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PROJECT INFO: Harry Wurzbach at Austin Highway

This page last updated May 12, 2018

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Project locationLocation: Harry Wurzbach at Austin Highway

Status: Construction starting soon

Description
This project will build an interchange connecting Harry Wurzbach to Austin Highway to replace the disjointed access there now. Exit and entrance ramps will connect from Harry Wurzbach directly to Austin Highway at the existing overpass location. The new interchange will include San Antonio's first Single Point Urban Interchange (SPUI).

A SPUI elongates a standard intersection so that the two intersections typically formed by the ramps on each side of the overpass are instead compressed into a single intersection located on the middle of the overpass. This allows the opposing left turn movements on each axis of the intersection to proceed simultaneously like they do in a typical surface street intersection, thus reducing the number of signal phases required (fewer phases = fewer delays.) Right-turns still take place at the same location as they do in a conventional intersection. However, with a SPUI, there is no straight-through traffic on the ramps (i.e. you cannot exit Harry Wurzbach and then re-enter it by going straight through to the entrance ramp.) (See the diagrams below for a visual depiction of a SPUI as compared to how a conventional intersection at this location would operate.)

As mentioned, this will be the first SPUI in San Antonio and, from what I can tell, just the third one in Texas. The other two are in North Texas in Plano and McKinney and a couple have also been proposed in the Houston area. The design, however, has been around since the mid-'70s and is in widespread use in many other states and overseas.

The diagrams below show what the traffic flow for a conventional intersection would be versus that for a SPUI.


CONVENTIONAL INTERSECTION
TRAFFIC FLOW FOR AUSTIN HWY.

Conventional flow


SINGLE POINT URBAN INTERCHANGE (PROPOSED)
TRAFFIC FLOW FOR AUSTIN HWY.

SPUI flow


CONVENTIONAL INTERSECTION
TRAFFIC FLOW FOR HARRY WURZBACH LEFT TURNS TO AUSTIN HWY.

Conventional flow


SINGLE POINT URBAN INTERCHANGE (PROPOSED)
TRAFFIC FLOW FOR HARRY WURZBACH LEFT TURNS TO AUSTIN HWY.

SPUI flow


To accomodate the SPUI, the existing overpass will be expanded. The extent of that expansion varies by quadrant.

In addition to the ramps and SPUI, this project will also make the following other changes:

  • The signal on Eisenhauer just west of Harry Wurzbach at the current access road will be removed and left turns at that intersection will be prohibited and blocked by a concrete island. (With the new ramps to and from Harry Wurzbach, there should be little need for those turns anymore.) The right-turn ramp from the access road to Harry Wurzbach will also be removed; drivers will still be able to make two right turns at that location to get onto southbound Harry Wurzbach.

  • The signal on Harry Wurzbach just south of Austin Highway (providing access to the Lowes and HEB) will be removed. A dedicated left turn lane into the rear driveways of both stores will be provided. Motorists can also use the new ramps to Austin Highway and then access those stores via Austin Highway.

  • The current "front" driveway for Lowes at Harry Wurzbach will become an exit-only drive as it is located where the southbound entrance ramp from Austin Highway will be; traffic coming from there will turn onto that ramp.

  • The driveway to Discount Tire on Austin Highway will be closed as it is too close to the new intersection and there is sufficient and convenient access just around the corner.

  • The driveway on Austin Highway closest to Advance Auto Parts will be closed, also because of its proximity to the new intersection. 

  • The existing signals on Austin Highway and on Harry Wurzbach at Eisenhauer (with the exception of the one discussed in the first point above) will remain.

  • There will be no right turn allowed from the northbound Harry Wurzbach mainlanes to Eisenhauer Road due to the proximity of the entrance ramp there.

  • Sidewalks, bike lanes, hike/bike (aka "shared use") paths, crosswalks, and bus stops are included throughout the project.

  • Previous proposals to close Thrush View Lane at Eisenhauer and to construct a connector to Harry Wurzbach instead have been eliminated.


How this project will help
The new interchange will allow traffic to more easily get from Harry Wurzbach to Austin Highway and vice-versa. The SPUI will allow the new intersection there to function more efficiently than a conventional intersection would. The two separate intersections in a conventional configuration cause left turn movements (for all approaches) to conflict with each other. As a result, each left turn movement requires its own protected green for a total of four signal phases. In a SPUI, the ramp approaches are bent inward so that the left turn movements can cross each other like they would at a typical surface street intersection (i.e. with opposing turning traffic passing on your right side.) This means that they can proceed simultaneously, which results in only three total signal phases being required.

Timeline
Construction is set to begin in early 2019 and take about two years to complete.

FAQ:

  • This looks difficult to navigate. It will cause lots of crashes.
    While it may look complicated on the schematics, it's actually fairly easy to navigate on the ground. A 1996 study of SPUI implementations found no significant difference in accident rates or severity compared to conventional intersections.

  • So if this isn't safer than a conventional intersection, why do it?
    Because the design means fewer signal phases, which means less delay for traffic. Since its also no more dangerous than a conventional intersection, the benefit of the reduced delay easily prevails.

  • Are the new left turns flyovers?
    No, the new turning lanes are at the same level as Austin Highway. Harry Wurzbach will continue to pass under Austin Highway as it does today.

  • Will there be turnarounds?
    No, there will be no turnarounds at this intersection


  • Who came up with this cockamamie design?
    The SPUI design has been around for several decades and is in widespread use in the US and internationally and has a proven track record of improving traffic wherever it has been implemented. It is one of several types of "innovative" or "alternative" intersection designs being implemented across the country at intersections to improve their efficiency. Since this is a completely new intersection, the city and TxDOT were able to select the best design for the location. Since there is no reason for north-south through traffic at this intersection, a SPUI was a perfect fit.

Schematic
Below is the detailed project schematic with annotations as well as some renderings of the completed project. The project schematic is courtesy of the City of San Antonio and is subject to change. Annotations are my own. For an animation of the SPUI, see the links at the bottom of this page.


DETAILED PROJECT SCHEMATIC

Click above to see the detailed annotated schematic for this project
 

RENDERINGS

Looking NE along Austin Highway


Looking SW along Austin Highway


Looking from SE corner


Looking along northbound exit from Harry Wurzbach


Close-up of SPUI intersection


Looking northbound along Harry Wurzbach



Other sites of interest

City of San Antonio - Harry Wurzbach at Austin Highway
http://www.sanantonio.gov/TCI/Projects/Harry-Wurzbach-at-Austin-Highway#199141583-about-the-project
Harry Wurzbach & Austin Highway SPUI animation
https://vimeo.com/225152712
TXDOT - Single Point Urban Interchange Fact Sheet
http://ftp.dot.state.tx.us/pub/txdot/my35/capital/concepts/spui-factsheet.pdf
FHWA - Alternative Intersections/Interchanges Informational Report
https://www.fhwa.dot.gov/publications/research/safety/09060/
Wikipedia - Single-point urban interchange
https://en.wikipedia.org/wiki/Single-point_urban_interchange




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This page and all its contents are Copyright 2018 by Brian Purcell

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