100' isn't much, about 5 or 6 car lengths.
When the center lane becomes a left turn lane, I wonder how soon you can get into it before the paint lines show it as a turn lane. I've often pulled into one way before it becomes a turn lane to get around a dozen cars at a stop light. That gets me out of the way of cars who want to go straight. I could probably get a ticket if the cops didn't do it, too.
RC, I am pretty sure Idaho law is that it is illegal to cross any solid yellow line, except to exit the roadway.
And I am as guilty as anyone of crossing into center turn lane on the solid yellow, and traveling a block or two before the actual intersection.