California glass expert has been involved with evaluation automobile windshields. One way car manufacturers can increase automobile mileage is to decrease weight. A part of this effort has been to reduce the weight of automobile windshields. This is done by reducing the thickness of the windshield. Upper left is a representative cross section of a modern windshield. The glass is only 2mm thick, and the polyvinyl butyral (PVB) inter-layer is only 0.07 mm thick. On the positive side, this reduces weight and improves the mileage. On the negative side, this windshield is prone to cracking. Upper right is a photograph of a windshield failure. The crack that was started by an impact from a rock. This crack formed immediately and has grown continuously for several months. Older windshields were thicker and were less prone to this sort of cracking.