So one thing which is still giving me a real hard time is incoming traffic from outside the city limits. I just can't seem to figure out a good way to deal with it and I always end up with a ton of gridlock. Here I've got a 6 lane one way road going around, and I'd think that would alleviate congestion some but it sure doesn't, besides which everyone seems to use only one lane anyways.
I've tried actual roundabouts but seem to have trouble getting them to work right.
A single highway connection is usually not nearly enough for a city. Don't have one single connection to the highways - have several at all corners of your city.
That way the import- and export traffic is distributed over the whole city and doesn't bottleneck near the one highway exit, gridlocking traffic in the whole area around it.
Another strategy is to try to minimize import- and export traffic so the highway connections experience less traffic. Commercial buildings regularly receive shipments of goods, which can either be imported or provided by your own generic industry. Your generic industry requires oil and ore products, which can either be imported or provided by your own oil- and coal industries. When any of your industries have an over-production, these goods will be exported. You can use the "Outside Connection" overlay to check which buildings export or import. Try to balance your economy to be mostly self-reliant and far fewer trucks will have to enter or leave your city.
Also consider to connect your city to the train network and build cargo train terminals near any clusters of importers / exporters to provide an alternative route for goods to leave / enter your city.
