If you crash into something going only 50 kilometres per hour, things that are the same weight as your average smartphone will have enough force on impact to kill you.
I've been told on motorcycle hazard awareness courses that if your body hits a solid object at 50 kph, it's 50% mortality risk - it's enough deceleration force to rupture your aorta. Take something like a sign post to the chest and you'll be lucky to survive.
I guess it depends on the kind of accident.
In a head-on collision or if you run into a tree/concrete wall, that's pretty close to being stopped dead.
In the more common rear end or "T" collisions, probably not.
The original point was about unsecured objects in the car becoming deadly.
Whether it's enough to make a difference, we should make calculations. I'd expect 50kph is where marginal increases in mortality start to become larger, so it wouldn't take much to reduce harm, I believe.
You're travelling at 50 kpm (let's call it 14 m/s). You strike a brick wall. The cell phone on the rear deck of your car flies off the rear deck with minimally deceleration before it leaves the rear deck, travelling 14 m/s through mid-air. Let's make a linearizing approximation and say your body is stopped in 0.8m with constant acceleration. Let's assume the phone started out 1.5m behind your head. By the time your head is stopped by the crumple zone, the phone has travelled 1.6 m, meaning the phone is still 0.7 m behind your head, still travelling 14 m/s (50 kph).
Granted, there are a bunch of simplifying assumptions here (linear deceleration via crumpling, frictionless rear shelf, zero air resistance), but it shows there are plenty of realistic scenarios where something placed on the rear shelf of your car strikes you in the head only minimally slower than the speed at which you were driving.
Sad to see that it ended.