I'm pretty sure this is a pretty heavily requested feature, but there isn't an easy way quite yet that I am aware of. There's this blog post that shows one way: HERE and I've used that before with success.
Most of the time though, if I'm doing a "quick and easy" cul-de-sac, I just do a 4 point "diamond" on the end of the road. You can change styles as shown on the blog link to remove the inner circle. Sometimes I'll just put a tree in the middle to make it look intentional! Maybe in the next couple releases they'll have this feature sorted out. I would imagine a modification of the roundabout coding could make it possible.
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