0
The default draw level. Roads with draw level 0 are to be displayed first, with draw levels 1, 2 and 3 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.
2
Used 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.
3
Used when there are several coincident levels of road bridge or overpass. Roads with draw level 3 are to be overlaid on top of draw levels 0, 1 and 2.