Quite, so where is kCr20 for an interstellar trip coming from?
Most plebs will not be travelling, but the people who are travelling will probably travelling more than once. The amount of traffic is determined by the number of trips, not the number of individuals making those trips.
Yes, "international travel" is a lot more than air travel.
How many cars pass the US-Mexican and US-Canadian border every day?
The ferries between Sweden and Finland carries more than the total population per year, IIRC.
But as other have pointed out, that is because they can.
Far Trader says well over 100 000 passengers on a population of 2 million, mostly to/from Collace.
Have you got a citation for that. I only got 50-100 thousand from the table. Tarsus gives the population closer to 3 Million, so 1.6-3.2% per year. But that is just to and from Collace. We'd need to assess the situation for every system. We can easily imagine travellers between systems 4 parsecs distant, 6 parsecs is the limit of a single jump, but it is not incredible to imagine travellers having to cover a greater distance.
Collace has a million passengers on a population of a billion.
By the logarithmic nature of Far Trader low pop worlds have a larger proportion of passengers, but the vast majority of the Imperiums population lives on high pop worlds, so Collace is more representative of the Imperium as a whole.
Whilst you may be correct that the majority of population lives on high population worlds (due to the logarithmic progression of population numbers), your conclusion is not necessarily true.
Lets assume we have a major population centre like Collace (pop 9). We would need a thousand population 6 worlds like Tarsus to just double the overall population. So with respect to the ratio of passengers to overall population they add very little to the denominator. So far so true.
However they are vastly more influential on the numerator. If we assume a simple 1 in 3 distribution of systems then within J-1 there are 2 that will add their (say) 50,000 passengers (the huge population of Collage provides a big chunk of BTN). Within J-2 there are a 12 more hexes and 4 more systems. They only add 10,000 passengers each (due to distance modifier) but that is still another 40,0000 Between J-3 and J-5 there are 72 hexes and 24 systems each adding 5,000 etc.
By the time we get 98 systems and have added barely 10% of Collace's Population to the total we are at 9 parsecs and there are 320,000 passengers a year travelling to and from Collace. After this it becomes a law of diminishing returns*.
None of the above takes into account the traffic between each of those Tarsus like worlds (which is an addition to the numerator only). BTNs between the Tarsus-like worlds will be a couple of points lower than the BTN to the Collace-like world, but the outer systems will be closer to each other than to it and after J-5 the Distance modifier effectively eliminates the extra contribution Collace-like's higher population contributes to BTN in attracting passenger traffic.
I'd need to write some clever code to work it out (and frankly if I was going to do that I'd have done it by now for the actual system of interest to me), but I think it is over 4700 combinations of trade pairs and each of those is likely to generate between 5 and 10 thousand passengers. Conservatively there are going to be several million passengers and is going to swamp the hundreds of thousands travelling between all of those systems and the Collace-like system. This tells me that the Hi-population worlds need not be the main contributor to interstellar traffic.
*With 992 systems the combined population of them all is approaching that of Collace and we getting passengers from 31 parsecs out. Each system is only contributing 50 passengers a year but there are so many are now getting half a million passengers a year in Collace from them.
The passenger traffics finally peters out at 300 parsecs (where the average drops below 1 passenger a year). Even at this low level the almost 50,000 systems between 200 and 299 are contributing almost 50,000 passengers. By this point however they are also contributing 50 Collace equivalents to the overall population. It is also difficult to accept that someone will be making a 299 parsec return journey.
