Interchange Forms

An interchange is a system of interconnecting roadways (ramps) in conjunction with one or more grade separations (bridges) that provides for the movement of traffic between two or more roadways on different levels. Interchanges are used to eliminate at-grade conflicts, maximize capacity, and improve safety on high-speed roadways.

Common Interchange Types

Interchanges are classified into several standard configurations based on their layout and operational characteristics.

1. Diamond Interchange

The diamond interchange is the simplest and most common type, typically used where a major freeway intersects a minor arterial or collector road.

• Layout: Consists of four one-way ramps. Right turns are relatively free-flowing, while left turns terminate at two separate intersections on the minor road.
• Variations:
  • Compressed Diamond: Ramps are pulled closer to the freeway to reduce the right-of-way footprint.
  • Tight Diamond Interchange (TDI): The two ramp terminals on the minor road are spaced very closely (typically $200$ to $400\text{ ft}$). This requires highly coordinated traffic signals to prevent queues from blocking the adjacent intersections.

2. Single-Point Urban Interchange (SPUI)

A SPUI is an efficient variation of the diamond interchange designed for space-constrained urban areas.

• Layout: All left-turn and through movements on the minor road meet at a single, centralized traffic signal located directly above or below the freeway.
• Key Advantage: Left-turning vehicles from both directions can make their turns simultaneously without crossing paths, providing very high left-turn capacity.
• Disadvantage: Requires a very large, expensive bridge structure to span the wide intersection area, and has a large clearance area for turning paths.

3. Diverging Diamond Interchange (DDI)

Also known as a Double Crossover Diamond (DCD), the DDI is a modern design that improves safety and capacity at congested diamond locations.

• Layout: The two directions of traffic on the minor road cross over to the opposite side of the road between the ramp terminals.
• Key Advantage: Left turns onto the freeway ramps are made directly from the left side of the road, eliminating the need to turn across oncoming traffic. This eliminates the left-turn signal phase and reduces conflict points.
• Safety: Reduces conflict points from 26 (in a conventional diamond) to 14. It is highly effective at reducing angled and left-turn crashes.

4. Cloverleaf Interchange

The cloverleaf interchange is used at the intersection of two major freeways where continuous, free-flowing movement is required for all turns.

• Layout: Uses four outer ramps for right turns and four loop ramps for left turns.
• Key Advantage: Free-flowing traffic in all directions; no traffic signals are required.
• Key Disadvantage (Weaving): The exit loop ramp is located immediately after the entrance loop ramp, creating a short weaving section where entering and exiting vehicles must cross paths. This limits capacity and increases rear-end and sideswipe crashes. It also requires a massive amount of right-of-way.
• Partial Cloverleaf (Parclo): Eliminates some loop ramps to remove weaving sections, terminating those ramps at signals on the minor road.

5. Trumpet Interchange

Used at three-leg intersections (T-intersections) where one freeway terminates at another.

• Layout: Uses a loop ramp for one left-turn movement and a semi-directional flyover ramp for the other.

Comparative Matrix

Choosing an interchange configuration requires balancing traffic demand, constructability, safety, and cost.

Worked Case Study

A state department of transportation is upgrading a congested conventional diamond interchange.

• Problem: Peak-hour left turns from the minor arterial onto the freeway entrance ramps are causing long queues that spill back through the intersections, blocking through traffic.
• Constraints: The area is highly developed; acquiring additional right-of-way is cost-prohibitive. The existing bridge deck across the freeway is $80\text{ ft}$ wide and in good structural condition.

Compare three alternatives—Full Cloverleaf, SPUI, and DDI—and determine the best selection based on cost, capacity, and right-of-way.

Alternative Analysis

1. Full Cloverleaf:
   • Feasibility: Rejected. A full cloverleaf requires massive right-of-way for the loop ramps, which violates the primary constraint. It would also introduce undesirable weaving sections on the freeway.
2. Single-Point Urban Interchange (SPUI):
   • Feasibility: High capacity, but requires replacing the existing bridge with a much wider, single-span bridge to accommodate the turning radii of the centralized intersection. This would lead to very high construction costs and long detour periods.
3. Diverging Diamond Interchange (DDI):
   • Feasibility: A DDI can typically fit within the footprint of an existing conventional diamond. The existing $80\text{ ft}$ wide bridge deck can be re-striped to accommodate the crossover lanes. The DDI eliminates left-turn conflicts, resolving the queue spillback issue, and has a much lower construction cost because it does not require bridge replacement.

Conclusion

The Diverging Diamond Interchange (DDI) is the optimal choice. It resolves the left-turn queue capacity issue, fits within the existing right-of-way, and avoids the high cost of bridge replacement associated with a SPUI.

References

• A Policy on Geometric Design of Highways and Streets (AASHTO Green Book), 7th Edition, 2018, Section 10.1 & 10.2.
• FHWA Diverging Diamond Interchange Informational Guide, 2014.