The size of the United States has not increased since 1970, but the number of people has. So yes, no shit, (US pop / US land area) has gone up. But the question is, "is the average neighborhood more dense than it was in 1970", and that's not a question you can answer from that number, because obviously cities & towns have spread since then.

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.

That's not the density I was referring to. That doesn't measure city/suburb density. We have many more suburbs (therefore more density overall) but the number of houses within a suburb (where a child might be riding around) is not likely to have increased.