perhaps a naive implementation solution would be for the radius to auto-scale based on density - I wonder if there's some interestingly evocative user experience here, if you could see a distance-direction-time vector which showed the distance in spacetime in the UI in a phenomenologically rich way somehow (with colour or size or something...) - then the "feel" of the density becomes part of the experience of what the user is experiencing. Using the app in a dense place lets you feel proximity, using the app in a sparse place lets you feel distance