Mercedes-Benz SLK Operator's Manual page 191

The warning vibration occurs earlier if:
you approach the outer lane marking on a
R
bend.
the road has very wide lanes, e.g. a
R
freeway.
the system recognizes solid lane markings.
R
The warning vibration occurs later if:
the road has narrow lanes.
R
you cut the corner on a bend.
R
Driving systems
Z
189