A jackknifed semi can bring traffic to a halt, especially on a slick mountain pass. I remember being glad the cattle hauler was empty because if it had been loaded, the process to clear the road would have been more involved and the delay much longer. Unexpected incidents like this are good reasons to always have a tank full of gas when driving in cold weather.
May 1979, I was traveling with a group from Wheatland, WY to a conference in Jackson, WY. That morning we left Wheatland's sunny skies and temperatures in the 60s and encountered snow and slick roads on Togwotee Pass. Wyoming's weather is notoriously unpredictable and can change quickly even in the summer.
The road over Togwotee Pass was recently improved (widened, realigned, and resurfaced), but even with the improvements, a jackknifed truck would still bring traffic to a halt. Previous posts of Togwotee Pass can be found here.