How can anyone be legally charged with "Obstruction of a Highway" instead of a DWI if there was no obstruction?

I completely understand the idea of persuading people to plea guilty to a lesser charge in order to simply secure an easy guilty plea. Obviously, this approach behooves both a defensive lawyer and a state prosecutor. What I'm having difficulty with, is understanding how someone could be charged with something completely unrelated to what they are being accused of. If you were not obstructing a highway or other pageway at the time you were pulled over for a DWI, how can you possibly be charged with obstruction? Shouldn't the lesser offense somehow pertain to driving or at least intoxication at some point? This is ridiculous to me