Human-vehicle-environment system; driver characteristics

Human-vehicle-environment (HVE) system

The traffic stream is the combined behaviour of three interacting elements: the Human (road user), the Vehicle and the Environment (road and surroundings). Safe and efficient flow needs all three to match.

        +--------------+
        |    HUMAN     |  driver, pedestrian,
        | (perception, |  cyclist, passenger
        |   reaction)  |
        +------+-------+
       controls|    ^ feedback
               v    |
        +------+----+--+      +--------------------+
        |   VEHICLE    |<---->|    ENVIRONMENT     |
        | size, power, |      | road geometry,     |
        | brakes, load |      | surface, signs,    |
        +--------------+      | weather, lighting  |
                              +--------------------+

• Human: receives information (mostly visual), decides and acts (steering, braking). Errors in perception, judgement or reaction are the main cause of crashes (about 80-90 percent involve human error).
• Vehicle: its dimensions, weight, power, braking and visibility set the road width, gradient, curve radius and pavement strength required.
• Environment: road geometry, surface friction, traffic control devices, lighting, weather and adjacent land use affect how the driver sees and responds.

A mismatch in any one (for example a fast vehicle, a sharp curve and a wet surface) causes crashes, so the design must fit the "design driver" and "design vehicle" to the road.

Physical characteristics of the driver that affect traffic performance

• 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.