While at first I thought to agree with you, I have a counterexample. Bigger car is safer if it collides with a wall, small ones and bikes are more dangerous in this case, because they result in quicker deceleration of the driver.
So longer cars are also safer, and longer car tends to be heavy ones. So if all cars will be large we will get safer driving environment.
So it is easy to imagine "absolutely" safe car: it would be large, heavy and very slow (like 30 miles per hour).
Maybe bigger cars are safer if they hit walls, but it's not entirely obvious to me. The more massive the car, the faster it will still be going when its front crumple zone finishes crumpling and wall and windscreen make contact; isn't that bad? Slower driver deceleration isn't a good thing if the still-fast-moving driver starts colliding with bricks.
Anyway, most car accidents don't involve hitting walls, and many car accidents endanger people other than the occupants of the cars, which means more mass => more danger.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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