>So, cyclists, please change my mind: when you have a wide shoulder to occupy, and you choose to ride so close to the left of it that your handlebars are in serious danger of being clipped by vehicular traffic, why? Why not the middle of the shoulder, or, god forbid, the right side of it?Cyclists need much more road width than you might intuitively expect, because we're less able to safely ride over debris or potholes. An obstacle that you would barely notice in a car could cause a cyclist to crash. The shoulder is often littered with debris, because the wheels of vehicles tend to sweep debris out to the edge of their usual track; the shoulder doesn't benefit from this sweeping action.
We know that we might need to swerve unexpectedly for reasons that aren't obvious to following traffic, so we want to make sure that we have sufficient space to swerve away from traffic rather than towards it. Riding well to the left gives the cyclist more control over their distance from passing traffic, because they maintain a buffer of safety that they can use if an inattentive or aggressive driver passes dangerously close.
Some cyclists might take this principle to an extreme, but the opposite is usually true - timid and inexperienced cyclists tend to ride right in the gutter rather than staking out a survival zone, severely limiting their options if they need to avoid a hazard or a vehicle is passing dangerously close.
On narrower roads without a shoulder, safe cyclists will take a variety of road positions based on the conditions; the reasons aren't always obvious to following drivers. We always keep at least four feet away from parked cars, because of the risk that someone unexpectedly opens their door in our path. Hitting a door is often fatal, because the top edge of the door is at around head height to a cyclist. We might tuck in to the right to maintain a safe distance when being passed, but we might also move out to the left of the lane to indicate to motorists that it would be unsafe to squeeze past us in the same lane on that section of road. Riding in the tracks followed by car wheels is usually safer, because those parts of the road are swept clean of debris and less likely to be contaminated with spilled oil or diesel.