I think it is more the size of the rocks. Semis usually have wide deep thread that pickup larger rocks than the small rocks in the tighter narrower thread that most sedans have. Big 4x4 trucks with their lifted off-road mud tires are a problem too. I hate being behind cement or gravel trucks the most. Once had a gravel truck that didn't have the gate closed properly and was layering gravel all down the freeway at 70+mph. It was a real nightmare because the choice was either slow way down to give extreme distance or speed way up to try and get past him as quick as possible.Did anybody have their glass broken from rocks flying from under regular cars? I've had my windshield replaced twice during my driving history and both times rocks came out from under semis. I try to avoid driving behind trucks/semis since then. I've heard rocks hitting windshild multiple times when I was behind cars, but it never resulted in cracks. Not sure if it's luck or has more to do with the weight of the car that launches the rock.