And that's probably why the U.S. regulation has settled upon 2 years old. It is a middle ground that balances the drive vs. fly fatality data with aviation safety/evacuation and inflight comfort factors.

There are a couple cities for us that were flights with no kids that are now drives with 2 kids. (Minneapolis, Chicago) We took a lot of trips with infants, although we got pretty good and selecting such off-peak flights that the kids almost always had a seat to themselves. But I completely see how the regulations alter the mix of fliers and drivers. It certainly did for us.

If they really wanted to create a complex regulation, they'd restrict the infant rules to flights within a fairly narrow distance range - say, 300 to 600 miles. That's where the real drive vs. fly decision is made. Infant rules aren't causing people with 3-year-olds to drive SFO-NYC. When we want to go to the East Coast (say, 950 miles to DC), it's always a flight - period. When we want to go to St. Louis (250 miles), it's always a drive. It's really these 6-8 hour drives where you think about the pros and cons of drive vs. fly.

(No, I don't actually want them to create a complex regulation. )