Status Completed The SPUI
intersection went into operation on January 17th, 2023.
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
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.
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.)
SINGLE POINT URBAN INTERCHANGE
TRAFFIC FLOW FOR AUSTIN HWY.
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.)
SINGLE POINT URBAN INTERCHANGE
TRAFFIC
FLOW FOR HARRY WURZBACH RAMPS TO AUSTIN HWY.
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.
Timeline Construction began in March 2020 was completed in
early 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. 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.
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
Videos
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.
NOTICE
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
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