Even if your city itself doesn't have traffic problems, it's possible that the region does. Trade export trucks come from the region only - if you have a backup at the entrance to your city, or if one of the cities in your region has major backups that is blocking the traffic flow to your city, the trucks may be stuck there or taking so long to get to your city that they don't have enough "time" to do the pickup.
The fact that you have tourism probably plays a big part in the issue. All of those regional sims flooding in to your city will cause the regional traffic flow to slow down to a crawl. Tbh, anything that causes regional traffic can cause major problems. I have just taken out all of my regional bus terminals from every city, and have only community colleges or universities for education so there are no bus stops. It seems to have reduced the amount of missing students I have (they were "commuting" out of town, never to return). Although it doesn't seem like this is related, when you think about it in terms of regional traffic, they are actually the same issue.
Try watching the trade map to see if you can spot the incoming freight trucks. If it's stuck at the entrance to your city, you may be able to rework your entrance to get traffic to flow in better. I sometimes put a road off to the side right at the first entrance connection, with nothing on it. A lot of those backed-up cars will run to the end, loop around and go right back out. If the traffic has been backed up, there might be a lot of stupid incoming things that never got to their destination in time. The AI system is pretty wonky, I seem to have the best luck with letting the ones that were in line and technically missed their deadline to enter the city, turn around and leave to trigger the rest. If there are a lot of them, it could take 24 or more sim-hours to clear up, but it's worth a shot.
If you can see freight trucks back up at a different city entrance, you will need to solve the other city's problem to clear up the traffic flow. If you own the other city, transfer over there and try the same thing with the entrance. If you're playing multiplayer and the other city is owned by someone else, this can be a huge problem, especially if the other city is abandoned. You can at least try visiting the other city to see what the problem is, perhaps you can solve it or contact the other mayor to get them to work out their problem.
A few other things to try, that seem to help me -
1. Place your trade depots as close to the entrance as possible, with nothing else on the road - clear shot to the exit highway.
2. Demolish the freight warehouse that comes as a default with your depots and ports.
3. Use separate depots for imports and exports.
4. Demolish all the truck garages in every depot except for the import ones. EXPORT uses trucks from the global market (they come in via regional highway); USE LOCALLY uses trucks from the facility that is supplying them (processor factory, oil drill, etc), IMPORT sends trucks out to the global market via regional highway and is the only one that uses the depot trucks.
5. If you have access to any water, plop a trade port and use the trade dock for exports. The water transit doesn't get clogged and sends out up to 200 tons of product at a time.
6. For delivery between local resources (like an ore mine to a smelting factory), you can use service roads from the dump or oil well to connect them directly. Only the delivery & service vehicles will use these roads, so no traffic jams and almost immediate delivery.
7. Demolish the old trade depots and rebuild. This only works if your regional traffic is not an issue, and your regional freight trucks have disappeared (which can happen - they go out to the global market, never to return). To try to do this without losing all the resources in the existing depots - Build a new trade depot. Building a new trade depot creates 2 new regional trucks to handle exports. Demolish the freight warehouse & add one warehouse with a resource you don't normally use. Import one delivery (you'll need a freight garage for this), then set it to export. If it successfully exports, you know that you've solved whatever the original issue was. Leave this depot running, set to export, with no product. If you're lucky, the old full depots may "borrow" the 2 new freight trucks and start exporting your product. If that's the case, turn off local deliveries to the old depots so they don't refill, and demolish them once they're empty. Rebuild to create new regional trucks. You can then either demolish the test one, or just demolish the unused storage lot and build what you need.
8. This won't help you because of your tourism, but in case it helps someone else: get rid of everything you can that causes regional transport. This means no regional bus terminals (you can use shuttle buses, they only run in the city), no trains, no airports, no elementary or high schools that use bus stops, and no tourism or gambling facilities. Stop sharing vehicles between cities. Anything that causes vehicles on the regional highway will slow your region traffic. Once you get the traffic flowing again, you can add some services back in, one at a time, just keep an eye on the regional traffic and time between exports, and back off if they start to slow or stall. This also works to combat the problem with students, workers and shoppers commuting out and disappearing.
Bottom line is regional play, even in offline mode, is basically broken. The services like power and water seem to work pretty well, I've never had a problem with those. And sharing the various unlocks between the cities works well also. But anything that involves transfers of AI agents just doesn't work reliably. They will head out from one city and never return, whether they actually reach the other cities or not.
This has become a much longer post than I expected. Let me know if any of these suggestions were useful or helped your situation.
