Loose stones on R21 Highway

Every time I drive on the R21 highway from Benoni to Pretoria and vice versa, my windscreen is being shattered or chipped by loose stones flying up from the road surface. You can actually see large "bald" patches where the surface stones are gone. This has now happened 7 times in the last few months. Is anybody else experiencing this, is SANRAL aware of it. and are they doing anything about it? I am concerned that if I break another windscreen, my insurance cover may be compromised.