Traffic engineering; road user characteristics and HVE

Definition of traffic engineering

Traffic engineering is the branch of transportation engineering that deals with the planning, geometric design, operation and control of roads, their networks, terminals and adjacent land use, so that people and goods move safely, quickly, comfortably, economically and with least harm to the environment. (Institute of Transportation Engineers definition, as used in Khanna and Justo.)

Its scope covers traffic characteristics, traffic studies (volume, speed, O-D, parking, accident), traffic regulation and control, geometric design, traffic signs, markings and signals, road lighting, and traffic planning and management, including the Nepal Motor Vehicles and Transport Management Act and the Department of Roads (DoR) rules for enforcement.

Road user characteristics

1. Physical: vision (acuity, field of vision, colour vision, glare), hearing, strength and agility. They fix sign sizes, letter heights, lighting, sight distances and footpath gradients.
2. Mental and psychological: attention, judgement, emotional stability and attitude. They influence overtaking decisions, gap acceptance and compliance with signals.
3. Reaction time (PIEV): about 2.5 s is used for stopping sight distance, headlight sight distance and intersection sight distance (IRC:SP:23, IRC:66, Nepal Road Standard 2070).
4. Pedestrian characteristics: walking speed about 1.2 m/s (IRC:103 and IRC:93 use 1.2 m/s; slower, about 0.9 m/s, for children and the elderly); space needed about 0.5-0.75 m² per person; they need footpaths, crossings, refuge islands and signal time.
5. Cyclists: speed 10-25 km/h, need separate lanes and safe turning at intersections.
6. Age and gender, fatigue, intoxication and disability: these add to variation, so design uses the 85th-percentile or a conservative value (for example the 15th-percentile pedestrian speed for crossing time).
7. Behaviour and compliance: lane discipline, helmet and seat-belt use, jaywalking and parking habits vary by country and must be reflected in the design and enforcement plan (in Nepal mixed traffic is common).
8. Mixed traffic: in Nepal slow and fast vehicles share the carriageway, so wider shoulders and segregation are required.

Human-vehicle-environment 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 design must fit the "design driver" and "design vehicle" to the road.