Let's say we have an adult traveling in the back seat of a car with shoulder belts. This attaches the adult to the seat. They could have their child against their front in a soft carrier. Something like:

This seems like it should be safer for the baby than a seatbelt is for an adult, because the infant would have much better support, especially of the head. Does anyone make something like this? Would it be as safe as current designs? Are safety seat regulations written to allow seats that demonstrate a given level of protection or do they require independent solid-bodied seats?

Update 2014-04-07: Now that we have an infant, another advantage of a carrier like this could be that the kid would like it more. Lily is quite content riding the bus in her front carrier and falls right asleep, while she hates her car seat and tends to cry the whole ride.