Critical characteristics of the driver

Driver characteristics are the personal attributes that decide how a driver senses, decides and acts in traffic. The critical ones are physical, mental and psychological.

Physical characteristics

• Vision: acuity (normal acuity is 6/6), the field of vision (clear vision within a cone of about 3-5 degrees, fairly clear up to about 10-12 degrees, peripheral vision up to about 120-180 degrees), colour perception (red/green colour blindness is critical for signals), glare sensitivity, glare recovery and depth perception. Speed narrows the field of vision (at 100 km/h it is only about 40 degrees).
• Hearing: detecting horns and sirens; deafness is partly compensated by vision, but it reduces awareness of emergency vehicles.
• Reaction ability: muscle coordination, strength and speed of hand/foot response; fatigue, age and disability reduce it.
• Age and health: reactions slow and vision/hearing fade in old age; illness, epilepsy and heart conditions are hazards.
• Fatigue, alcohol and drugs: these lengthen reaction time, impair judgement and cause drowsiness; Nepal's Motor Vehicles and Transport Management Act 2049 prohibits driving under the influence.

Mental characteristics

• Intelligence and judgement: the ability to judge speed, distance, gaps and the intentions of other road users.
• Knowledge and training: familiarity with traffic rules, signs and the route; drivers unfamiliar with a route hesitate and make errors.
• Attention and concentration: a driver can attend to only a limited amount of information at a time; distractions (mobile phone, passengers) reduce attention.
• Memory and learning: help in recognising signs and routes and in forming habits.
• Perception-reaction process (PIEV): perception, intellection, emotion and volition, which together determine the total reaction time.

Psychological characteristics

• Emotional state and temperament: anger, stress, worry or elation lead to aggressive driving, overtaking in unsafe places and over-speeding.
• Attitude and behaviour: attitude towards rules, other users and risk; over-confident or risk-taking drivers have more crashes.
• Anxiety and fear: a nervous driver may brake suddenly or hesitate at intersections.
• Social habits: lane discipline, courtesy, and the tendency to follow the crowd.
• Driving habits and experience: young drivers take more risks; experienced drivers anticipate hazards better.
• Response to stimuli: drivers react quickly to expected events but slowly to unexpected ones (for example a stationary vehicle on a bend).

Design implication

Because drivers vary, design values are taken for an average or conservative driver: a perception-reaction time of 2.5 s for sight distance (IRC:66, IRC:SP:23; Nepal Road Standard 2070), sign letter sizes matched to visual acuity, and lighting and markings to help night vision.