteries and nerves – albeit fractured and mangled. I was 23 at the time, the proud owner of a Yamaha FZ and I dreaded taking the motorcycle out for a few weeks. The kid survived, but his arm was amputated.

This begs the question – is there anything that can be done to ensure occupant safety in case of a crash? Where do we start? We start in our heads.

How to modify a car crash?

First we need to rejig our perspective. We need to stop referring to crashes as accidents. You don’t have to take my words for it. BMJ - British Medical Journal, is one of the iconic communicators in medicine. In 2001, they banned the word ‘accidents’ to be used in association with traffic incidents like these. Referring to crashes as accidents doesn’t tell you how to prevent them. That takes away the predictability factor. Realise that it is a crash. It is predictable. There are people, objects and scenarios that are involved. There are modifiers that can change the outcome.

Remember when I said that speeding is the cause for almost 75% of car crashes? That is the first and most important modifier. Speed, or in better terms, velocity! The impact and the resultant shockwaves disrupting the car and occupants are all a by-product of the kinetic energy trying to redistribute itself. How do we calculate kinetic energy? Your high school physics teacher says “hi”. KE = ½ mv2. It is directly proportional to the square of the velocity.

When you’re involved in a 50 km/h crash in a 1.5 ton vehicle, the energy released is close to 150,000 Joules. That is like a 1000 people trying to punch you all at once. Double your speed and you are left with 4 times this energy to dispose off. Imagine fending off 4000 people trying to have a go at you. That is the size of an academy award crowd! And Chris Rock could hardly fend off a slap from Will Smith.


Imagine if he had to face this

Believe me, no matter how strong you think you are, that is just way too much energy to handle. Reducing your speeds to 40 km/h decreases this kinetic energy to just 2/3rd of its initial value. Reducing speed also gives us enough reaction time to apply brakes and slow down even further. Now, the kinetic energy might look more manageable, but it can still cause considerable damage. That is where engineering come into play.
Just to have some fun, I decided to play around with a simple experiment that I found in the crash science section of IIHS (Insurance Institute for Highway Safety) website: The eggselerator!
For those who are interested, this is their URL. Their resources are really well laid out.

My rendition of this experiment was a little different from theirs. I decided to use a toy car instead of building one out of wheels and mini axles. The idea is simple. To design a mechanism using paper that can prevent the egg from cracking when it hits a stationary wall. I also decided to have some fun with my phone’s slo-mo cameras while at it. And these are the results.


What I ended up doing was create a rudimentary crumple zone.

Whatever energy that is generated during a crash can be diverted onto other structures away from the passenger compartment with the help of crumple zones. Modern cars are engineered with crumple zones to absorb as much energy of the impact as possible and allow sufficient time for the occupants to decelerate so as to minimize injuries. Remember collision 1? Crumple zones help in mitigating this. This is why choosing cars that are specifically designed to absorb this impact energy is important. It is not exactly the weight of the car that determines its crash worthiness. It is the way in which everything is designed and fitted to divert that energy away from the occupants that matters. Cars like these are the ones which usually score well in various crash tests.

The second collision, where the occupant’s body hits against an object inside the car, can be taken care of by using restraint systems. That is why buckling your seat belts are very important. In the event of a crash, the seat belts slow down the occupant from hitting on the inside of the car. But simply wearing seat belts is not enough, you have to wear them properly.
There’s a really good thread (Guide: How to fasten your seatbelt properly)already on this topic in team-bhp.

Seat belts anchor us to the car at two of the strongest points of our body-the chest and the pelvis. The lap belt should be close-fitting to your hips, the shoulder belt should be centered over the opposite collar bone. If the belts are not positioned properly, they can wander off into the nearby soft tissues like neck or abdomen, which can prove to be disastrous in case of a crash. The next time you see someone walking off a car crash with a broken collar bone, ask him to thank his stars, because the seat belt just did its job!
Then we have the supplemental restraint system – the airbags. When the sensors in the car detect that the crash is severe enough, the airbags are inflated in an explosive manner. The occupants are cushioned against impact and they slowly deflate providing ample time to dissipate energy. Here also, wearing seatbelts are important as

1. Most cars enable airbags only if the seat belts are fastened
and
2. The occupant should contact the airbags only after complete inflation.

If not, the occupant will be subjected to the considerable amount of force that is produced while inflating an airbag leading to injuries.

Regarding the third collision, there’s nothing that can be done to avoid the organs bumping into each other. That is inevitable. But there is one thing that can be done. Reduce the energy, reduce the speed. Reducing speed mitigates all 3 types of collisions that occur in a car crash.

If you are among the really tolerant ones who have read this long thread till this very moment, you might have noticed that in many instances I have mentioned about how the extent of injury varies with the magnitude of force, which ultimately depends on the amount of energy to be dissipated. And this energy depends on…you guessed it….the velocity times itself.

Remember – 2 x velocity, 4 x energy; ½ x velocity, ¼ x energy.

Engineering prowess has enabled us to find ways for reducing impact energy and extending impact time. But the best solution would be to prevent crashes in the first place by following safe driving practices. No matter how skilled we are as drivers, or how capable our machines are, we still cannot beat physics or its effects on human physiology.

Remember: Drive slower to live longer.

[ruzbehxyz]

Thanks for sharing this. Wonderful compilation of various details and aspects.
Normally cars are tested at 64 kmphr, so I’m wondering what would happen if one crashes at 100 kmphr which is the official speed for most highways.

[vedirah]

Thanks for the video! I've circulated it among my family and friends. I have a few cousins who are almost repelled by the idea of wearing seat belts for some reason. The other day we were going to Varkala and I told her to wear seat belt. She had such a frown on her face like I asked her to jump off a cliff or something. They labelled me 'lame' for wearing my seat belt. Eventually they budged and agreed to wear seat belt when I told them I wouldn't pay the fine if we got caught.

Today she got her own new car, a Swift. I sent her this video and many others describing the importance of seat belts. Now, my question to this forum - is there any way to make the seat belt reminder beeps louder or more persistent? I would like the sound to stay on permanently until the time she wears the seat belt.

[GKR9900]

Originally Posted by ruzbehxyz

Normally cars are tested at 64 kmphr, so I’m wondering what would happen if one crashes at 100 kmphr which is the official speed for most highways.

I had also wondered about the same. The only saving grace in such cases could be that the outcome also depends on what one crashes into, whether that structure yields to the forces, the run off areas available etc. so that maximum energy can be dissipated before finally entering into the occupants. Sharing a video of a Huracan crash from Hungary at speeds of 300km/h. Of course, the safety rating of a Huracan would be higher, and the collision wasnt exactly head-on. But just as importantly, the barricades, run-off areas and even the bushes and shrubs have played a role in the occupants coming out alive.

https://youtu.be/Jvk2hSIJd9A

That's why road infra needs to be laid down scientifically, which may not always be the case in India. We have too many intrusions and variables that can make driving at high speeds multiple times risky than in a developed country. We have far too many examples in team bhp itself where poorly designed roads have been a culprit in many crashes. The only variable that we can effectively control lies under our right foot!

Originally Posted by vedirah

She had such a frown on her face like I asked her to jump off a cliff or something. They labelled me 'lame' for wearing my seat belt. Eventually they budged and agreed to wear seat belt when I told them I wouldn't pay the fine if we got caught.
Now, my question to this forum - is there any way to make the seat belt reminder beeps louder or more persistent? I would like the sound to stay on permanently until the time she wears the seat belt.

I can totally relate to this. When any of my friends or family refuses to wear a seatbelt while I'm at the driver's seat, I just turn the car off and firmly says that I will not be moving another inch. Annoyed by this, they usually comply. Not sure about whether there's a way to make the belt reminder go louder though.

[V.Narayan]

Dear @GKR9900,

Thank you for a well written and well illustrated explanation of this important topic. Going by the number of readers who have visited this thread sadly it is a subject not on the minds of most. More tragic given the India is the global leader in road accidents.

At 120 kmph your kinetic energy is 2.25X that at 80 kmph and 3X that at 70 kmph and 4X that at 60 kmph. This fundamental point is missed by most. Combine this with the fact that at 120 kmph our reaction time available is significantly lower than at 80 kmph and we understand the challenge better.

Several members argue in favour of 120 kmph or higher speeds. But till we remain a low income country our roads will serve both the rich {almost all on this forum} as well as the rural folks, the wheels of trade and animal drawn vehicles. That is a fact of life. One usually does not reach a destination earlier by driving faster. Not in India at least. That toll plaza, that cow, that tractor, that micro traffic snarl on the road all ensure the seconds gained by driving faster get lost in a flash.

I drive inter-city a fair amount and in all my years I find 80 kmph to be the ideal speed that balances safety with reaction time and time to destination. To each his own.

[locusjag]

Originally Posted by GKR9900

When the head suddenly decelerates as in the case of a car crash, the brain suspended in the CSF continues moving forward and hits the front side of the cranial cavity, then rebounds to hit the backside.

There is a major cause for brain injuries that most folks don't realize - the insides of the skull aren't a smooth surface. When the brain collides against the Cerebrospinal Fluid sack, it gets pressed against several hard ridges, undulations and needle-like sharp surfaces that the skull has on its insides.

This URL explains why our skull has ridges and holes - to hold the brain in one place: https://braininjuryhelp.com/cribrifo...hazards-brain/

And alas, for I've seen a brilliant answer on Quora by a Neurologist and I'm unable to locate it right now; he had shared a pic of a skull's internal surface that has proper bony needles in it. For a brain to collide against that would be outrightly horrific. We must think way beyond concussions with the pin-cushions that our skulls have in some of its internal surfaces!

[GKR9900]

Originally Posted by V.Narayan

Thank you for a well written and well illustrated explanation of this important topic. Going by the number of readers who have visited this thread sadly it is a subject not on the minds of most. More tragic given the India is the global leader in road accidents.

At 120 kmph your kinetic energy is 2.25X that at 80 kmph and 3X that at 70 kmph and 4X that at 60 kmph. This fundamental point is missed by most. Combine this with the fact that at 120 kmph our reaction time available is significantly lower than at 80 kmph and we understand the challenge better.

Several members argue in favour of 120 kmph or higher speeds.

Thank you for the kind words sir. If I can strike sense into at least one reader through this thread, I would be a happy man.
There's also one more important byproduct of speed that I had left out for the sake of simplicity. It is called the tunneling effect. As the speeds increase, the extent of our peripheral vision decrease drastically. At around 50km/h, our peripheral field of vision is about 100 degrees. As the speeds climb, this reduces. At about 65km/h, it's 70 degrees. At 150 km/h, it's just 18 degrees. We need our peripheral vision to make out any elements that may veer into our driving field. 18 degrees is too narrow for this purpose.

Originally Posted by locusjag

There is a major cause for brain injuries that most folks don't realize - the insides of the skull aren't a smooth surface. When the brain collides against the Cerebrospinal Fluid sack, it gets pressed against several hard ridges, undulations and needle-like sharp surfaces that the skull has on its insides.

This is an important aspect that many miss out on. Kudos for bringing it up! These sharp, indentated surfaces are meant to tether the brain in normal scenarios. A crash can render them harmful as much of the force gets concentrated on these points causing tremendous pressure on various areas of the brain leading to catastrophic damage.

[chandrda]

Amazing Write up, very informative

Apart from educating myself, I can try to explain it to my friends and family as well through these examples.

I had couple of questions regarding tunnel vision and tiredness/fatigue relative to speed while reading the first post. You have addressed the tunnel vision in your second post with peripheral field of vision explanation.

Do you have any examples/information related to tiredness/fatigue in relation to increased speed?

Thanks in Advance

Regards

[Small Bot]

Wow, this is so informative and I spent a good 20 minutes reading and pondering about it.

Over the last few months, I've convinced all the men in the family to wear seatbelts whenever they enter a car - purely by refusing to drive off until they're buckled up. I'm facing an uphill task with the womenfolk but I'm confident that I'll win them over to the safer side soon. It helps that modern cars have the intentionally annoying beeps for seatbelt-wearing.

Having been in quite a few accidents in my life, I believe that all of us have an inflated estimate of our own ability to handle a car safely at speeds >100 kmph, just in varying degrees of inflation. Life is all thrilling at high speeds, up until one loses it suddenly.

I'm sharing this thread to my friend and family circles right away, for more awareness.

[Arun Varma]

Originally Posted by GKR9900

TAs the speeds increase, the extent of our peripheral vision decrease drastically. At around 50km/h, our peripheral field of vision is about 100 degrees. As the speeds climb, this reduces. At about 65km/h, it's 70 degrees. At 150 km/h, it's just 18 degrees. We need our peripheral vision to make out any elements that may veer into our driving field. 18 degrees is too narrow for this purpose.

This is a brilliant thread and essential reading even for experienced drivers. Thank you for this.

This point above on the reduction of the peripheral vision on increasing speed is a great reinforcer of why speed is the single biggest influence on outcomes in a crash. Too often and in too many threads, there is whatboutism regarding infrastructure, pedestrians and stray animals, etc. cited which distract from the importance of keeping our road speeds sane. When, if anything, those additional reasons are why we should keep the speeds even lower!

Thanks again for sharing!

[GKR9900]

Originally Posted by chandrda

Do you have any examples/information related to tiredness/fatigue in relation to increased speed?

I had read about this a long time back, but from what I understood speed has a mixed relationship with fatigue. If you were to drive fast for a prolonged period of time, you would be subjected to cognitive overload as driving warrants undivided attention for a majority of the time. Driving faster requires one to be extra vigilant of other road users. Add to that the constant vibrations and bobbing around of the body due to speed and that can increase fatigue as well. But then again, there are other articles which quote a reduced speed or in better terms, reduced speed variability, where the driver doesn't vary his speed much leading to cognitive underload and resultant boredom and drowsiness. This could be wrongfully cited as a reason to speed by many. But when you bring in the other factors like increased kinetic energy, tunneling effect, poor road infra and driving etiquette etc, the odds are heavily in favor of driving within sane limits.

[am1m]

Originally Posted by GKR9900

If I can strike sense into at least one reader through this thread, I would be a happy man.

You have certainly influenced many with this excellent thread and your brilliant explanations. Not everyone will respond or react, in fact the majority of people who read will tend not to. But they will have read and absorbed for sure.

While I've never been one to overtake rashly, I'm certainly guilty of pushing speeds on empty roads. I'm going to consciously try not to henceforth. Thank you for your post.

[rpunwani]

Many thanks Dr. Gokul S R for the brilliant & informative video. Most of us know that speed kills - many of us are not aware of what damage can be caused to the body.

I used to drive fast (and occasionally rash) in my youth. Couple of factors have worked to reduce mitigate my fast driving to a sedate & safe driver. They are:-

1. Driving the car with occupants
2. Progression in age
3. Upgrading to more expensive cars
4. Increased potential of speeding fines

At times I have been chided by my wife that I do not talk to her when we drive long distance. I prefer to have 100% concentration on the road when driving on highways & tricky roads.

Also have been sticking to the speed limits specified always. One often sees others driving rash - best to slow down and let them pass you.

[RunGaDa]

Very precisely and technically explained. Thank you so much.

Incidentally one hour ago, I was browsing my Facebook group and found a boisterous post of a twin picture, man with car in each one. On the left was Paul Walker and in the right the post creator. With a tagline “Wanna race?”
Such abysmal ignorance I thought.

I hope people pay attention to each and every precious word from your informative post.

[SS-Traveller]

Thanks, Doc, for a brilliantly well-written thread that explains the intricacies of the human body in a crash situation in simple terms for the non-medico layman.

It would be nice if you could write about reaction times and motor control under various situations, especially when driving under the influence of alcohol and various drugs, even such common ones as cough syrups. Many think that one small drink does not make a difference, or that having OTC medicines such as antihistamines do not hamper their driving ability.