People are rightly pointing out the physics of this, so I thought I'd try to give you a bit of an example. My math is a bit rough here, but basically it would work out something like this:
For a newborn, goto the store and buy 4 liters of say Pepsi. That will weigh about 4kgs, or about 9 lbs. For a six month old, get about 6 liters (about 15 lbs).
Goto the second floor of a building, about 20 feet off the ground (for a quick comparison a double decker is about 14 bus feet tall). Sit down and have the Pepsi in your lap (not clutching it, just sort of in your lap).
Close your eyes and than have someone push you off the edge (because you never know when an accident would happen).
Did the Pepsi break? Were you able to hold it in your arms?
At 20 feet, by the time you hit the ground you'd be going about 25mph (40km/h), which is probably the average speed on many roads, but well below the freeway speeds. When you hit the ground, your newborn 'baby' would be thrown with the force of 250lbs out of your arms. It would be basically be a projectile ripped from your arms and thrown into the windshield of your car.
This isn't a perfect example but might give you an idea of some of the forces you'll be dealing with trying to hold a baby in your arms during a car crash.
Please, get a car seat. Take no comfort whatsoever in the fact that the government has failed to mandate you to do so.
Get a car seat, please.
(others who didn't sleep through physics lessons can feel free to update my math).
More on the math:
http://www.wku.edu/~david.neal/116/Unit2/FallingObject.pdf
http://www.physicsforums.com/archive/index.php/t-208805.html
http://answers.yahoo.com/question/index?qid=20070920020229AAfLIfp