A good scenario for this situation would be roads got more dangerous and cars were not improving for more people to survive car crashes. in this world they became more common. what would our physiology be like?
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Cars evolve way quicker than humans.
So your question should be more "how would humans look if they were forcibly adapted to survive current-model car crashes?".
I'm afraid there are too many ways of doing that. Piercing trauma requires the more visible modifications, because you need some sort of biological armor.
Blunt trauma and acceleration damage, including diffuse axonal damage, can be ameliorated without too visible alterations by adding internal struts holding the organs together, and less incospicuously by adding what would look like layers of fat - and actually would be sacs of deceleration fluid. In short, biological airbags designed to spread decelerations over a slightly greater time scale.
Internal safety belts - bands of muscular and tendon tissue that would increase the natural reaction of "cringing" when expecting an impact.
But as the wise man said, "presence of spirit is good - absence of body is better". Increased reflexes, situational awareness and muscle sprint can allow you to survive an accident by not being involved.
If the worst happens, two modifications at almost the cellular level - hibernation to save energy, providing the first half of the Vulcan healing trance; and heightened regenerative powers, providing the second half.
So, no necessarily too visible modifications, but if you need to, you can make the shock-resistant humans look like this guy:
Back in 2016 there was a project undertaken by the Transport Accident Commission of Australia. A trauma expert as well as the artist Patricia Piccinini collaborated to create 'Graham'. An artwork based somewhat in reality, the example of what physiological features that might be present in humans if they had evolved to withstand the forces involved in crashes.
Things such as the removing of the neck, which in turn sacrificed mobility were deemed necessary to make him more resilient to injury in the event of a crash.
As well as this, a larger cranium with surrounding crumple zones was introduced, also with the inclusion of more fatty tissue in the forehead to dampen impacts.