Need of crash studies; highway lighting design

Need of crash studies

• To find the extent, trend, severity and types of crashes and their cost to the nation.
• To identify black spots and the contributing causes (human, vehicle, road) so that money is spent where it saves the most lives.
• To design suitable remedial measures (engineering, education, enforcement) and rank them by cost-benefit.
• To judge the effect of measures through before-and-after studies, and for road safety audit and policy (for example Nepal's Road Safety Action Plan and the DoR black-spot programme).
• For legal, insurance and research purposes.

Steps to design a highway lighting system

1. Classify the road and fix the need: arterial, collector or local; traffic volume, speed, crash record, intersections, bridges, curves and pedestrian crossings. Fix the required average illuminance and uniformity (IS 1944: about 30 lux on major roads, 15 lux on other main roads, 4-8 lux on secondary and residential roads; uniformity min/avg $\ge$ 0.4 on main roads and 0.3 on minor roads).
2. Choose the arrangement: single side, staggered, opposite or central (twin-arm on the median) according to the road width.
3. Select the mounting height $H$ and luminaire type: cut-off or semi-cut-off to limit glare; $H$ about 8-12 m on highways, 6-8 m on local roads.
4. Select the lamp: LED (now preferred), high-pressure sodium or metal halide; get its lumen output $F$.
5. Find the utilisation factor $U_f$ and maintenance factor $M$ (0.7-0.8).
6. Compute the spacing by the lumen method:

$$S = \frac{F \, U_f \, M}{E\, W}$$

7. Check $S/H$ (not more than about 3 for cut-off and 3.5 for semi-cut-off luminaires), the uniformity ratio and glare; adjust the lamp or height if needed.
8. Locate the poles: lateral offset at least 0.6 m behind the kerb (1.5 m or more on high-speed roads), use breakaway or frangible poles, and place additional lights at junctions and curves.
9. Electrical design and economics: cable size, voltage drop, switching (timer or photocell), power and maintenance cost.