I find it kind of funny that in 4 years we changed trade route models
(CT 1977 vs 1981).
Recap: In the original, trade routes were based on starports, not
trade classifications. Actually the 81 edition doesn't mention trade
routes, just communication routes.
Currently, trade routes are based on the trade codes of worlds within
four parsecs of each other, which can lead to the very strange
happenstance of a class A port trading with a class X or E. Such
things can be explained as "just drop at the producer and pick up the
cargo", but...
I suppose we can classify the two models. The old one could be called
the "Hub" model, where starports only grow if there is trade volume to
support them, while the new model is the "Interdependent" model and it
assumes that planets will specialize in goods and trade for the ones
they don't produce.
While both are accurate, I see more advantages to the hub model. Now,
I do have ideas for combining the two (Primary routes being based on
starport class within 4 parsecs, as the original 77 table, secondary
routes being based on trade code within 2 parsecs), but what are your
ideas?