GMC 2010 Yukon XL Denali Owner's Manual page 59

WARNING:
It is extremely dangerous to ride in a cargo
area, inside or outside of a vehicle. In a collision,
people riding in these areas are more likely to
be seriously injured or killed. Do not allow people
to ride in any area of your vehicle that is not
equipped with seats and safety belts. Be sure
everyone in your vehicle is in a seat and using a
safety belt properly.
This vehicle has indicators as a reminder to buckle the
safety belts. See Safety Belt Reminders on page 4 27
for additional information.
In most states and in all Canadian provinces, the law
requires wearing safety belts. Here is why:
You never know if you will be in a crash. If you do have
a crash, you do not know if it will be a serious one.
A few crashes are mild, and some crashes can be
so serious that even buckled up, a person would not
survive. But most crashes are in between. In many of
them, people who buckle up can survive and sometimes
walk away. Without safety belts, they could have been
badly hurt or killed.
After more than 40 years of safety belts in vehicles,
the facts are clear. In most crashes buckling up does
matter ... a lot!
2-27