0
The default draw level. Roads with draw level 0 are to be displayed first, with draw levels 1 and 2 overlaid on top.
1
Used for road bridges and overpasses. Roads with draw level 1 are to be overlaid on top of draw level 0 roads.
2
Used in very rare cases when there are several coincident levels of road bridge or overpass. Roads with draw level 2 are to be overlaid on top of draw levels 0 and 1.