I have my computer set to calculate average speed based on riding time, not elapsed time. That way I don't feel pressured to run stop lights. I generally keep breaks to a bare minimum by eating while I ride. I stop only to answer the call of nature, to change a flat tire, or for red lights (traffic, not the Amsterdam style). However, I'm fat and slow enough (or the area is hilly enough, take your pick) that I don't see myself doing a 5 hour century any time soon either way.
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The rider in my avatar is David Etxebarria, not me.