Using distance as a negative modifier sounds like the right way to go.
I used Importance for guesstimating traffic flow between any two ports.
index = sqrt( I1 * I2 ) - (distance/2).
I've played with subtracting the distance from both values first, or subtracting half the distance, and so on.
The target index is a traffic "size" as an exponent of ten for passengers. The cargo traffic, in tons, is that value times some constant in the range of 2 and 10.
Two Importance 5 worlds, one parsec apart, would have a trade index of 5. That implies:
100,000 to 900,000 passengers per week.
400,000 to 3,600,000 tons of cargo per week (assuming cargo = passenger count x 4).
Two Importance 2 worlds, one parsec apart, would have a trade index of 2. That implies:
100 to 900 pax per week
400 to 3600 tons of cargo per week.
Similarly, two Importance 5 worlds, six parsecs apart, would have a trade index of 2. sqrt(25) - (6/2) = 2.
Everything scales to suit how you view the universe. If you like a low traffic universe, subtract [distance] instead of [distance/2], for example.
