When you have a lot of big residential buildings on one street, that street becomes congested and people won't want to live there anymore. You'll need to upgrade your road. New roads are two lanes, but you can upgrade to 4 or 6 lanes.
It's a waste of money to upgrade roads before you have to, but once you get messages about bad traffic, tap the road icon on the right side of the screen, then click the Upgrade option (the middle of the three). It'll highlight the problem road in red, and when you tap it, it will show you how much it costs to upgrade that street to a 4-lane street.
Longer roads cost more to upgrade. I'm trying to layout my town so that no individual road segment is too long. That way whenever I have to upgrade, I don't have to spend as much money all at once.
Edited to add: If you don't have the cash to upgrade the roads, you can move around your buildings to lower the demand for that segment of road. I just swapped in a smaller house where a big skyscraper was, and that made the people on the road happy again, so I didn't have to upgrade it yet. Upgrades from 4 to 6 lanes are expensive!
