At this stage it’s a trope that people imitate, perhaps without really thinking about it. Originally it was almost certainly an ironic joke about the value of the medals, playing on the old-fashioned bite tests that would be used for for items of dubious worth
It would have a massive effect. Transport (car) emissions are one of the larger - and growing - sources of emissions.
And we can’t hide behind “But the corporations…” because ultimately what they produce gets used by us.
So to answer your question: riding a bike when Global Capital wants you to keep buying cars and pumping oil into them is one of the best acts of defiance you can make