GMC 2003 C4C042 Owner's Manual page 17

Why Safety Belts Work
Q:
Aren't safety belts for kids?
A:
Yes. And they're for adult truckers, and anyone
else who rides in your vehicle.
Here's why: when your vehicle goes, say, 30 mph
(50 km/h), so do you and your passengers.
1-10
If the vehicle hits something, it stops
But nothing stops the people. They keep moving.
--
right then.