I ride alone for a bunch of reasons. First, most of my miles are commuting miles, and I've yet to see a single other rider on 90% of my route into the city.
On rec rides, I avoid groups because (a) I'm not competitive, (b) I obey the traffic laws, and few group rides seem to agree with me on that, and (c) I feel far more endangered by other cyclists' behavior and lack of skill than I do by cars and other forms of traffic.
I get talked into going on (usually small) group rides every now and then. I prefer riding solo.
RichC
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Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)