In case anyone is wondering: this is a consequence of our exploding population combined with steep hill sides.

When the new highway was punched through, allowances needed to be made on those steep hillsides for a much wider road (nearly two and a half times the width of the old 2-lane highway), and a much straighter road (for higher safe speeds).

This means that much more of the hillside needed to be blasted away, at many more locations than previously. The old highway had its largest exposed rock face at about 100ft of height, today’s highway has over 500ft of exposed rock face at its greatest point. This gives many more “points of failure” for rock to spall, crack, and collapse.