We don't get much in the way of traffic where we live, but we are situated near where the army practices landing helicopters in an open field 'touch and go' style. Coming in very low and leaving very fast, often in groups. The best way i've found to get horses used to that is to progressively tie them closer to where they land.
All you need is for the horse to be able to stand tied in a relaxed environment (which every horse should know anyway), and the person to be responsible enough not to tie them too close too soon, risking a massive wreck.
Works for me, and i've never had a problem with horses spooking at helicopters. Even when they think it's fun to swoop me while i'm out riding.
I imagine it would work for roads and cars too.
Eta for safety keep a fence between the road and where you might tie the horse.