I am afraid that the guinea fowl need to be trained not to go on or near the road. The guinea fowl can be trained but it may take some time to break the habit as guinea fowl are creatures of routine and tend to follow set routes around the garden each day.
When you see the guinea fowl ranging near the road, it is best to go out and steer them back into your garden. Long sticks for driving the guinea fowl into a certain area can be helpful, but a word of warning don’t wave the sticks too close to the guinea fowl as this will frighten them and send them flying into the trees.
Obviously going out each time the guinea fowl go near the road will be time consuming, so I would try enticing the guinea fowl back into the area that you want them to range in, with some millet. Jeannette Ferguson’s book Gardening with Guineas suggests that if you sprinkle the millet near a flower bed then the guinea fowl will eat the millet and hopefully notice all the bugs in the flower bed that they enjoy eating.
I am sorry that I don’t have a quick fix to stopping your guinea fowl from straying onto the road but hopefully with some training your guinea fowl will learn to stay within the area you train them to range in, thus keeping them safe and all the bugs and pests that eat your flowers will be eaten by the guinea fowl. It sounds like a win, win situation!