Supposedly the per-turn trade benefit of a route is:
Route = ((raw trade arrows of City A) + (raw trade arrows of City B)+ 4)*modifiers)/8
Both cities are yours -50%
Airports in both cities +50%
Superhighways in home city +50%
*Cities connected by road +50%
Cities connected by rail +50%
**Freight instead of caravan +50%
**Cities on different continents +100%
* - I think this is just the individual square benefit
** - I think these are actually applied just to the one time bonus.
(this might belong on the strategy forum...

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