Something to consider:
Attached is a post I did in 2001 when a rock hit my glass windshield for the second time and I decided to replace it with Lexan.
Now 8 years & 18K miles later the windshield looks as good as it did when I installed it (no scratches, "tested" at speeds up to 125mph . I've heard a number of rocks hit it since and each time I just count the dollars the Lexan just saved me
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Here's a link to my original post
Lexan Windshield for Cobra
good luck,
mike