Adults and Seat Belts
Since I often write about car seat safety, some of my friends have come to me with their questions about car seat recommendations, laws and best practices. A question comes up often, especially for people who either know my best friend, or happen to know another smaller adult. "If the law says that you have to be 4'9" to be out of a booster seat, does that mean short adults shouldn't be allowed to drive? Aren't they breaking the law? Shouldn't they be in the backseat in a booster?" My best friend is just under 4'9". She's a perfectly proportioned adult, just a small one. Unless you see her next to someone, you would think she was of average height. But, when people joke about this particular law, she's the one. When I talk to someone about the "5 step test" to determine if their big kid is ready to sit with only a seatbelt, her frame technically doesn't pass.
The short answer is, no, of course she doesn't need a booster. And yes, she can legally drive. The long answer is all to do with skeletal development. Adults have a skeleton that is fully formed and full strength. Kids in the 8-10 year old range (when many kids are itching to ditch the booster) don't. Even if children are the same height as their parents, they have immature skeletons that are not completely ossified. That means that their bones are not as strong as our adult skeletons are.
When a child uses a booster until they can consistently pass the Five Step Test, the booster seat helps take some of the forces off of their young skeleton. By the time they've grown enough to be out of a booster in nearly every vehicle, they're getting toward puberty, which brings another spectacular growth spurt for the skeleton.
Ideally, a child meets all of those criteria before they move out of a booster. An adult is well beyond puberty. We've got strong bones that can take more of an impact from another object or the airbag better than a child's skeleton. Luckily, most cars this day have adjustable seats and seat belts in the front seat, so every adult, from tall to small, can get the right fit to keep them safe.