I've known people from whom the "have a 3rd kid, or stop at 2?" question probably was tipped over to "stop at 2" by the cost and hassle of having to buy a bigger car to comfortably & safely fit 3 car seats, on top of all the other painfully-high expenses of having a kid. And I've repeatedly heard needing to get a minivan or largish SUV mentioned in conversations about whether to have more than 3 kids, as a downside to going higher.

I'd absolutely believe that car-dependence has played some measurable role in suppressing birthrates, by causing step increases in transportation costs at certain counts of kids. 1 or 2 is no problem with almost any car, but 3 means at least a large sedan, and 4-5 means a van or SUV. More than 5 and you're looking at a fullsize van or a minivan with dual bench seats or something.