Keep in mind that your increment length affects civilian performance. They only do at most one trade run per increment. I've found civilian shipping is essentially worthless when running in 30-day increments, as they don't make enough money to replace ships, and don't contribute much to my economy. They make lots of profit on 1-day increments, and can expand quickly, but more ships slows turn processing, and running 30 1-day increments is already slower than running 1 30-day increment. And with civvie's not giving equal weight to all contracts (or even prioritizing contracts over trade location run), some contracts never get filled. Because of these, I've been kind of turned off civilian shipping and have been handling it myself. Honestly, I don't see a point in making the mechanism explicit. It seems like something that could be abstracted, where it could be more effective without impacting game performance.