>We're there 1/5 the number of roads back then?

Probably more like 1/2 or 3/4 depending on how you want to count dead end office park and residential subdivision roads of which there are many.

Pretty much every road in the US and Europe that isn't in the above category or a purpose built highway existed in 1900, and likely 1850 if you're looking at europe or the american northeast.

Obviously size and quality was lesser, many times they weren't even paved. But they existed because they were the roads between towns and points of interest.

I doubt this. The interstate project in US added a massive number of roads. Not just the interstate, but new tributaries. Those office parks and subdivisions have to hook up to stuff too.