The act of driving is a complex task requiring constant vigilance, physical coordination, and rapid cognitive processing. While most people associate impaired driving only with substances like alcohol or drugs, temporary non-substance-related states—like being ill, angry, or depressed—routinely compromise a driver’s ability to operate a vehicle safely. Understanding how these temporary conditions affect the brain and body highlights why it is unsafe to get behind the wheel when feeling unwell.
Physical Impairment from Illness
Physical sickness, even a common cold, can seriously degrade a driver’s physical and mental performance. Symptoms such as fever, congestion, and muscle aches restrict the movements required for steering and braking. This physical discomfort demands a portion of the driver’s attention, pulling focus away from the dynamic road environment.
Medications used to treat these symptoms are a major concern. Many cold and allergy remedies contain antihistamines, particularly older generations, which cause significant drowsiness, blurred vision, and mental fog. One dose of certain antihistamines can affect driving ability comparable to being above the legal blood alcohol limit. Decongestants, while not sedating, can cause jitters and an increased heart rate, interfering with focus and steady control of the vehicle.
High-Arousal States and Aggressive Driving
States of intense negative emotion, such as anger and high stress, create a high-arousal condition that fundamentally alters driving behavior. This state is associated with aggressive and impulsive actions that increase the risk of collision. Drivers experiencing road rage or high stress are more likely to engage in dangerous maneuvers like excessive speeding, tailgating, and rapid lane changes.
This emotional state causes “tunnel vision,” where the driver’s focus narrows intensely on a perceived threat or source of anger. The driver fails to adequately monitor peripheral hazards, traffic signals, and the overall context of the road. High-anger drivers have been found to have twice as many accidents in driving simulations. The danger stems from deliberate infringements and a failure to regulate impulsive actions toward other road users.
Low-Arousal States and Cognitive Slowing
In contrast to anger, emotional states like depression or apathy create a low-arousal condition that impairs driving through mental sluggishness and internal distraction. Depression is frequently associated with diminished cognitive abilities and psychomotor retardation. The driver’s attention is often internally focused, leading to rumination and preoccupation with personal thoughts rather than external events.
This internal distraction results in a tendency to miss traffic signals or fail to register unexpected changes in the driving environment. Mental fatigue, a common symptom, contributes to slow reaction times and an inability to maintain consistent vehicle control. Simulator studies show that depressed drivers are more likely to exhibit difficulty with divided attention, lane weaving, and maintaining a constant distance from the car ahead.
The Universal Impact on Reaction Time and Judgment
Regardless of whether the impairment is physical, high-arousal, or low-arousal, the consequence for safe driving is a degradation of two core functions: reaction time and judgment. Safe operation of a vehicle requires a rapid sequence of perception, decision-making, and execution. All three states compromise this sequence.
Physical symptoms and sedating medications delay the execution phase, creating a measurable lag between seeing a hazard and physically reacting to it. Emotional states, whether aggressive or lethargic, impair the perception and decision-making phases, leading to misjudgments of distance, speed, and hazard risk. The resulting delay in braking or maneuvering demonstrates the universal safety risk posed by any temporary state that compromises a driver’s full functional capacity.