This image of Zion Canyon shows the steep ascent vehicles must take to exit the valley floor. The switchbacks on the road at center left allow slow progression up the steep slopes without creating too much challenge for larger vehicles.
The description states:
ZION-MT. CARMEL HIGHWAY
ZION NATIONAL PARK, UTAH
Starting from the floor of Zion Canyon and zig-zagging up the talus slopes of Pine Canyon in a series of six switchbacks, the Zion-Mt. Carmel highway plunges into a 5,607-foot tunnel, from which galleries have been broken out at six selected intervals.
–This card likely dates from the 1950s.