Pedestrians and velocity


John Whiteside made a perceptive comment about my post “Cycling to work – why people hate cyclists, part 3“. I completely agree with him – what he said in his comment is what I intended to say in my post. Sorry it wasn’t clear.

The post talked about collisions between cyclists and pedestrians. I said

The cyclist will see a pedestrian many metres ahead and will calculate his own trajectory and the pedestrian’s to ensure there is no collision.

John correctly commented that pedestrians don’t have a “trajectory”. What they have is a “radius of occupation” – a theoretical circle around them that they could be anywhere in when the cyclist reaches them. The radius of the circle is a function of the pedestrian’s maximum velocity and the distance the cyclist has to travel before impinging on the pedestrian.

Here’s what I meant to say: the cyclist unconsciously calculates this “radius of occupation” and avoids it.

Of course, there are some parts of the circle that the pedestrian is unlikely to occupy. The sophisticated cyclist might try to calculate the probability distribution across the entire “radius of occupation” and choose to clip a part of the circle where the pedestrian is unlikely to end up. This sort of calcuation will be correct most of the time, but ultimately will result in a few collisions.

I think such collisions are the fault of the cyclist. My diatribe against pedestrians was intended to include only those collisions that the cyclist could not possibly avoid.

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