) allow some convertibles without roll bars at tracks that allow it, but said convertibles must have the pop-up protection, a hard top or a note in the owners manual describing the front windshield frame as "rollover protection". The only two cars I know of with this note in the owners manual are the Honda S2000 and Porsche Boxster. Of course we highly recommend a roll bar!
This being noted, several tracks are restricting us on this and I think it's only a matter of time before all tracks mandate roll bars in all convertibles. Personally I think it's a good idea to have a roll bar in any convertible you plan to track.
Call me at (314) 249-3770 if you need clarification.