The Expert will be specific knowledgeable people at the Department of Transport, Arrive Alive Communications, the Road Traffic Management Corporation or others who might have the expertise to answer the question.
Non-OEM glass can be a huge risk. A modern flush-bonded windscreen can provide up to 30% of the vehicle's structural strength in a rollover. Also, if a window is cheap, why? The manufacturers may have used inferior-quality laminate, non-spec glass, or sub-standard manufacturing processes. I have driven in several vehicles where deformations in the windscreen have been apparent. This is particularly bad for visibility at night.
Also, with toughened glass, if the manufacturing process is incorrect, the glass may splinter instead of crumbling into harmless fragments.
Some non-OEM glass may also not have the correct tint or infra-red treatments applied, meaning higher cabin temperatures, increased aircon use and thus higher fuel consumption. And at the very high end, vehicles with a heads-up display require a special windscreen for the HUD to work properly and these must be made to very tight specs.
