The problem with drafting someone you don't know on a commute is that it is unlikely they are going to be indicating hazards to you. This is important as if you are genuinely drafting (not sitting 3 metres back, for example) you are going to have a limited view of the road conditions ahead. What's more, you are also obscuring the view of the cyclist giving you a tow when he shoulder checks. In a chaing-gang, club ride etc cyclists are used to riding in close order and use hand signals and calls to keep eath other safe. On a commute in the rush hour, I don't want my focus to be distracted by someone who might plough into the back of me because he cannot see a problem developing. If people just sit on my wheel, I either speed up or slow down and wave them through. That said, I will draft and give people tows if I know them or they introduce themselves and seem savvy.
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