Have you ever found yourself running along a pavement or a towpath, somewhere with limited width, and seen your way obstructed by pedestrians or other malingerers walking two or three or four abreast? You expect that they'll accommodate you, make way, move over a little, share the space. You make eye contact. You hurtle towards them. And nothing happens. The block your way. You slow down. You slow down more. You stop. And then they issue a big smile and clear some room.
Once this has happened a few dozen times you begin to puzzle about it. Are they stupid? Do they think that you like to stop? Are they incapable of judging your speed, and thus inadvertently move over too late? At least with dogs you know what will happen: they look over their shoulder and invariable drift into your path. Dogs are reliable, and thus can be negotiated. People are blockish.
Now, you may have read the odd letter in Runners' World, or the odd personal email for that matter, complaining about runners spitting. It's an unpleasant habit, apparently. I suppose that in most contexts I would agree. However, I can assure you that nothing gets the obstructive pedestrian's attention than a throat clearing followed by a modest expectoration into the pathside. Nothing substantial or portentous, just a functional vacating of the airwaves. It's not clear to me why this should be the case. It's not as if spitting runners are faster, or need more room than others. But there's something about the confident and focussed spit, performed with a rhythmic indifference, or even insouciance, that declares to obstinate path-blockers, 'excuse me, could you move over a little in order to let me through?' Even if you think spitting in public (or in private?) an unpleasant activity, it is one you need to master. Other columns can supply you with mere rudimentary details.
So ignore the social opprobrium attached to the business. Regard conspicuous spitting as an essential training technique, especially when running intervals. And the good news is that in these days of swine 'flu panic it is more effective than ever.