I have had that problem before with some of my ponies.
There's basically two ways I do this; there's an easy fix for horses who just don't want to go out (like just backing or just not wanting to move their feet) and there's one for the more dangerous ponies who do what you described.
The easy way: when they try to back up when you're cueing them to go on, turn them around so they're facing the way they
wanted to go, and then back, back, back, back, back.. Making her back the way you wanted to go (in your case, onto the road). If she tries to go forward in one direction or rear, do tiny tiny circles until she gives up and just lets you back her up to where you wanted to go. I've found that after a few repetitions of this; they do what you ask
The more complicated, but safer way that I use when I'm riding a horse that reacts badly, such as Eagle, who once flipped over on me while she was insisting on backing down a steep ditch (good thing she's 13hh or I would've been toast); is to make the barn hard work, and make the road easy work.
Basically, just work her HARD by the barn where she wants to be, then take her up to the road (even if you have to lead her) just let her rest there, feed her some treats, rub her, treat her nice. Then go back down to the barn and work. Then go back up to the road and rest.
Soon they will be anxious to get away from the barn!
Both ways work for me, but the second one just takes longer