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

This page last updated January 16, 2023

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Project locationLocation
Harry Wurzbach at Austin Highway

Status
Substantially complete
(est. 98% complete)
The new SPUI intersection went into operation on January 17th, 2023.
Engineers will monitor the intersection and adjust signal timings as necessary.

Description
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 ramps are conventional at-grade entrance and exit ramps, not flyovers. 

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, which 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.


CONVENTIONAL INTERSECTION (TYPICAL)
TRAFFIC FLOW FOR AUSTIN HWY.
(This is how traffic would have flowed if they had used a conventional intersection for the new interchange.)

Conventional flow


SINGLE POINT URBAN INTERCHANGE
TRAFFIC FLOW FOR AUSTIN HWY.

SPUI flow


CONVENTIONAL INTERSECTION (TYPICAL)
TRAFFIC FLOW FOR HARRY WURZBACH RAMPS TO AUSTIN HWY.
(This is how traffic would have flowed if they had used a conventional intersection for the new interchange.)

Conventional flow


SINGLE POINT URBAN INTERCHANGE
TRAFFIC FLOW FOR HARRY WURZBACH RAMPS TO AUSTIN HWY.

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 current access road has been removed and left turns in all directions at that intersection are 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 ramp 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 go up to Austin Highway 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) has been 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 current northern driveway for Lowe's on Harry Wurzbach has become 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 has been 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 is no right turn 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.


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.

Timeline
Construction began in March 2020 and is substantially complete.
The new SPUI intersection went into operation on January 17th, 2023.

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. This design is in widespread use in other states 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 the new left turns flyovers?
    No, the new turning lanes are at the same level as Austin Highway. Harry Wurzbach continues to pass under Austin Highway as it always has.

  • 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, and below that is a video from the City of San Antonio.


DETAILED PROJECT SCHEMATIC

Click above to see the detailed annotated schematic for this project
 

RENDERINGS

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

VIDEO

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



Other sites of interest

COSA - Harry Wurzbach at Austin Highway
http://www.sanantonio.gov/TCI/Projects/Harry-Wurzbach-at-Austin-Highway#199141583-about-the-project
COSA - Innovative Intersection Ends Decades Old Traffic Headache on Harry Wurzbach and Austin Highway (Video)
https://www.youtube.com/watch?v=gePvybQwurM
COSA - 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


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

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