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Other San Antonio Area Roads
PROJECT INFO: Harry Wurzbach at Austin Highway

This page last updated July 18, 2023


Project locationLocation
Harry Wurzbach at Austin Highway


The SPUI intersection went into operation on January 17th, 2023.

This $17 million project built an interchange connecting Harry Wurzbach to Austin Highway to replace the disjointed access there previously. Exit and entrance ramps connect Harry Wurzbach directly to Austin Highway at the existing overpass location. 

The new interchange includes San Antonio's first Single Point Urban Interchange (SPUI). A SPUI elongates a standard access road intersection so that the two junctions typically formed by the access roads on each side of the overpass are instead compressed into a single intersection located in the middle. To do this, the access road approaches are curved inward so that the left turn movements can pass by each other like they do at a typical four-way intersection. This allows the opposing left turn movements on each axis of the intersection to proceed simultaneously like they do in a typical four-way intersection, instead of individually like they do at a standard access road intersection. This reduces the number of signal phases required. Fewer phases means shorter red times, which means less delay. A picture is worth a thousand words, so see the diagrams below to help better visualize this.

What is a signal phase?

A signal "phase" is the green time assigned to a specified movement or collection of movements in a traffic signal cycle. In other words, when the signal is green for a specific movement (straight through, left turn, etc.), that's a signal phase. When it changes to red and another movement gets a green signal, that's another phase. The complete rotation through of all of the phases is a cycle.

Right-turns at a SPUI still take place at the same location as they do in a conventional intersection. Note that at a SPUI, there is no straight-through traffic possible on the access roads, i.e. you are not able to exit Harry Wurzbach and then re-enter it by going straight through the intersection to the entrance ramp. At this location, since these are entrance and exit ramps instead of true frontage roads, there should be no need for anyone to go straight through anyway.

Overview of new interchange

This is the first SPUI in San Antonio and appears to be the fifth one in Texas. There are three in North Texas in Plano, McKinney, and Frisco, and one in El Paso, and a few have also been proposed in the Houston and Austin areas. 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 have been versus that for a SPUI.

(This is how traffic would have flowed if they had used a
conventional intersection for the new interchange.)

Conventional flow


SPUI flow

(This is how traffic would have flowed if they had used a
conventional intersection for the new interchange.)

Conventional flow


SPUI flow

To accommodate the SPUI, the existing overpass was expanded. The extent of that expansion varies by quadrant.

In addition to the new interchange, this project also made the following other changes:

  • The signal on Eisenhauer just west of Harry Wurzbach at the previous access road was removed and left turns in all directions at that intersection are now 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. Motorists on Harry Wurzbach wanting to get to businesses located along that access road, such as the bowling alley, should instead exit to Austin Highway, then turn onto the access road from there.) The dedicated right-turn slip lane from the access road to southbound Harry Wurzbach has also been removed. Instead, drivers now have to make two right turns at that location to get onto southbound Harry Wurzbach, or instead go up to Austin Highway, turn left, and use the new entrance ramp to southbound Harry Wurzbach.

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

  • The northern driveway for Lowe's on Harry Wurzbach was converted to an exit-only drive. This is because it is located where the southbound entrance ramp from Austin Highway is; traffic coming from either Lowe's driveway on Harry Wurzbach must turn right.

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

  • There is no right turn now allowed from the northbound Harry Wurzbach mainlanes to eastbound Eisenhauer Road due to the proximity of the entrance ramp there. Motorists on northbound Harry Wurzbach wanting to get to Eisenhauer Road should instead exit at Austin Highway, turn right, and proceed to Eisenhauer from there.

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

  • Previous proposals to close Thrush View Lane at Eisenhauer and to construct a new access point to Harry Wurzbach were dropped from the project after public input.

How this project will help
The new interchange allows traffic to more easily and intuitively get from Harry Wurzbach to Austin Highway and vice-versa, reducing congestion at the Eisenhauer intersections and eliminating the rampant cut-through traffic in adjacent parking lots.

The SPUI design allows the new intersection created by this interchange to function more efficiently than a conventional intersection would have. The two separate intersections in a conventional "diamond interchange" 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 time for a total of four signal phases. Because opposing left turns in a SPUI can proceed simultaneously, this results in only three total signal phases (Harry Wurzbach left turns, Austin Hwy. left turns, and Austin Hwy. through) being required instead of the four in a conventional intersection, thus resulting in at least 25% less wait time on all approaches.

Construction began in March 2020 was completed in early 2023.


  • 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. A 2005 study found the same thing, although it did find that SPUIs had fewer collisions with injuries or fatalities. This design is in widespread use in the US and overseas and has a good track record.

  • 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. With safety being equal (no more or less dangerous), the reduced delay still produces a net benefit for this intersection type over a conventional intersection.

  • Are there turnarounds?
    No, this intersection does not have turnarounds, but traffic wanting to turn around can easily do so at the intersection.

  • Why hasn't there been a "real" intersection here before?
    When Harry Wurzbach was first built back in WW II, its purpose was to connect Fort Sam Houston to Camp Bullis. There was no I-35 at that time, so Austin Highway, which was US 81, was the main highway from San Antonio to Austin and points north, so it was quite busy. To avoid conflicts between military convoys on Harry Wurzbach and civilian traffic on Austin Highway, the overpass was built to separate that traffic. Because the area around this intersection was sparsely populated at the time, there was little demand to get from Harry Wurzbach to Austin Highway or vice-versa, so the ad-hoc routes to connect the two roads via Eisenhauer and the short access road to the west was determined to be sufficient, and that configuration endured. (You can read more about Harry Wurzbach Road's history here.)

  • 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 it 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. As this was a completely new intersection, the city and TxDOT were able to evaluate and select the best design for the location. Since there is no need to facilitate north-south through traffic on Harry Wurzbach at this intersection (as it goes under Austin Highway), a SPUI was a perfect fit.

Schematic and renderings
Below is the detailed schematic for this project from the City of San Antonio with my own annotations added to help clarify and explain the various elements. Click on the image below to open the schematic in a new window that you can scroll and zoom. Below that are some renderings of the completed project.


Click above to see the detailed annotated schematic for this project


Looking north along Harry Wurzbach

Looking south along Harry Wurzbach at Eisenhauer
Notice the exit ramp to Austin Hwy. just past the intersection.

Looking north over the new SPUI intersection

Looking southwest over the new SPUI intersection

Turning left from Harry Wurzbach to Austin Hwy.
This is the left turn from southbound Harry Wurzbach to northbound Austin Hwy., but the opposite turn is similar.

Turning left from Austin Hwy. to Harry Wurzbach
This is the left turn from southbound Austin Hwy. to southbound Harry Wurzbach, but the opposite turn is similar.

Nighttime view of intersection


The City of San Antonio released a video about the project with video renderings of the completed project and how to drive through it.

And here is my video driving through it shortly after it opened to traffic.

Other sites of interest

COSA - Harry Wurzbach at Austin Highway
COSA - Innovative Intersection Ends Decades Old Traffic Headache on Harry Wurzbach and Austin Highway (Video)
COSA - Harry Wurzbach & Austin Highway SPUI animation
TXDOT - Single Point Urban Interchange Fact Sheet
FHWA - Alternative Intersections/Interchanges Informational Report
Wikipedia - Single-point urban interchange

This page and all its contents are Copyright 2023 by Brian Purcell

The information provided on this website is provided on an "as-is" basis without warranties of any kind either express or implied.  The author and his agents make no warranties or representations of any kind concerning any information contained in this website.  This website is provided only as general information.  The author expressly disclaims all liability with respect to actions taken or not taken based upon the information contained herein or with respect to any errors or omissions in such information.  All opinions expressed are strictly those of the author.  This site is not affiliated in any way with any official agency.