Did you know that the forces a body feels at 30mph are the same as if you were to jump from a 3rd floor balcony?
This fabulous 1970s public information film from the UK asks this: If you think it would be suicide to jump onto a glass roof 30 feet below, why drive at 30mph without a seatbelt?
Just like you couldn't catch a child who fell from 30 feet up, so too you can't hold onto that child in a crash. The forces are such that things in a crash weigh their weight multiplied by the speed of a crash - so a 30 pound toddler in a 30mph crash would weigh 900 pounds (30 x 30 = 900).
Perhaps you think to yourself, I'm just going in a taxi for a short trip - if I were going on the highway I'd use a car seat for my child and buckle my own seat belt. Studies show that the majority of crashes involving children happen on roads with speed limits of <45mph and within 10 minutes of your destination. Make sure your child is protected on EVERY trip.
Chen IG, Durbin DR, Elliot MR, Kallan MJ, Winston FK. Trip Characteristics of Vehicle Crashes Involving Child Passengers. Injury Prevention 2005; 11:219-224.