Prolonged monotonous driving is associated with hypovigilance and reduced alertness, yet its influence on subsequent visual attention and driving behavior remains insufficiently understood. This study investigated how prolonged monotonous driving influences highway control, subsequent lane change performance, visual attention to lane change signs, and the relationship between subjective state and objective driving control. Sixteen licensed drivers completed a simulated drive consisting of a pre-highway lane change task, a prolonged monotonous highway segment, and a post-highway lane change task. Driving performance and eye-tracking metrics were assessed. The findings suggest that prolonged low-demand driving weakens lateral control, while visual attention to lane-change signs became faster and more extensive in the post-highway session. This pattern is consistent with compensatory visual reallocation following underload when task demands become visually explicit and behaviorally structured. These results highlight the importance of multimodal assessment for road safety, as subtle changes in control stability and visual attention may not be captured by vehicle dynamics alone, despite their relevance for driver readiness in attention-critical situations.