9. In a past life, I raced mountain bikes, road bikes and off-road motorcycles competitively and trained solo all the time. I would go on 9 hour training rides in the Oregon Cascades backcountry on my mountain bike with nothing but myself, my bike and some redundant gear (tools, spare tubes, patches, enough water and food)- I find it interesting how different sports and activities treat solo participants differently.
To be fair, this is an underrated security aspect in these sports. In my group we remind our buddies to be very careful when riding alone. With an enduro motorcycle, you can very easily reach spots where nobody else ever goes, and there's lots of ways to screw up yourself, the bike, or a combination thereof.
In my opinion, the different treatment of solo activities has to do with the perceived difference in the near-instant death of scuba diving, versus sitting in the dirt with a broken leg, thinking somehow you might fight your way out of this.