Originally Posted by SethAZ

I can think of several responses to this.

1) In a strong crosswind, the passing of a vehicle can result in a very sharp change in the sideways forces acting on me, the bike, and the modestly deep carbon wheels. This can result in sudden swerving, etc. That's especially true if the vehicle is a large truck, and it is passing me in such a way as to complete block the wind coming from that side. Knowing the vehicle is about to pass allows me to mentally "brace myself for impact", as it were. I'm not saying this is a huge factor, but it's definitely a factor, and is responsive to your question.

2) I try to keep a good situational awareness of the road conditions directly in front of me, at intermediate distances, etc. Despite that, when I hear a vehicle coming up behind me it focuses me more than usual on what's immediately ahead of me, how much margin there is to the right of where I'm riding, etc. in case I do feel the need the move further to right or even pull off the road entirely. I've actually been run off the road before by an assclown who buzzed me way too close. Whether I actually move further to the right or even off the road entirely for every car or truck I hear coming up behind me or not, knowing that it's coming at least gives me the chance to focus on the road conditions and what my options may be. Again, not a huge factor, but it's a factor for me that I value, and is responsive to your question.

3) "It seems to me that if you do anything differently based on whether or not a car is behind you is just a recipe for disaster, because you're going to eventually get it wrong." Yes, eventually I'll probably get it wrong. Not knowing a car or truck is coming up behind me, however, increases the likelihood that I'm already wrong and just don't know it until it's too late. If I get it right even just a few times where not knowing would have meant I was already wrong and just didn't know it, that's a win.

Some of my cycling routes have this one awful choke point where a small bridge passes over a usually dry creek bed. Whoever designed that bridge deserves to be drawn and quartered, because they made zero allowance for any type of bike lane, safety margin, etc. On that road you go from the lane of travel, white stripe, then 2-3 feet of pavement, to the bridge where it suddenly constricts to the lane of travel, the white stripe, and like 3 or 4 inches and then a hard concrete barrier on the side of the bridge. I really hate passing over that little bridge, especially because the road in question is a fairly major north/south artery road here where the speed limit is 45mph which means in practice people are buzzing you doing 50-60 mph. I very much want to know, approaching that bridge, whether there are cars coming up behind me as I assume the safest position I can just on or inside the lane of travel from the white line. Sometimes when I hear cars coming up behind me but still a ways back I'll drift over a foot or so into the lane of traffic from the white line to prompt the drivers to pull left a little more or even change lanes, and then just as I cross the bridge I'll hug that line as closely as I dare to give myself the best chance of not being hit. Others might be fine having music blaring in their ears while navigating this particular road feature, but it would drive me insane. Again, that's maybe a "me" problem and not a "you" problem, but there it is.