One second can change your life forever. In fact, most car collisions last only 1/10th of a second. That's too fast for a human being to react, but it's not too fast for your seatbelt which can activate in less than 1/100th of a second.
A collision at 15 mph can kill an unbelted passenger because there are actually two different impacts. The first is when your car hits something, the second is when your body hits something. Wearing your safety belt protects you from this deadly second impact. They also prevent you and other occupants from being thrown into each other. Instances where seat belts do more harm than good are very rare.
If your vehicle is equipped with an automatic shoulder belt system, keep in mind that it is designed to work in unison with the manually operated lap belts. Many people are lured into a false sense of security by the strap across their chest. In an accident the momentum of the unrestrained lower body can still propel you into the dashboard.
Motor vehicle accidents are the leading cause of injury and death to children. Car seats not only protect children from physical harm, they protect you legally and financially. Properly restrained children are also less likely to distract you into making an error in driving judgment. (Note that there are many programs that provide car seats for families that can not afford them. Midas Muffler and St. Anthony's hospital in St. Petersburg are two local participants in such programs.)
Failure to wear seatbelts can affect your insurance coverage and even prevent your recovery of money damages in a law suit. This is because in Florida Law there is a concept of "comparative negligence." Comparative negligence means if you have contributed to the severity of your injuries (by, for instance, not wearing your seatbelt), then compensation for medical expenses, pain and suffering and lost wages can be severely reduced or even eliminated.
For example, suppose you have been in an accident caused by someone else, but you were not wearing your seatbelt. You were badly injured in the crash, so you sue the other party in the accident to recover money damages for your injuries. In court at trial, the jury may decide that you were 50% at fault in causing your injuries so your money award will be cut in half. If the jury had found that you were 100% at fault for not wearing your belt, you would receive nothing.