These were installed a few years ago. The city limits extend quite a ways from the built up city itself (like Ottawa does), so you'll see the sign that says "welcome to Thunder Bay" and by the time you actually get to Thunder Bay you'll probably have forgotten that you saw a sign welcoming you here.
They're actually one of the better things the city has designed, since the logo is pretty crappy.
The northern lights one is pretty cool at night.
These were installed at three of the five major entrances to the city (Highway 61, Highway 11/17 eastbound, and Highway 11/17 westbound), the other two (Highway 102 and Oliver Road) were supposed to be installed a couple years ago but I think they forgot?