Why? See retreating ice from a capricious drive. The Going to the Sun Road somehow carves an 80km long furrow through the mountains of north west Montana. It s also notoriously difficult to clear of winter snows. Portions of it remain open year round but the very earliest in the season that the entire length might be accessible is mid June.
Then by late September parts are off limits all over again leaving a brief window for a traverse. It s worth trying though: the road slices through the parks most dramatic sections with hair raising hairpins and a haul over 2 026m Logan Pass.
You can also see the park s rapidly retreating Jackson Glacier from the tarmac which you should certainly pull over for it s estimated that Glacier NP may be entirely glacier free by 2030.