• Move It!
  
• Network Extensions
  
• Some road anarchy mod

• Ploppable Asphalt (or some similar decals)
  
• Prop and Tree Anarchy

• Cars will probably freak out when they drive to these circles. Probably best not to do these in some areas if you expect a lot of cars to use them. I expect most of these will rarely be used by cars as they don't go anywhere.
  
• This does not work as well with larger roads.
  
• The road's name will look weird. Replace it with a period if you cannot hide it.

Build your alleyway, or find the alleyway you wish to add a cul-de-sac circle to.

Build a small section of road off of it.

use Move It! to move the nodes of the larger road together, forming a circle. It may be a little finicky to get the perfect shape, but you can keep moving it until you are satisfied.

If you find that there are unwanted textures from the road, you can cover them with a decal such as ploppable asphalt.

Example from my city.