I was reading on wikipedia that the TTC streetcar system has a minimum turn radius of 36', which translates into about a 5" radius curve in HO scale. My previous layout had minimum 15" radius curves; far too large for a convincing representation of the street. I laid down a lumpy loop of track - almost an oval at one end - of 5" radius to run some tests with my fleet.
It turned out only one end was more-or-less circular and the other was a bit too oval, but it worked out to be a fairly ok 5" radius test track. Only the Bachmann Birney single truck tram could handle that tight curve. None of the PCCs could.
I was crazy when I built that loop. I figured on all members of the fleet successfully negotiating the 5" radius curve. I got a little smarter when I had to do tests with 6" and 7" radius turns. I drew a guide on my cork board and temporarily thumbtacked down a strip of track. It turned out to be a good approach: a 6" radius curve was also too tight, but 7" worked just fine. To be a little more specific: the Bachmann Peter Witt could handle a 6" radius curve, but the Bowser and Con-Cor PCCs couldn't, although they could handle a 7" radius turn. It looks like 7" will be the minimum radius for curves on the new layout.