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Harry Wurzbach at Austin Highway SPUI

This page last updated February 20, 2024

SPUI location map
SPUI location map

From the construction of Harry Wurzbach Rd. in 1943 until 2023, there was no direct connection between it and Austin Highway. Instead, Harry Wurzbach simply passed under Austin Highway with no interchange, and traffic wanting to transition between the two roads had to use an ersatz access road to the west or Eisenhauer Rd. to the east, and many people simply (and illegally) cut through the adjacent parking lots.

To finally remedy this disjointed connection, a project was started in 2020 and completed in early 2023 to provide a direct interchange between the two roadways. The new interchange includes San Antonio's first (and, to date, only) Single Point Urban Interchange (SPUI), and what appears to be the fifth one in Texas. The design, however, has been around since the mid-'70s and is in widespread use in many other states and overseas.

What the heck is a SPUI?
A SPUI (pronounced "spooey") is an innovative intersection that improves the efficiency of the intersection by adjusting how left turns are handled. In the case of a SPUI, it 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 per cycle. Fewer phases means shorter red times, which means less delay. In fact, a SPUI allows for a 25% reduction in red time on each approach.

As they say, a picture is worth a thousand words, so see the diagrams below to help better visualize this.

Question mark sign
What is a signal phase?

A signal phase is the green time assigned to a specified movement or collection of simultaneous 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.

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.

SPUI diagram

SPUI overview

Why did they choose a SPUI here?
The SPUI design was chosen for this location instead of a conventional intersection because it is more efficient and because there was no need for straight-through traffic on the ramps from-to Harry Wurzbach. In short, a SPUI was a good fit for this location.



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Traffic flow through a SPUI
The diagrams below show what the traffic flow for a conventional intersection would have been versus that for the 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 intersection diagram

SINGLE POINT URBAN INTERCHANGE
TRAFFIC FLOW FOR AUSTIN HWY.

SPUI diagram

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 intersection diagram

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

SPUI diagram

FAQ



Video

Here is a drive-through video of the intersection shortly after it was opened to traffic:



Other sites of interest

Wikipedia - Single-point urban interchange
https://en.wikipedia.org/wiki/Single-point_urban_interchange
TXDOT - Single Point Urban Interchange Fact Sheet
http://ftp.dot.state.tx.us/pub/txdot/my35/capital/concepts/spui-factsheet.pdf