You might even observe a vehicle straddling a center or lane line . That is, the vehicle is moving straight ahead with either the right or left tires on the wrong side of the lane line or markers. Drifting is when a vehicle is moving in a generally straight line, but at a slight angle to the lane. The driver might correct his or her course as the vehicle approaches a lane line or other boundary or fail to correct until after a boundary has been crossed. In extreme cases, the driver fails to correct in time to avoid a collision.
Course corrections can be gradual or abrupt.
For example, you might observe a vehicle to swerve, making an abrupt turn away from a generally straight course, when a driver realizes that he or she has drifted out of proper lane position or to avoid a previously unnoticed hazard. A related DWI cue is almost striking a vehicle or other object. You might observe a vehicle, either at slow speeds or moving with traffic, to pass unusually close to a sign, barrier, building, or other object. This cue also includes almost striking another vehicle, either moving or parked, and causing another vehicle to maneuver to avoid a collision. Turning with a wide radius or drifting during a curve is the final cue in this category of driver behaviors.