Aah, so that's what this is

I can't tell you how many times I've driven past this (easily in the four figures) and wondered what it was, but then did stuff all to actually find out, other than knowing it had something to do with sport as it is next to athletics track.

And, it is an air dome containing an indoor pitch. So ... no big surprise there then. What slightly surprised me was to find out that the structure is actually a large rectangle, and not the circular dome I believed it to be. 

It is called, rather grandly, Cardiff International Sports Campus, and includes the athletics track.

I also like the random cone levitating by the traffic light.