L.,
Consider the laws of physics: A body in motion stays in motion until it is stopped.
Would your child be best stopped by a lap belt with shoulder harness, or 5 points? The only portion of their body that is able to continue moving forward is their head, arms and legs.
This is why speeding is dangerous. A sudden stop could flex your baby's neck too far. Arms and legs have a bit of give and take, the neck is not so good.
I hadn't gotten this far thinking about going to a booster, but I am glad we have a seat with 5 points that goes to 55 pounds.
Good luck,
M.