In regards to inertia affecting a person who is not wearing a seatbelt: A Seatbelt is mean't to keep someone from flying through the window. In physics an object in motion will stay in motion, and because we are in the car we share the same speed of the car. When we stop the seatbelt is mean't to act as a force to counteract the inertia exerted from the stop. However if we do not wear a seat belt aside from an airbag, the next thing that would stop your inertia is a windshield. If that doesn't work your inertia will carry you right on through the windshield.... Don't drive without putting on a seat belt.
Kinetic energy is heavily influenced by the speed at which an object is traveling, the faster it is the more force is needed to counteract the object. An object weight also influences the Kinetic energy, making a smaller car faster, but easier to stop and a big truck slower, but the stopping force needed increases.
The best thing to do when taking a curve is to slow down. Going fast can cause the forces acting on the car to flip it over. This is especially useful if you have a higher center of gravity.
Depending on kinetic energy, the objects involved (their movement, force, weight). You can have very varying degrees of injury. If you have a small car, and were driving super fast for example, you may slow down fast enough that you simply rear end a truck without so much as a sprained finger. If your traveling at a higher velocity, and smack right into a telephone pole, which is a stationary object you can be sure all the forces acting upon you at that moment can cause heavy injuries.