My general observations on vehicle pathing:
-Sometimes they turn left/right and bump right into something, when a curve in other direction would have cleared the obstacle.
-On terrain edges, often this happens:
Vehicle takes the corner too much on the inside, gets stuck.
I see the idea how slopemod is supposed to keep vehs further away but does not seem to work. Screenshot from v1.0.10.8
Slopemod probally only makes a difference on "soft" terrain edges:
But those soft hills pathed okay-ish before.
-In a group, pathing is sometimes a bit better: The vehicles push each other around corners. Though sometimes all clump up.
-Buildings as obstacles behave similiar to terrain edges but bump the vehicles away. Can be good or bad: Generally vehs are less likely to come to a complete hold but sometimes "scrape" along buildings (eg windmill lines) which makes them superslow, also endless bump circle.
tl;dr:
imo in none of these situations changing slopemod helps.