perhaps not the fairest, but i have to hack hal out so we tackle the problem head on.
i take him along fairly busy roads (thankfully its only about 1/2 a mile) and the traffic cant over-take which helps, thats his fear, he doesnt like been trapped in a situation (eg between tall hedges with cars behind and infront)
it does scare him (shallow fast breathing, excessive sweating, bulging eyes etc) but luckily he trusts me so he carrys on.
if he meets something huge (buses and tractors mainly) then he does have to escape the situation there is a few drives, private roads and carparks we can run off into thankfully.
so i wait for it to pass (he shakes as it passes and runs on the spot- his legs have to move, but he doesnt physically need to move if you see what i mean (usually anyway) then we carry on.
once we are off this road, he has run out of adrenaline and he settles right down. and he copes with the smaller things better aswell (like road markings and single cars passing)
its not going to fix anything (bit does help which is why i do it), and you need a unflappable rider (im not fussed about riding on roads, its second nature to me which helps him alot so his quirks dont fluster me and i can deal with the situation calmly) and a good road to do it on.