You use the word "track" which implies you were cycling at a high speed, so you are quite likely correct.

Studies show quite a clear benefit for wearing helmets if you are travelling above 20km/hr.

The benefits of a helmet are much less significant if you are biking less than 20km/hr.

By track I meant a rural unsurfaced road - the accident was completely my own fault, but I fell off and my head hit a post at the side of the road. My helmet broke into fragments and although I was unconscious for a while (I had no idea how long) it was my helmet rather than my skull that broke into pieces.

There are more requirements for "safe" than "traffic free". In fact, there are arguments that 'safe' and 'traffic free' are semi-orthoganal, that is you can have safe conditions with traffic, and unsafe conditions without it. For instance, an unsurfaced road provides many ways in which a bicycle can easily be toppled - loose gravel/dirt, weird bumps caused by rocks, washouts, etc, hence probably not 'safe', at least compared to a smoothly paved surface which is kept obstacle free.

You probably realize this but I just want to note that helmets and skulls have very different failure modes (by design in helmet's case) and just because your helmet split into pieces doesn't necessarily mean your skull would have as well.

Indeed, I can't find the link to the video where they show many different brands of cycle helmet splitting into pieces when heading a football (soccer) dropped from a certain height (which causes no injury for someone heading it without wearing a helmet).

(Pro-choice and anti-compulsion on the helmet issue. I choose not to wear one. I cycle 5,000 miles a year - 10,000 miles in a good year - cycle commuting in London, UK and leisure riding or touring.)

Probably, but as I was knocked out by my head hitting the post as I skidded along the ground still attached to my bike I hate to think what would have happened if I wasn't wearing a helmet. It's not exactly an experiment I want to re-run.

Bear in mind that a requirement like that may actually kill more people through dissuading people from the exercise a bike provides.

I wear one too since I had a close call, but I wonder whether that is actually an improvement. That time I only grazed my head because of martial arts falling training that told me to turn in midair and roll - but I wonder how that move would have ended with a few centimetres more material on my head.