If I remember correctly roundabouts and trafic circles are not the same thing. Traffic circles are much larger and are essentially a series a T-intersections connected by a circle shaped road. They where invented in the US and predate roundabouts by quite a bit.

However it turned out that these traffic circles where inefficient and dangerous, so the US public came to hate them and the idea ended up being phased out.

Meanwhile in Europe the idea evolved into the modern roundabout. These are smaller in size, better shaped, yield on entry, etc. Basically they fixed the issues resulting in quite a nifty piece of road infrastructure design. But because the US already had the bad experience with traffic circles, these never really became popular there.

In short traffic circle is the original name for a crappy circle shaped intersection, but some people keep using the name to refer to roundabouts.