Which law would you be referring to?
To see a list of the state laws you can go to
www.highwaysafety.org/saf...train2.htm
However, these laws don't account for the law of physics which requires a child to be in a harnessed seat until they reach the upper weight/height limit of the seat (40 lbs for all but the Fisher Price Futura which is 60 lbs and the Britax Super Elite which is 80 lbs). At that point, they NEED to be in a booster seat until they are about 4'9". There is some great information about booster seat usage and how to tell if your child is big enough to use the adult seat belts in a car at SafetyBeltSafe USA at
www.carseat.org - there is a tab at the top of their homepage for booster seat information.
Several states, including Washington, California, and Arkansas have passed or are trying to pass better laws that would require booster seat usage up to higher ages and weights. If you are considering going merely by what your state law says, make sure you check often to see if they have changed it to better protect the children.
Krista