At-grade requirements; grade separated intersection

Basic requirements of an intersection at grade

1. Safety first: reduce the number and severity of conflict points; give drivers clear sight of each other (sight triangle, IRC:66 and IRC:SP:41).
2. Adequate capacity: the junction must carry the design hour volume of all movements without long delays.
3. Simple, clear layout: drivers should understand the movements at once; avoid complex or multiple-road junctions; avoid skew (angle between roads should be about 90 degrees, not less than 60 degrees).
4. Minimum conflict area: keep the paved area small and channelise movements with islands, so vehicles cannot wander.
5. Good visibility: approach sight distance and sight triangles should be free of obstructions.
6. Separation of conflicts by space (islands, channelisation), by time (signals) or by level (grade separation).
7. Suitable geometry: correct approach gradients (flat, not more than about 4 percent), corner radii for the design vehicle, adequate turning lanes and pedestrian facilities.
8. Proper traffic control devices: signs, markings, signals or rotary where needed.
9. Pedestrians and cyclists need crossings, refuge and signal time.
10. Economy: the cost of land, construction and operation should match the benefit.
11. Drainage and lighting for safe all-weather and night use.

Grade-separated intersection

Types

• Overpass / underpass (simple flyover or underpass) with no connection between the roads; used for crossings of major roads, railways and pedestrians.
• Trumpet interchange: for three-leg (T) junctions; one loop ramp.
• Diamond interchange: four-leg junction; four ramps, two at-grade junctions on the minor road; compact and economical.
• Cloverleaf interchange: four loops for left (right in left-hand traffic) turns; all turns are free-flowing, large land need and weaving problems.
• Partial cloverleaf, directional (stack) and rotary interchange: used for high volumes; stack has fully directional ramps and is the most expensive.

   Overpass            Diamond
   ---------- main      \   |   /
   ====//==== road       \  |  /
     /  \ minor        ---+----+---
   cross road           /  |  \

Advantages

• Removes crossing conflicts, so fewer and less severe crashes.
• Greater capacity and higher speeds; free-flow without stopping.
• Reduces delay, fuel use, noise and vehicle emission over time.
• Gives safe pedestrian crossing if an underpass/overbridge is included.

Disadvantages / limitations

• High cost of land and construction, and often needs a long construction period.
• Large land requirement and displacement of buildings.
• Needs space for ramps and may harm the appearance of the city.
• Long detour for some turning vehicles; weaving on short sections; heavy maintenance.
• Not justified at low volumes (usually considered when volume exceeds roughly 10,000-15,000 PCU per hour at the junction, IRC:92).