Other San
Antonio Area Roads
PROJECT
INFO: Harry Wurzbach at Austin Highway |
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| This
page last updated May 1, 2020 |
 Location
Harry Wurzbach at
Austin Highway
Status
Under construction (5% complete)
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 Harry Wurzbach
directly
to
Austin Highway at the existing overpass location. The ramps will be
conventional at-grade entrance and exit ramps, not
flyovers.
The
new
interchange will include 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 halfway between. 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.
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Right-turns
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 will not be able to exit
Harry
Wurzbach and
then
re-enter it by going straight through to the entrance ramp. At this
location, since these will be 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
will be the first SPUI in San Antonio and appears to be 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
(TYPICAL)
TRAFFIC
FLOW FOR AUSTIN HWY.
SINGLE POINT URBAN INTERCHANGE (FUTURE)
TRAFFIC FLOW FOR AUSTIN HWY.
CONVENTIONAL INTERSECTION
(TYPICAL)
TRAFFIC
FLOW FOR HARRY WURZBACH RAMPS TO AUSTIN HWY.
SINGLE POINT URBAN INTERCHANGE (FUTURE)
TRAFFIC
FLOW FOR HARRY WURZBACH RAMPS TO AUSTIN HWY.
To
accommodate the SPUI, the existing overpass will be expanded. The extent
of that expansion varies by quadrant.
In
addition to the new interchange, 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. Motorists on Harry Wurzbach wanting to get to businesses
located along that access road, such as the bowling alley, will instead
exit onto Austin Highway, then turn onto the access road from there.)
The dedicated right-turn ramp from the access road to southbound Harry
Wurzbach
will also be removed. Instead, drivers will 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 parking lots) 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" or northern driveway for Lowes on Harry Wurzbach will
become an
exit-only drive. This is because it is located where the southbound
entrance ramp
from Austin Highway will be; traffic coming from there will turn south
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 eastbound Eisenhauer Road due to the proximity of the
entrance ramp
there. Motorists on northbound Harry Wurzbach wanting to get to
Eisenhauer Road will 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 are included throughout the project.
- Previous
proposals
to close Thrush View Lane at Eisenhauer and to construct a new access point to
Harry Wurzbach have been eliminated.
How
this
project will help
The new interchange will allow traffic to more easily and
intuitively get
from
Harry Wurzbach to Austin Highway and vice-versa, reducing congestion at
the Eisenhauer intersections and eliminating cut through traffic in
adjacent parking lots.
The
SPUI design 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 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 being
required instead of the four in a conventional intersection, thus resulting in less waiting on all approaches.
Timeline
Construction began in March 2020 and is expected to be
complete in early 2022.
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.
- Are the new left turns flyovers?
No,
the new turning lanes will be 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, this intersection will 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 the 1940s, its
purpose
was to connect Fort Sam Houston to Camp Bullis. Austin Highway was US 81 at that time and
was the main highway to Austin and points north, so it
was quite busy. To avoid conflicts between military convoys on Harry
Wurzbach and 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 has endured.
- 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. As this is a completely new
intersection, the city and TxDOT were able to select the best design
for the location. Since there is no need to facilitate north-south
through
traffic 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. 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
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