Traffic engineering; road users and vehicular characteristics

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 users

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; they 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.
8. Mixed traffic: in Nepal slow and fast vehicles share the carriageway, so wider shoulders and segregation are required.

Vehicular characteristics

Static characteristics

Examples: the 2.5 m width and 11 m length of a bus fix the minimum bay size; the 4.75 m height fixes bridge clearance; the large turning radius of trucks fixes the radius of intersection corners; the axle load fixes pavement thickness.

Dynamic characteristics

• Power and performance: engine power, tractive effort and weight-to-power ratio decide acceleration, gradient ability and the need for climbing lanes.
• Speed and acceleration: the typical range of speeds and accelerations fixes overtaking sight distance, the length of acceleration/deceleration lanes and signal timings.
• Braking and braking efficiency: deceleration depends on tyre-pavement friction (f about 0.35-0.40); it fixes stopping sight distance.
• Resistances: air, rolling, grade and curve resistance decide fuel use and the speed on gradients.
• Turning and stability: centrifugal force limits speed on curves, so superelevation and an adequate radius are provided; vehicles with a high centre of gravity may overturn.
• Headlight range and visibility: fix night sight distance and vertical-curve length.
• Vehicle mix: the share of heavy vehicles sets the PCU factors (IRC:64-1990 for rural roads: car 1.0, bus/truck 3.0, motor cycle 0.5, bicycle 0.5, cycle rickshaw 2.0, horse-drawn vehicle 4.0, bullock cart 8.0).