I was curious about the claims about Eno so I Googled a bit. Eno actually is responsible for a lot of these ideas being put in use, if not inventing them himself. He basically pioneered the concept of standardized traffic rules.

As for the stop sign, Eno proposed the idea in 1900:

In a 1900 article titled “Reforming Our Street Traffic Urgently Needed,” for Rider and Driver magazine, he proposed placing stop signs at intersections. It was a civilizing notion.“That was a new concept and really did introduce the idea that you had to watch out for other people,” Schank says.

Harry Jackson created his octagonal sign 14 years later, in 1914:

Stop signs were invented in Detroit in 1914, which seems the obvious place considering it’s the car center of the universe. Detroit police sergeant Harry Jackson was working an intersection where one lane came in at an odd angle and so he continuously had to stop the cars coming that way. He took a board, cut the corners off and wrote “stop” on it. It did the trick and soon he was telling the other cops about it and it soon caught on.

The invention of the roundabout is a bit more complicated. Circular intersections of some kind predated Eno, but they wouldn’t all be considered roundabouts by our definition today.