Now, there are several reasons this bothers me. First, imperial measurements don't really fit well with science fiction, in my opinion, unless we're talking about Victorian sci-fi, or Flash Gordon/Buck Rogers style pulp sci-fi. All these measurements should be in metrics. Second, population is determined by 2D-2, with no DMs, so there is no level 11 or 12. But perhaps most importantly, the population score is represented elsewhere in Book 3, as well as later in Book 6, as a general population value and not a population density. Therefore, a small planet would have a higher population density than a larger one with the same population score. Furthermore, since people live primarily on land, planets with larger oceans will have higher population densities than desert planets, houseboats, platforms, and underwater domes notwithstanding.
So, in a fit of gamer ADD, I needed to do some fiddling around in general terms. Nothing I've done here is really ground-breaking, but this kind of thing should be taken into account by Traveller GMs, or GMs of similar games like Stars Without Number.
First and foremost, on any planet where humans can't just hang out on the surface in street clothes, none of this applies. If there's no atmosphere, not enough atmo, or a toxic atmo, all settlements will be built like space stations, with airlocks and air processing and not a whole lot of privacy. In these situations, the economics of building safe habitations and providing appropriate life support will ensure relatively high population densities in settled areas, and wilderness everywhere else. Similarly, on planets with 100% of the surface covered by water, settlements will be artificial structures, and economics will tend to ensure high population density (although if a water world has a nitrogen/oxygen atmosphere, a lot of people might opt to buy their own watercraft to live on and get away from the crush). Those planets with breathable atmospheres have more options, though...high density city living, or a lonely cabin out in the sticks.
What I've done here is calculated out the approximate land area of different sized planets with different hydrographics ratings. Since this is just a thumbnail sketch rather than a comprehensive rule system, I just went with default planet sizes (no variations from the standard) and default hydro ratings (ten percent increments). I also did these calculations in square kilometers (!)
This next table is barely of interest. Just a rough estimate of people per square kilometer, based on population rating and a few benchmark planetary land areas. Because population rating is just an estimate in the first place, I didn't bother getting into a whole lot of detail, and anyone who can divide by ten can easily recreate the table. I also didn't bother working out numbers for less than 1 per sq km...it's basically all wilderness at that point.
So, finally, I looked up some representative countries with these approximate population densities. At 0 (less than 1 really) we have Greenland...essentially small settlements in the most appropriate areas (most efficient resource exploitation, most comfortable weather, most scenic areas; you have a whole planet to pick from, so why not?) with the rest of the planet as uninhabited (possibly uninhabitable) wilderness.
At about 1 per square kilometer, there's Mongolia, with some decent sized cities and widespread, low-level habitation everywhere else (basically wilderness).
At about 10 per sq km is Russia; large cities and industrialized areas but still large unexploited wilderness areas.
At about 100 is Turkey (and most industrialized modern countries for that matter). Cities, some sprawl, but still a lot of wild areas and low density agricultural areas as well.
At about 1000 people per square kilometer, we have Bangladesh. Densely populated cities with some very tall buildings, cities and towns tend to blend seamlessly into each other, but there are still some rural areas punctuated by smaller cities and towns. The Hawaiian island of Oahu has about this same level of density, if you've ever been there.
At about 10,000 people, our example is Singapore. Once you start getting up this high, rural areas really get crowded out. Lower levels of overall density have pockets of highly dense settlement, but it's getting pretty uncomfortable now; people spread out as much as possible.
The smallest planets with the highest populations can reach up to 100,000 people per square kilometer. There are some places on Earth now with that level of population density, but they're not well known...basically some neighborhoods in some already densely populated cities. I can only imagine a planet-wide version of this would be the Traveller equivalent of Coruscant, with city everywhere, and the only variation being how tall the buildings are.
And now that itch of mine is scratched. More accurate population densities in Traveller's world generation. I've been thinking about working up some system using Zipf's Law to figure out more detailed settlement patterns for science fiction games. Whether I come up with anything worthwhile remains to be seen. And of course, I can't seem to stay on task for almost anything anyway. Today's post came about when I intended to work on another Patron for DCC. We'll see what happens I guess.