How much have increasingly tight laws on where your children may sit in your car–laws requiring children to sit in car seats until higher ages and weights, and laws barring young children from the front seat–affected birth rates?
The effective consequence of these laws is that, if you wish to transport three or more young children, you need a car with three rows of seats. This imposes a cost barrier, because minivans and full-size SUVs are more expensive than sedans (and car seats themselves cost money). It also imposes a lifestyle barrier–getting around with a lot of young kids becomes more annoying than it needs to be, and not everybody wants to drive a minivan.
Different jurisdictions adopted more stringent car seat requirements at different times. It seems like it ought to be possible to use a cross-jurisdictional comparison to figure out how these laws have affected birth rates. If the effect is non-trivial, that would be a good argument for relaxing car seat requirements, even though child auto fatalities would rise.