What are Grade Separated Intersections?
Grade separated intersections eliminate crossing conflicts between traffic streams by separating them in the vertical dimension — one road passes over or under another without any at-grade crossing. This is achieved through overpasses (where the highway passes over an intersecting street) or underpasses (where it passes below).
An interchange is a grade-separated intersection with connecting ramps that allow vehicles to transfer between roads. Interchanges are mandatory at all crossings of access-controlled highways (national expressways) where maintaining uninterrupted high-speed flow is essential.
Four Types of Interchanges
1. Trumpet Interchange
A 3-leg interchange (T or Y intersections) where two right-turning movements are provided — one via a loop ramp and another via a semi-direct connection. Diagonal ramps serve left-turning movements. For heavy traffic volumes, ramps should preferably use direct connections. Suitable for highway ends or T-junctions.
2. Diamond Interchange
The simplest and most economical 4-leg interchange for a major-minor road crossing. Ramps provide one-way movement and are elongated along the major highway. Ramp terminals on the minor road are at-grade intersections (may need signals). Diamond interchange requires minimum land and involves only a small extra travel distance — ideal for most urban and rural cases. Its demerit is limited capacity due to at-grade terminals on the minor road.
3. Cloverleaf Interchange
Used for 4-leg intersections where two major roads of equal importance cross, particularly in rural areas. All crossing conflicts are eliminated and continuous movement is provided to all interchanging traffic. Weaving sections are created between exit and entry points. Cloverleaf requires a large area and involves appreciable extra travel distance for right-turning movements — making it impractical in urban areas where land is expensive.
4. Directional Interchange
The most operationally efficient interchange — ramps follow the natural direction of movement for right-turning traffic. All conflicts are eliminated. However, it is avoided in practice because of its very high construction cost and complicated multilevel structure.
Interchange Comparison Table
| Type | Cost | Conflicts | Best Use | Demerit |
|---|---|---|---|---|
| Trumpet | Medium | All eliminated (3-leg) | T or Y junctions | Complex ramp system |
| Diamond | Least costly | Partial (at-grade on minor) | Urban & rural major-minor | At-grade terminals on minor road |
| Cloverleaf | Very high | All eliminated | Equal major roads, rural | Large space, extra travel distance |
| Directional | Highest | All eliminated | High-volume expressways | Complex multilevel, very expensive |