THIS Is Why Winter Coats Are Dangerous When Combined With A Car Seat
According to the Intact Health & Natural Parenting Public Page, bundling up a child up in a coat before setting them into a car seat seriously prevents the harnesses from working properly. What happens is that the extra layers of fabric do not give the straps time to tighten closely enough to the body in case of an accident. In other situations, the coat will compress, leaving a huge gap between the harness and the child.
Experts recommend that you remove the coats before putting your child in a car seat, but you can still use a warm blanket to cover them up afterwards.