Single-point urban interchange
A single-point urban interchange (SPUI), also known as a single-point diamond interchange (SPDI), is a grade-separated highway interchange design in which all ramps from a freeway or highway converge at a single signalized intersection on the crossing arterial road, allowing for consolidated traffic control under or over the mainline roadway.[1] This configuration typically features a bridge spanning 160 to 280 feet to accommodate the intersection, enabling opposing left-turn movements to proceed simultaneously without crossing paths, which reduces the number of conflict points from 14 in a conventional diamond interchange to just 6.[1] By utilizing three-phase signal timing instead of four or more, the SPUI minimizes delays, increases green time for through movements, and supports higher vehicle throughput in urban or suburban environments with limited right-of-way.[2] Developed in 1970 as a variant of the compressed diamond interchange to address capacity constraints and operational inefficiencies in dense traffic areas, the SPUI was first constructed in 1974 at the intersection of State Road 60 and U.S. Highway 19 in Clearwater, Florida.[1][3] Since its introduction, the design has been widely adopted across the United States, with over 20 states implementing SPUIs at high-volume interchanges to enhance safety by limiting vehicle path crossings to one location and reducing crash risks associated with offset left turns.[4] Notable advantages include a smaller environmental footprint due to minimized land acquisition needs, easier coordination with adjacent signals, and improved pedestrian accommodations through centralized crossings, though it requires longer clearance intervals for signals owing to the intersection's width.[1][2] Examples of successful implementations include the interchange of U.S. 60 and KY 4 in Fayette County, Kentucky, and multiple locations along New York Route 390, where SPUIs have demonstrated capacity increases without expanding the overall footprint.[2][4]Overview
Definition and Principles
A single-point urban interchange (SPUI) is a type of grade-separated interchange that consolidates the ramp terminals of a conventional diamond interchange into a single signalized at-grade intersection, typically located beneath or above the crossing of a freeway and an arterial road.[5] In this design, all vehicular movements—including left turns, right turns, and through traffic—are accommodated at one central point, with the freeway passing over or under the arterial via a bridge structure that spans the intersection area.[6] This configuration is particularly suited for freeway-to-arterial connections in dense urban environments, where space limitations and high traffic volumes create challenges for traditional grade-separated junctions, such as congestion and inefficient signal phasing at multiple intersection points.[7] The basic principles of a SPUI revolve around centralizing traffic control to streamline operations and enhance safety. By merging the two typically separate signalized intersections of a diamond interchange into one, the design reduces the total number of conflict points from approximately 26 in a conventional diamond to fewer through consolidated phasing, minimizing opportunities for vehicle collisions across crossing paths.[6] A single traffic-actuated signal controller manages all movements, often using a three-phase system (or four phases if frontage roads are present), with protected left-turn phasing and large turning radii (150–300 feet) to accommodate high-volume flows and larger vehicles like trucks.[5] This setup allows opposing left turns to occur simultaneously in the same phase via inverted paths under the overpass, further simplifying progression along the arterial while requiring less right-of-way than full cloverleaf or parclo designs.[8] Conceptually, a SPUI features a freeway overpass (most common) spanning the arterial roadway, with entry and exit ramps looping from the freeway to converge at the central signalized intersection below; alternatively, an underpass variant positions the arterial above.[5] The bridge typically requires a span of 200–220 feet to cover the intersection without intermediate supports that could obstruct traffic lanes.[6] First conceptualized in the mid-1960s, the SPUI was developed specifically to address space constraints in urban areas, where expansive designs like cloverleaves are often infeasible due to limited right-of-way and high land costs.[5]Comparison to Other Interchange Types
The single-point urban interchange (SPUI) differs from the conventional diamond interchange primarily in its consolidation of all turning movements at a single signalized intersection under or over the freeway, which reduces the number of conflict points compared to the diamond's two separate intersections.[1] This design allows for simpler signal phasing—typically three phases instead of four—enabling opposing left turns to proceed simultaneously without crossing paths, thereby increasing capacity by up to 25-30% over diamonds in high-volume scenarios.[9] However, the SPUI requires a wider bridge structure with spans of 160-280 feet to accommodate the looping ramps, demanding more right-of-way in the longitudinal direction than a standard diamond, though its overall footprint remains more compact for urban retrofits.[1][10] In contrast to the cloverleaf interchange, which relies on high-speed loop ramps and weaving sections that can limit left-turn capacities to around 1,200 passenger cars per hour per lane, the SPUI eliminates weaving entirely through its signalized control and at-grade turns, making it more suitable for dense urban environments where land availability is constrained.[11] Cloverleafs, while effective for rural or suburban freeway-to-freeway connections, consume significantly more land—often two to three times that of a SPUI—due to their expansive loops and acceleration/deceleration lanes.[10] The SPUI's centralized intersection also simplifies traffic management, avoiding the high-speed merges that contribute to rear-end collisions in cloverleaf designs.[9] Compared to turbine or roundabout interchanges, the SPUI employs traffic signals for precise control rather than yield-based or spiral geometries, which can handle higher volumes in turbine designs (up to 2,000 vehicles per hour per direction) but require multi-level structures and more extensive right-of-way.[11] Roundabout interchanges, such as dumbbell variants, promote continuous flow without signals but demand larger circular footprints and can pose challenges for heavy vehicles or pedestrians, areas where the SPUI's signalized approach offers better accommodation for urban pedestrians despite longer clearance intervals.[1] Turbines excel in non-urban settings with balanced flows, whereas SPUIs are optimized for high-volume urban corridors with limited space, providing a cost-effective alternative to full grade-separated options like stack interchanges.[1]| Interchange Type | Conflict Points | Land Use | Suitability |
|---|---|---|---|
| SPUI | Fewer (centralized at one intersection) | Compact footprint; wider bridge span | High-volume urban areas with space constraints[1][10] |
| Diamond | More (two intersections) | Moderate; shorter spans | Urban/suburban with moderate volumes[1][9] |
| Cloverleaf | High (weaving sections) | Large (extensive loops) | Rural/suburban freeway connections[11][10] |
| Turbine/Roundabout | Moderate (yield-based or spirals) | Large (multi-level or circular) | High-volume non-urban or balanced flows[11][1] |