Cars generally can only fit 3 carseats. Regardless of equipment costs, that makes it infeasible for parents to have a 4th child. They wouldn't be able to go anywhere.
This by itself has prevented thousands of births.
I've known people for whom this was a major factor in a decision to not have another child. They are always the most thoughtful, low time preference people whose kids would benefit society the most.
Just going from 2 to 3 kids shocked me at the cost increase, and we already had a minivan (hotel rooms sleep 4, so now we have to get 2 rooms or a suite). There are a lot of other things that go up a lot more than you would expect when you have more than 2 kids. Kids eat free is normally each parent can have one free kid.
Overall kids are not very expensive, and I'm glad I have them. However there are a number of things we can't do anymore. Family memberships are well worth it for us since most of them are one price no matter how many kids you have, and thus cheap entertainment for the rest of the year.