Time
Mostly anymore I commute when it is light enough for a 'daytime morning commute' -- leaving the house at 6:30-7:00. So the sun has to be up before that. Half asleep morning drivers are bad enough, and I refuse to do it when it is dark out - even though I have both bright front and rear blinky lights!
OK, so more on-topic, I have a 'goal' door-to-door time for my commute -- that is to try to beat my best. My commute is 12.5 miles one way, on relatively flat terrain. Looking at elevations, it is about 160' difference - downhill to work. My best time heading to work -- including stopping at traffic lights -- is 39 minutes flat. That's averaging 19.2mph! I don't think that's all that bad for a 60-yr-old 'semi-Clyde' riding a 30-40-year old, 25-30 pound bike!
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'75 Fuji S-10S bought new, 52k+ miles and still going!
'84 Univega Gran Tourismo
'84 Univega Viva Sport
'86 Miyata 710
'90 Schwinn Woodlands
Unknown brand MTB of questionable lineage aka 'Mutt Trail Bike'
Plus or minus a few others from time-to-time