1: https://www.census.gov/data/tables/time-series/dec/density-d...
To clarify, transportation is a means to get you to a destination. I don't know where you live, but I haven't lived in or ever even seen a suburb that provides all the destinations that a child (assuming they're old enough to ride a bike alone) would want to bike to.
Friends live in different neighborhoods. The mall certainly wasn't in my neighborhood. The video store, my church, the woods, the local pool, the public library, all required crossing streets which have become busier and busier.
If you want an intellectually honest comparison, take a look at the District of Columbia, which is basically 100% city and has been for many decades. It's gone down since 1970.
The other commenter and I were talking about cars.
Car ownership rates increased slightly, number of households nearly doubled, and average population density went up in every state except DC. There are more cars. Cars do not stay in one place, especially in the case of suburbanization.
Also, I'm not sure why/how the DC piece is intellectually honest. The Washington Metropolitan Statistical area has more than doubled in population since 1970[1]. Do you think all of the people who moved to PG County stay out of DC? That must be why the beltway is so easy to maneuver!
1: https://en.wikipedia.org/wiki/Washington_metropolitan_area
This was the original mention of density. Sure, cars don't stay in one place, but if we're talking about kids walking/biking around their suburban neighborhood, how big is the impact if there's a new 50k suburb on the other side of the urban core? Even commuters from the exurbs are taking the dastardly 45mph stroads, not the stopsign-laden 25mph streets through your neighborhood.
The common parlance around here is that "greater density" means smaller houses closer together or multi-unit structures. If you build a new subdivision outside town, nobody says "oh wow the town got so much denser", it just got broader. Waving at "57.5 average people/square mile in 1970, growing to 93.8 in 2020" says absolutely nothing about the experience of the average person on the average streets near their homes.
