I have seen many people ask this question and I assume if you are new to CXL, this tutorial will be of use to you.
I have seen a few different ways to build circular roads. While this method may not give you a perfectly circular road, it is quick.
Here is how to build roads in perfect circles:
1. Start by making a crossroads. Place an even number of low density buildings on each side, starting from the center.
2. Make a small road at the end of each line of buildings, protruding from each side just a little bit.
3. Bulldoze all the excess road and the low density buildings.
4. Choose the curved road tool, and connect each end.
That's all there is to it. I hope this helps. Here's the final result: