I don't think you will have any problem. Organic matter that turns to compost will act as a buffer...but 500 feet is a 10th of a mile. You can grow anything that is suitable for your climate and exposure. Food plants absorb very little lead from the soil...at least according to what I read. If lead in the soil that far from a highway were a problem, the corn crop in the midwest would be not fit for human consumption.
Linda C