The trick is to anticipate the dogs reaction without giving the dog the clue that somebody is coming.....
You can see people coming before the dog does, you are higher up and your horizon is further away. You then have time to get ready - get the lead to a comfortable length before you go out - so the dog is not able to run towards people and scare them but you still have a relaxed, loose lead when normally walking. Walk slower, this will calm the dog and give you both more time to think.
Another thing you could try is the technique for getting a dog to stop pulling on the lead that I have described before - this will concentrate the dog on you and make it easier to appear that you haven't noticed anyone else as a threat.
Start this in a quiet area - every time the dog gets two paces in front of you - walk backwards, patting your thigh and saying in an encouraging lively voice, "come close". This should be fun and once the dog has the idea of it use it out on walks ALL the time. As soon as you are aare of anyone coming towrds you start the "Come close!" routine and keep it up ( you will look daft, but you'll get used to it!!) until the person has passed or the dog starts to bark. If the person passes - go wild, praise, treats and hugs. If the dog barks stop straight away and leaving the lead at the length it was try not to haul the dog in.
This is so frustrating!!! If I could get to you I could explain it a whole lot better but I'm no good at trying to explain without a dog!! I do hope this is helpful and good luck, keep us up to date, please