I posted this a while back; here’s the story:
I rolled my ‘01 Crystal Blue in April. No roll bar. I got my head and face gouged and cut up BAD, and came real close to bleeding to death pinned upside down in the car. Hospital, 7 weeks off work, and now facing 1 to 2 years of facial plastic surgery to repair the bloody meat that the trauma team did their best to sew up. Nasty. (Update: 3 surgeries so far, 9 weeks off work total.) And with all that, I was extremely, extremely lucky. Should have died or worse. When I replace my Miata, it will have a roll bar, period. No hurry, because I can’t be out in the sun for the next couple years.
As others have said, Miatas are hard to roll over, but they’ll obediently do it when the laws of physics say they must. All it takes is a small mistake, or lapse in attention, in the wrong place. In my case, it was the wrong line through a downhill hairpin, getting one wheel off the edge of the road, causing the right side of the car to go up a mountainside. I don’t remember the flip. All I know is I slid on the asphalt with a ton of Miata strapped to my back, leaving about a 12-foot skidmark of blood and skin and tissue on the road. The windshield folded down flat. I’m about 6 feet tall, and the seatbelt and the 01’s high seatbacks somehow kept me from breaking my neck, but they couldn’t protect me from getting hacked up real good.
I had considered the argument that a roll bar can cause rear-of-head injury in a rear-ender, and that you’ll probably never roll a Miata. There’s some merit to that, but from my experience, I’ll never drive the twisties again without a roll bar. The answer might be different for those whose driving is mostly in traffic, but for me, the answer is the tallest bar that will clear the soft top, with real good padding and/or tall seats from an ‘01 or later Miata.