MBPerformance

Hi, has anyone figured out how to read the MBUX approaches turn 2D overlay? It looks like a simplified version of the map, all in gray and some roads are shown thicker and white. I can't figure out what the thick white lines are trying to tell me. I'm pretty sure they have a meaning, but I can't find anything in the manual. Does anyone have any ideas? Does it maybe mean these are streets I can turn to (if the junction were gray would it maybe mean it‘s a one way street?)