NO! The primary job of a safety belt is to keep you from being ejected from the vehicle. The younger and smaller the child, the less effective the safety belt will be in both preventing injury and ejection from the vehicle, because the safety belt was designed for a 50th percentile adult male.
We realize that many NYC toddlers ride in taxis without a car seat, often held on the lap of an adult, so we feel it is important to note that holding a child is the worst option of all. No one is strong enough to hold onto a child in the instant of a crash—where the child becomes much heavier than normal. In this situation a child will fly forward either into the divider screen or out the windshield.
Therefore, if you find yourself in a taxi with your toddler but no car seat, put the child in his own seat belt. Pull the seat belt snug and try to make sure the lap portion of the belt is resting very low and very snug on the tops of the child’s thighs. Never let the child place the shoulder belt behind her back or under her arm, since this increases the chance of serious head, abdominal, and spinal cord injuries; the shoulder belt rubbing the neck is not a source of major injury in a crash.
NO! Never use one seat belt for two people.
Rule of thumb: One person, one seat belt.
Why? In a crash, everything will feel much heavier due to the G-forces. A 30mph crash typically has about 20-25G’s. If you usually weigh 100 pounds, you will feel like you weigh 2,500 pounds in that 30mph crash (100lbs x 25G’s = 2,500). If there’s something between your body and the seat belt (like a child sitting on your lap), your body will crush the child.