Depending on what exactly you have and haven't tried, these ideas may or may not be helpful...
Have you tried taking her out in the company of a horse who has a lot of experience with roads and is comfortable with the traffic? Sometimes a quiet buddy does a lot for stressful situations.
If you have to tackle the problem yourself, I recommend finding a fair-sized field that borders a road (obtaining the permission of the owner, of course) to practice in. The field should be large enough that you can work far enough away from the road itself that your horse is comfortable and able to focus, and you can extend figures closer to the road and safely extend her comfort zone. From here, I'd tackle the road like any other spooky object: approach and retreat, and allow rest close to the spooky thing.
Another option, if it's cars in general or moving vehicles period that are the problem rather than heavy road traffic, is to start very small and follow a car around. Start leading the horse from the ground, have a friend drive ahead of you fairly slowly, and just follow the driver until the mare starts acting curious rather than fearful. When she's comfortable on the ground following at the walk and trot, try it mounted. The idea is to make the vehicle nonthreatening by having it "run away" from the horse, rather than behave like a predator to be feared. When she's good following, try riding alongside the vehicle, and eventually in front of it. It takes some coordination, but I've seen it done.
Other than that, controlled exposure is key. I'm "fortunate" in that I live on a busy road. I don't ride on the road as it is simply too busy to be safe, but my home arena is only a few yards away from highway speed tractor-trailer traffic - the sights and sounds become old hat pretty quickly with that kind of exposure. Not having immediate access to that kind of area makes the process harder.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown