What does this broken line indicate?

As in the case of the image in the question, a special lane will generally be bounded on its outer side by a continuous or broken white line noticeably wider than in the general case. The proximity of a continuous line will be indicated by a broken line with the distance between its dashes noticeably shorter than in the general case. In the case of a reversible lane, it will be bounded on its sides by double broken markings.