I have no-one else to go out with (and youngster is not backed), and there's no way I'm taking both ponies at same time (to have a reliable babysitter), so the way I'm doing it to handle it by myself is:
Take baby down to end of road, and let her see the traffic. Once she's calm, I'll take her down the road a little way, turn and come home. Building on this, to then go round the 30 minute loop, so we don't have to turn back the way we came
also if you can find somewhere with a field next to a busy road and turn out with others for a few weeks
IT really depends on what you have available. ie lorries tractors, cars, motorbikes.
On the yards where I have need to traffic proof I have taken horses up to vehicles when parked, then when engine running.
Next step was to get someone to drive past while I was leading/riding.
I would ride/lead out with safe horse and ride two abreast on the inside. or If I had to do it alone I would lead out in hand in a bridle with pocket full of treats.
The first horse I did it with I led out choosing a route that went down a quiet lane, around a housing estate and back up a main A road, with heavy lorries hurtling past. Gosh I was stupid, wouldn't do it now however it worked and horse NEVER EVER had a problem with any traffic. But it could have been such a disaster