There are a number of problems associated with growing edible plants near the road, as well as near cement, driveways and all that.
Fly ash is an important ingredient of most cement, as well as other things (including potentially the road). Fly ash contains heavy metals. Plants could absorb these heavy metals.
Here's what I would do: Put some kind of lining in the ground between the road and your soil to prevent your plants' roots from touching the substance containing fly ash. Never put cement, concrete gravel and stuff like that in your soil (like if you're shoveling snow and bits of the driveway come off with it, don't throw it on your lawn, as you may want to garden there some day).
Your neighbors and others happening by might eat or play with your fruit.
You never know what's in car exhaust.
All in all, if there weren't too much traffic, I would still do it if I were running low on places to plant, but I would put that lining between the soil and the road. This may also keep your tree roots off the road. If there were a load of traffic, I'm not sure what to say, although from what I've seen exhaust doesn't look terrible for plants in ways that would harm humans eating them, these days (although lead in the past was a problem). I could be wrong. You sometimes don't know what new changes come about until after the fact.
I wouldn't be that concerned about exhaust with the road being 20+ feet away, but that's me (and this isn't professional advice). However, you could always put a giant hedge close to the road to stop much of the pollution from getting into your yard, if it turns out to be a problem.
Edit: Michael's answer reminded me of the mosquito trucks. They spray stuff all over the roads to kill mosquitos. I'm not sure how far from the roads the stuff gets. Depending on which chemicals they're using and what you want in your garden (including things like insects, fish ponds, and toads), this may be a problem. One of the chemicals they are known to use is not good for amphibians and fish, and that chemical and others may or may not harm beneficial insects. The health impact on humans, I don't know. But I wouldn't ride a bike behind one of those trucks like a lot of kids used to do probably before my time. I wouldn't even be outdoors.