I run on the treadmill more than on the road, I guess you could say semi-by-choice. I full admit to being a weather-weenie as I don't enjoy running in either cold (anything below ~45F), rainy, or extremely humid or hot (anything above 80F especially if sunny) weather, which leaves a small window of where I feel really comfortable running outdoors. If I lived in the SW where it wasn't so humid, I'd probably run outside more ("it's not the heat, it's the humidity"). I also often choose the treadmill because I run during the week at the work gym at my lunchtime, and it's quicker and easier just to hit the treadmill than have to worry about the weather outdoors and bringing various layers of clothes -- I can just bring a singlet and running shorts for the gym, which is always 68F, a perfect running temp for me.
When the weather is nice, nothing beats a trail run, but I don't really enjoy running on the road with cars passing.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow