Absolutely priceless
I remember seeing a TV science program about this a long time ago . I'm pretty sure that at one point they were showing how tough the glass was by shooting a brake cylinder at the window at 100 mph.
The stuff they were demonstrating actually stopped it, but I don't imagine that particular type went into everyday use. There have been quite a few cases recently of objects dangled over bridges and tunnel mouths and they've all penetrated the cab, sadly.