Once you hit a population of 50,000, you start to get bad traffic. Density then ramps up pretty quickly from there. Quite soon you'll have a population of 200,000, and the traffic is quite a lot worse, but not terribly bad.
The real horror comes when you start a region's great work. A significant number of people will leave the city in the mornings to work at the great work. If you are making a great work you are probably sending dozens of trucks with resources to it as well. The end result is you get easily 10x more vehicles leaving/entering the city. The queue to get in through the single lane off-ramp from the highway then ends up stretching all the way to the next city, and it takes a good 24 hours for the cars at the end to get in. It then ends up stabilizing, taking maybe 4-6 hours for someone to get into the city.
Also, a large number of cars 'just passing through' will use your city entrance as a 'quick' way to do a U-turn on the highway, blocking traffic even more.
The real horror comes when you start a region's great work. A significant number of people will leave the city in the mornings to work at the great work. If you are making a great work you are probably sending dozens of trucks with resources to it as well. The end result is you get easily 10x more vehicles leaving/entering the city. The queue to get in through the single lane off-ramp from the highway then ends up stretching all the way to the next city, and it takes a good 24 hours for the cars at the end to get in. It then ends up stabilizing, taking maybe 4-6 hours for someone to get into the city.
Also, a large number of cars 'just passing through' will use your city entrance as a 'quick' way to do a U-turn on the highway, blocking traffic even more.