Hi @AnubisLaKingDon - The international airport brings in tourists to all connected cities, it will not be used for commuting between cities. This great work will effectively flood all of your cities with an additional layer of shoppers called "tourists", so unless you've got tourism as a specialty (lots of casinos and/or landmarks, stadiums, etc) it's very difficult to manage the influx. In order to build the GW, it also requires one city with a municipal airport that has brought in at least 100 tourists.
Now - that's the answer to your question, but there are a couple of things you should know about how the game actually handles commuting. Unfortunately, the game does not really handle commuting sims in the way it says it does. They tried, but a number of issues with syncing online really screwed it up. Some regional things directly interact, others just "look" like they're working. So if you have 100 unemployed in one City, when you're in that city's regional view, it will look at the numbers of the other city and show workers commuting, but 100 sims do not actually track to the city with open jobs. The game treats it as if they are, in regional view, but after all my years of playing I have never been able to balance jobs in this manner.
One of the bugs that is here to stay is Sims leaving a city for any reason lose their tracking and don't return. So it is best to balance population on a per-city basis. This applies to workers, shoppers and students.
If you're trying to get more workers and shoppers to a city, I would suggest building the Arcology GW. Once built, it supplies workers and shoppers to the connected cities and it works very well. It's also fairly easy to meet the requirements, you only need one city with 58,000 population to unlock it.
About building a Great Work - make sure you wait for synchronization in all cities after starting a GW build. Do NOT initiate a GW build from more than one city. It will corrupt the entire save and there is no way to recover from it.
You can create a horrible problem if you go to another connected city and click "build" after you've already started building from a different city. It's all about the regional synch. I usually use one city to build the GW just to avoid the issue. If you go to another city during construction or after it's built, don't panic if you don't see the GW from there. Give it some time. Eventually, you'll get notice that the GW has started and it will run through all the steps of building and you will be asked to do the ribbon cutting in the new city.
Whatever you do, do NOT confuse the server by clicking the build button in the other cities. If you do, when the game finally catches up, it will completely hose your entire region; there is no fix. So, safest way is to build the GW from one city, wait until it's completely finished and you've had the ribbon cutting ceremony, then load each of the other connected cities and wait for it recognize the GW. Go through the ribbon cutting ceremony on each, and you'll have the completed GW visible and actively supplying all the cities in the region.
I hope this helps - Happy Building!
