At-grade intersection requirements and night visibility

Basic requirements of an intersection at grade

1. Safety: few and simple conflict points, good sight triangles (IRC:66, IRC:SP:41), and low approach speeds.
2. Capacity: enough lanes and green time or rotary width for the design hour volume.
3. Clear, simple layout: roads meet at about 90 degrees (not less than 60 degrees); avoid more than four arms; flat approach gradient.
4. Channelisation and control: islands, markings, signs, signals or a rotary where needed; provision for pedestrians and cyclists.
5. Drainage, lighting and economy.

Factors affecting night visibility

• Headlight intensity, beam type (dipped or high) and alignment.
• Glare from opposing headlights, and the driver's glare recovery time.
• Street lighting level and uniformity.
• Contrast between object and background; colour and reflectance of the pavement (a wet or dark surface reflects little).
• Speed of the vehicle (less time to see and react) and driver age and eyesight.
• Weather (rain, fog, dust), cleanliness of windscreen, and retro-reflective signs and markings.

Lateral offset of lighting poles

Poles are placed at least 0.6 m (minimum 0.3 m on very restricted kerbed streets) behind the kerb face on urban roads, and about 1.5 m or more from the carriageway edge (outside the shoulder) on high-speed roads without kerbs, as in IS 1944 / IRC practice.

Reasoning: (1) a pole set too close is hit by vehicles whose wheels or mirrors overhang; (2) a larger offset gives the driver recovery space and reduces the severity of impact (frangible or breakaway poles are used where offset is small); (3) it keeps the footpath and sight lines clear; (4) too large an offset reduces the light falling on the carriageway and increases the bracket length, so the offset is a balance between safety and lighting efficiency.