I think I would have just run over the snake and tried my best to remain upright and in control. Was the snake in a position where hitting it was likely to cause a spill? Or were you just reluctant to harm it? What diameter does 3-4 feet of length correspond to? What a decision to have to make! What I might opt for with the underpass would be to get in front of the truck before the underpass, signal to move left, and, if the truck slows, move to take the middle of the lane through the underpass. That depends on a cooperative driver, but gives you the most safety margin in a narrow place.