I had brought that up over Christmas with the family, and there was much discussion. Well, today, I finally remembered to take a look at the sign closer, and found a web page.
I was right about the 4' 9" height limit, but what the sign didn't say, but the web page did, was there is also an age limit of 8.
I put in my son's information on the calculator (the best I could remember) and this was what it told me.
Children who are over 40 pounds should use booster seats until they are at least eight years old or 4-feet 9-inches tall. Install it in the back seat of your car. Booster seats position a child so the safety belt, which is designed for adults, fits properly and prevents injury.And for the record, my son does use a booster seat. To him, it's absolutely normal.
Recommendations are from the National Highway Traffic Safety Administration