Road user characteristics and their importance

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.

Importance in traffic engineering

• They are the basis for design values: perception-reaction time for sight distance, walking speed for crossing and signal timing, eye height and visibility for signs and lighting.
• They explain the cause of most crashes, so help to select safety measures (education, enforcement and engineering).
• They help in choosing traffic control devices (size, colour, location) and the timing of signals.
• They allow design for all users, including pedestrians, cyclists, children, the elderly and disabled.