OK, assume it happens, and at the worst possible point for you (e.g. longest walk to 'civilization').

It seems to me that there are 2 key questions:
1) how long do you wait in the traffic jam before you abandon the truck (and what I assume is more equipment then you can carry/drag along with you), and,
2) can you realistically safely walk out under bad conditions (the assumption automatically assumes some bad conditions, but maybe add bad weather) to 'civilization'?

Good Intelligence, that is, good real time information, about what is going on, could be key to this decision. How to get it? Rely on local news? CB radio reports? other? Do you trust the sources to get it right?

I was once trapped in my car by bad weather (snow) for about 12 hours. It was not clear if it was better to stay put, or try to walk out. It is a tough decision to abandon relative safety for the unknown. I stayed put: at the time, it seemed to me that cold and dry beat cold and wet.