Why Cyclists Slow But Don’t Stop (26th April 2012)
It hurts to start after stopping. Getting 50–100kg moving from 0–15mph in a short time takes a lot of effort. If they do that every 30 seconds (typical for a new rider in London rush-hour) it would become excrutiating. They’d simply grind to a halt.
Slowing a little, to ~10mph, then building back up to 15mph is much more comfortable.
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The pain comes from lactic acid building up near the muscles being used the most, in the legs. If you drive a car in stop-start traffic, you’ll know how it takes a lot more energy because you’ve seen how much more fuel the car burns per mile.
It isn’t motivated by aggression or a sense of superiority. They’re just trying to get from A to B like everyone else – minimising the pain of their effort along the way.