Lines on maps

What do the black lines on the maps represent? I had thought at first they might be ATC sectors, but some of the lines are darker black than others, and some of the outlines do not appear to be sectors. Shouldn’t the answer to this question be in the FAQ?

Those would be roads/highways/interstates. The darker lines are Interstates (look at TUL, the darkest line going diagional SW-NE is I-44, the dark to light to the west of TUL is US 75, the one to the East is US 169)