Last time, I had a city that looked like this.
Then I built some more, and now my city looks like this.
Nothing particularly interesting happened on the way here. I gradually added more neighbourhoods, each pretty much self-sufficient. I now have 75,000 citizens and no real reason to continue expanding this city.
The moment I unlocked office buildings, the challenge pretty much vanished. The idea was that high rise buildings would lead to more citizens in a given area and therefor more traffic, but offices get rid of heavy traffic completely, which makes traffic management easy.
Also, you can build offices wherever you want, because they don't cause ground pollution or noise pollution, nor are they bothered by pollution. This makes them ideal pollution buffers, but takes away a lot of the challenge. If you want to try the Gravel Road-challenge yourself, I suggest you add the rule that you're not allowed to build offices.
I'm ready for something new.