Here's what I found while poking around:
In order for the LDWS to work you must have:
The vehicle is driven near the center of the lane with the white or yellow line on the left and right sides, or on either side.
The vehicle speed is about 70 km/h (44 mph) or more.
The vehicle is driven on a straight road or road with gentle curves.
Causes for the LDWS not working:
The LDWS warning only becomes operable when the system detects the white or yellow lines.
If the steering
wheel, accelerator pedal, or brake pedal is operated abruptly and the vehicle moves close to a white or yellow line, the system determines that the driver is making a lane change and the LDWS warning is automatically canceled.
The LDWS may not operate during the period immediately after the vehicle has deviated from its lane and the LDWS has operated, or the vehicle deviates from its lane repeatedly within a short period of time.
The LDWS does not operate if it does not detect the white or yellow lines of the traffic lane.
Under the following conditions, the LDWS may not be able to detect white or yellow lines correctly and the LDWS may not operate correctly.
If an object placed on the dashboard is reflected in the windshield and picked up by the camera.
The white or yellow lines are less visible because of dirt or paint flaking.
The vehicle ahead is running near a white or yellow line and the line is less visible.
A white or yellow line is less visible because of bad weather (rain, fog, or snow).
A misleading line is picked up on the road such as a temporary line for construction, or because of shade, lingering snow, or grooves filled with water.
The surrounding brightness suddenly changes such as when entering or exiting a tunnel.
The illumination of the headlights is weakened because of dirt or the optical axis is deviated.
The windshield is dirty or foggy.
Back-light is reflecting from the road surface.
The road surface is wet and shiny after rain, or there are puddles on the road.
The shade of a guardrail parallel to a white or yellow line is on the road.
The width of a lane is excessively narrow or wide.
The road is excessively uneven.
The vehicle is shaken after hitting a road bump.
There are two or more adjacent white or yellow lines.
There are various road markings or lane markings of various shapes near an intersection.