SPUI comes to Texas

us75spui

Rendering of SPUI on US 75 at Parker Rd. in Plano

Work is underway on the North Central Expressway at Parker Rd. in the Dallas suburb of Plano on what, to my knowledge, will be Texas’ first Single Point Urban Interchange, or SPUI¬†(pronounced “spoo-ee”).¬† (If anyone knows of an existing one in Texas, please post it!)¬† Popular in several other states,¬†a SPUI is a type of intersection between a surface street and a freeway’s entrance and exit ramps.

A SPUI’s design allows each set of opposing left turn movements to be conducted simultaneously, thus eliminating extra signal phases typically required to accomodate¬†those movements¬†separately.¬† Because of the reduced number of signal phases, more traffic can be accommodated (much like the Super Street concept being implemented on US 281 in¬†far north¬†San Antonio.)¬† However,¬†to accomplish this, a¬†SPUI also eliminates the straight-through movement¬†between the freeway ramps– in other words, if you’re coming off the freeway, you have to either turn left or right onto the cross street; you can’t go straight through the intersection.¬† It’s because of this gotcha that SPUIs have yet to catch-on in Texas.¬† Most of Texas’ freeways have frontage roads, and traffic inherently needs to be able to continue through on those frontage roads to either reach businesses further down or to get onto the freeway from¬†a location further upstream.¬† However, the SPUI in Plano will accommodate frontage roads by having them pass under the cross street.¬† Several locations in Texas, including here in San Antonio, already have flyovers or underpasses for frontage road through traffic, so using them to accommodate a SPUI is¬†a logical progression.¬†

Schematic of a typical SPUI

Schematic of a typical SPUI

The “single point intersection”¬†of a SPUI is where all four left turns cross.¬† Typically at a freeway/cross street intersection, you have two separate intersections (one on each side of the freeway) where left turns are made.¬† With a SPUI, those are collapsed into one single intersection midway between where the two traditional intersections would be.¬† Drivers turning left pass to each other’s right, just like at a regular intersection, although the angle of the left turns is¬†notably “softer”.

The SPUI at Parker Rd. will replace an outdated four-quadrant jug-handle interchange.  In this case, the old jug-handle ramps will remain and will be modified to serve as the right-turn ramps.  In a typical SPUI, the right turns are accommodated much closer to the central intersection.  The Parker Rd. SPUI will also include another feature not seen in typical SPUIs but quite prominent and necessary in Texas: turnarounds.

Will SPUIs be in San Antonio’s future?¬† I guess that will depend on how well it does in the Dallas area, but I can already¬†envision a few locations where it would really help with congestion.

 

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