A liter of gasoline contains about 56 megajoules of chemical energy. This is more than in the explosion of the same amount of TNT. And this energy would be enough for a whole day of work of the toaster. Cars work by burning gasoline, which converts chemical energy into kinetic energy, helping to propel the car. Eighty percent of the energy is lost as heat in the engine, but 20 percent of 56 million joules is still a lot. It takes only 5 teaspoons of gasoline to accelerate a 2-ton car from 0 to 60 km/h. It doesn't seem like a lot of fuel, but the energy of a car moving at 60 km/h is comparable to that of an elephant, or rather a stegosaurus, thrown from the third floor. For the car to stop, all that energy has to go somewhere. If the brakes stop the car, they dissipate energy by heating (and then cooling) the brake pads and discs. And in the event of a collision, the energy is dissipated by the deformation of the front of the car. And since a slow stop is better than a fast one, the cars are carefully designed to crumple upon impact. This prolongs the collision time and stopping requires less decelerating acceleration.
Large acceleration has a very bad effect on human brains and organs. But people don't really like to drive cars with a long front. Most machines have 50 cm of creasable space in which they must dissipate the energy equivalent. The deformation of the front of the machine must withstand a force that is equal to a quarter of the thrust of the shuttle's main engine. More than half of the controlled collapse must be taken up by a pair of steel rails that connect the main body of the car to the bumper, which bend and deform to absorb energy and slow the car down. All the remaining energy must be absorbed by the deformation of the rest of the metal in front of the machine. This planned destruction allows the machine to slow down rapidly, but at a reasonable and stable speed.
If the machines were very hard, they would stop so quickly that the acceleration in them would be 15 or more times greater than what astronauts experience in training. Such huge overloads are not compatible with life. Engineers have learned to make machines with crushable parts that create a safe zone inside. Completely rigid cars are not suitable for the safety of the driver and passengers. In completely solid cars, even in a collision at a very low speed (30 - 40 km / h), people could die.