In order for you to see an image of the Sun coming off the windshield, that means that rays therefrom must be hitting that windshield and then reflecting off in the right direction to reach your eyes.
And the fact that rays are hitting that windshield means that also, given that it is mostly transparent, some, in fact most, of those rays are being transmitted, instead of reflected. That is, the Sun is shining into the inside of the vehicle, and hence will be near or in the driver's field of vision, likely greatly interfering with their ability to see thanks to the intense light.
Of course, depending on the exact angles involved, it might still be out of hir field of vision, or sie may have a visor down to block it. Nonetheless, the point is to use the precautionary principle: since it indicates a sizeable chance that something dangerous is happening, treat that chance as 100%, and act accordingly (i.e. don't just dart out into the road expecting they'll see you and stop, but wait for them to pass and there to be a large enough opening in traffic to make it through with considerable safety margins or, better yet, look for an intersection with traffic signals).