California, in particular, has a population density of about 100 people per square kilometer which is about the same as France. Most of California is either uninhabitable, factory farms, or military reservations. The population is highly concentrated near the coast.
People who haven't been there imagine that Los Angeles is "sprawl" but the truth it is very high density that is "sprawling" over a large plain that is boxed in by mountains. Millions of people have a line of sight to the Hollywood sign.
(I grew up in Southern New Hampshire which is based on Buckminister Fuller's diabolical geometry of airports and subdvisions -- where getting from one subdivision to the next is like going from the right side of your left lung to the right side of your left lung all the way up to the fork in your trachea. That is a self-inflicted wound that has nothing to do with how dense the population is.)
One approach is scale-dependent (divide the area into a grid of square kilometers, sort by population, pick the square kilometer where the median person lives.) but you would get different answers if you picked different scales.
The median of "How far away is your nearest neighbor?" seems parameter free at first but it is absurd if you live in an apartment. (e.g. ignores all the space around your apartment.) I guess you could look for the median (or other decile) of "What is radius in which N people live?" where N might be 100-10,000 but then you're back to having a spectrum instead of a single number again.
Maybe you can't avoid that.
And before anyone says I should just live in places that are like that, It's not so easy to just pick up my life and move somewhere else.