project will convert the Blanco/1604 intersection to a "Diverging
Diamond Interchange" (DDI).
DDI shifts traffic on the overpass to the opposite side of the road
using two signalized crossover intersections.
allows through traffic and left turns from one direction of Blanco and
left turns from one direction of the access road to move
simultaneously, which is not possible in a conventional intersection.
overlap essentially doubles the traffic that can move through the
intersection at one time.
road through traffic is blocked by a DDI, so a bypass will be built to
accomodate that traffic, which will completely remove that traffic from
it seems confusing at first, it's actually pretty easy to navigate.
of past DDIs show improved safety as well as reduced congestion.
have won widespread praise for their innovativeness.
Description This project will convert the access road
intersections at Loop 1604 and Blanco Rd. (FM 2696) from conventional
intersections to a
"Diverging Diamond Interchange" (DDI).
shift traffic in both
directions on Blanco to the opposite side of
the roadway just before crossing Loop 1604. Traffic will then travel
over Loop 1604 to the left of opposing traffic (i.e. on the "wrong"
side.) Traffic turning left onto Loop 1604 will make a free left turn
(i.e. no opposing traffic) while through traffic on Blanco will then
cross back over to the right
of the roadway. The two crossover
locations will be controlled by a traffic signal. (See
the diagrams below for a depiction of these changes.) Shifting
traffic over the left side allows for two left turn movements that
would normally be conflicting to occur simultaneously, thus allowing
the intersection to move more traffic at one time. (More on that below.)
DDI inherently blocks straight-through traffic on the access roads. To
accommodate this through traffic, an access road bypass (known as a
road") will be built below Blanco next to but
separate from the Loop 1604 mainlanes. Through traffic on the access
road will descend to this road before reaching Blanco, pass under
Blanco, then ascend back to the access road. This
bypass roadway will also serve Loop 1604 traffic headed to and
from Huebner and Stone Oak Parkway. This
will take all that through
traffic on the access roads out of the
Blanco Road intersections. All
of the entrance and exit ramps between Huebner and Stone Oak
will be adjusted to accommodate the new collector/distributor
addition to the above, the project also will
add the following:
additional through lane on Blanco (for a total of three)
Madera Parkway and Country Club Lane.
additional left turn lane (for a total of three) on both
roads at Blanco.
second right-turn lane from the westbound access road to
will be the first DDI in San Antonio and the seventh in
others are in El Paso, The Colony, Round Rock (next to
IKEA), College Station, and two in Austin. The design
has been used successfully in about 100 other locations
the US as well. Two other DDIs are planned for San Antonio: one at US
281 and Jones-Maltsberger and another at I-37
SE Military Dr.
FLOW FOR BLANCO RD.
DIVERGING DIAMOND INTERSECTION (FUTURE)
TRAFFIC FLOW FOR BLANCO RD.
CONVENTIONAL INTERSECTION (EXISTING)
FLOW FOR ACCESS ROADS
DIVERGING DIAMOND INTERSECTION (FUTURE)
FLOW FOR ACCESS ROADS
project will help The DDI will help reduce current
and expected future congestion at this intersection. Current and
projected traffic volumes are sufficient to cause level of service "F"
(congested) conditions during peak periods now and into the future. (You can
learn more about levels of service here.)
flyovers are not considered to be a good option for this
this intersection is considered to be ideal for an
"outside-the-box" (no pun intended) intersection design. Computer
modeling shows a dramatic improvement in this intersection's operation
with this project. For example, it is projected that it would take
through traffic on Blanco (either direction) 16 minutes to get through
this intersection during the afternoon rush hour in 2037 with no
improvements; with the DDI, it is projected to only take about four
minutes. The left turn from eastbound Loop 1604 to northbound Blanco is
projected to take 29 minutes during the afternoon rush hour in 2037
with no improvements, but also just four minutes with the DDI-- an 86%
Projected 2037 afternoon peak
period travel times
Blanco through traffic (NB and SB)
EB Loop 1604 left turn to NB Blanco
How a DDI
improves traffic flow:
A DDI increases the
throughput of an intersection by eliminating the conflicts caused by
left turns by shifting all traffic on the cross
street (in this case, Blanco) to the left
side of the road through
the interchange so that left turns from that street to the access road
no longer have to cross oncoming
traffic. This also creates space on the opposite side of the road that
allows the left turns from the adjacent access road to also turn at the
same time. This
reduces the number of signal phases from
the four required in a conventional intersection to just two, thus
allowing more traffic to move through the
intersection in the same amount of time.
What is a signal phase?
"phase" is the green time assigned to a specified movement or
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.
traffic turning left from the access road will encounter a red
signal at the crossover point on Blanco where they will wait for
through traffic on Blanco to complete their crossover. However, because
both movements started at about the same time, the Blanco through
nearing the completion of their green by the time the left-turners
that location, so
the wait will be brief and more than offset by the overall reduction
in delay that this intersection provides.
diagrams below should help to illustrate the description
Only one direction of traffic can go through the
intersection at a time.
This requires four signal phases to move traffic from all four
turns not shown.
One direction of Blanco and one access road left turn can proceed at
the same time. This requires only two signal
phases to move traffic from all four approaches. Right
turns from access roads are shown as they will be signalized. Right turns from Blanco will have a yield sign and are not shown.
each phase in the conventional intersection is 30 seconds, that's
a cycle time of two minutes for all four phases. If each phase in the
DDI is also 30
seconds, each cycle is just one minute, so two signal cycles
can be completed in the same two minute
period. In other words, each approach would get twice the amount of
green time every two minutes than they would in a conventional
intersection, meaning the intersection can move about twice as much
traffic. (See example timings illustration
below. Signal timings are for
illustrative purposes only and do not represent actual timings.
Timings are typically adjusted in response to traffic demand.)
that the description above is simplified to help explain the
concept; the actual phasing and timing of signals is a bit more complex
and may vary somewhat
from what is described above.
Simplified timelines showing example signal phase timings for each
movement in the
Loop 1604/Blanco intersection. Note that these timings are simplified
purposes only; they do not
represent actual timings, and the actual phasing is a bit more complex.
Be sure to read the explanation in the
Conventional intersection One direction can go at a time. If each direction
gets 30 seconds of green time, that's a total cycle time of 120 seconds.
intersection The DDI allows one direction of Blanco and one
access road left turn to move simultaneously, so it only needs two
phases to move all
directions. If each phase remains 30 seconds long, two complete cycles
can be completed in the same 120 seconds required for one cycle in the
Here is an animation that puts it all together. Note that this animation is
greatly simplified and represents the "typical" DDI trafficflow. The
phasing at Blanco and Loop 1604 could vary from this.
of typical DDI traffic flow Right turns not shown.
markings, and signals: To
properly control traffic and minimize potential
signals will be located at the two crossover locations as well as the
turns from the access roads. The right turns from the access
to Blanco will also be controlled by a signal.
lane markings and signage (including large overhead signs on Blanco)
traffic into the correct lanes.
crossover intersections will be
angled sufficiently to make the crossover transition much more
intuitive; engineers have learned the optimal
angles for this from previous projects.
Additionally, barriers will be placed between the
traffic lanes in the "wrong-side" area to obscure the oncoming traffic
from view and thus reduce
possible confusion or panic that might be caused by seeing
oncoming traffic to your right.
probably looks and sounds confusing, driving
through a DDI is actually far easier than it may seem from the
diagrams and descriptions. See the video
at the bottom of this page.
study of the DDI built in Round Rock in 2016 showed that despite a 50%
increase in afternoon peak period traffic volumes after the DDI was
travel times improved 44-58% over the previous conventional
is also improved
at DDI intersections. A DDI
reduces the number of potential crash points
from 26 to 14 with the worst type (side-angle collisions)
reduced from 10 to just two. Additionally, a DDI physically prevents
turning the wrong way onto the access roads, thus helping to prevent
head-on collisions. A
study at DDI
intersections in Colorado showed a 36% reduction in crashes, and a 60%
reduction was reported at a DDI in
Springfield, Missouri where 97% of drivers surveyed reported
the DDI was safer
than the previous conventional intersections.
main negative of this intersection is that straight-through traffic on
access roads will not be possible. While there will be
road for through traffic below the intersection, it will not be
accessible for traffic with
origins or destinations in close proximity to the intersection (e.g.
Starbucks, Jim's, Luby's, etc.) This
means many drivers headed to or from those locations via the access
roads may have to use
an alternate and possibly circuitous route. The impact of this
should be fairly
limited and is a
small trade-off for the significant overall improvements this change
possible drawback is that the DDI may be so efficient, it will cause
increased congestion at other downstream intersections on Blanco as the
additional traffic that gets through this intersection exceeds the
capacity at those intersections.
as with anything new and unconventional, there will be some skepticism
and initial confusion while drivers learn the new traffic patterns and
adapt. And there are always some naysayers who will perpetually
anything different, innovative, or outside their comfort zone or
understanding. It's worth noting that there
was much consternation on social media before the Displaced Left
Turn intersection opened at Loop 1604 and Bandera in 2019, but
acclaim after it opened both for the congestion relief it brought as
well as for it not being nearly as confusing as many had previously
Timeline This project was rolled into the larger Loop
1604 North expansion project.
Construction started in November 2021 and is expected to be
completed in mid 2025.
This looks confusing.
It will cause lots of crashes. While it may look complicated on the schematics, it's
actually easy to navigate on the ground. See the
video at the bottom of this page. Statistics from DDI
implementations show a significant reduction in crashes, and
practical experience from other DDI locations shows drivers quickly
adapt to it.
How does this improve
This intersection design improves traffic because shifting the traffic
to the left side of the road through the intersections removes the
conflicts between left turns and opposing traffic found in a
conventional intersection. As a
result, traffic in two of the four legs of the intersection can move
during a single green phase instead of needing four separate
phases. This reduces the total number of green phases required from
four to two, thus moving about twice as much traffic through the
intersection in the same
amount of time.
The new third lane on Blanco
needs to extend further or it will create a bottleneck.
The scope of this project was limited to this intersection and
immediate vicinity. The additional lanes that will be added on Blanco
are considered to be
"auxiliary" lanes intended to help traffic get
through the intersection and then smoothly merge back into the through
additional capacity is needed on Blanco beyond this point, that is a
much larger project requiring separate study and additional funding.
That said, TxDOT has said it will monitor the situation after
implementation. Keep in mind
that due to funding and other
constraints, road improvements often need to be incremental, much like
unkinking a hose one kink at a time.
looks like there will be no straight-through traffic on the access
roads. This will prevent access to nearby businesses, severely
inconvenience drivers, and cause
extra traffic on Blanco. This will also block westbound through traffic
headed to Huebner.
is correct that traffic on the
access roads will have to turn left or right onto Blanco. Keeping the
through movement on the access roads would cancel-out much of
improvement this design provides. However,
an access road
bypass will be provided below Blanco (next to the Loop 1604
mainlanes) that will allow most of the traffic that would normally go
through the intersections on the access roads to instead pass
stopping.This will have
the major benefit of taking traffic going to and coming from Huebner
out of the Blanco intersections.
The ramps to and from this bypass will be located about 1000
the Blanco intersection, so motorists going to or from businesses
between these points may need to turn onto Blanco and make a
left or U-turn to
get to or depart from those locations. This will most especially affect
the businesses right at the NE and SE corners. The shopping centers at
other corners have sufficient access points located away from the
intersection that will minimize the
affect of this change for them. While this is a bit of an
is no more so than what is required on streets with continuous medians
accessing or leaving
businesses along freeway access roads and is a
trade-off for the significant improvements this change will bring.
Will there still be turnarounds?
Yes, the turnarounds in both directions will remain.
Why not just add the extra
and through lanes?
While this would help move more vehicles through the intersection on
each cycle, the improvements would be short-lived as traffic increases
to fill the new lanes. The
DDI dramatically improves the underlying efficiency of the traffic
signals themselves which provides longer-term relief.
They should just widen the bridge
on the westbound access road approaching Blanco-- that's the bottleneck.
While it may seem like the bridge is a bottleneck because the road
widens to five lanes after crossing it (and bridges sometimes just
inherently seem constricting), the bridge is actually not the
bottleneck. If it were, then traffic would be free-flowing once you
get past the bridge, and that's obviously not the case. Instead,
traffic backs up from the Blanco intersection onto the bridge and
beyond. It's the intersection
that's the bottleneck. Converting the
intersection to a DDI will allow more green time for all approaches,
which will increase the throughput through the intersection, thus
reducing congestion on the approaches, including the bridge. Note that
the same lane
configuration without a bridge is present on the eastbound
road approaching Blanco, and it also suffers similar levels of
Why not build flyovers instead
like the ones at Bandera and Loop 410?
This intersection does not have current or projected traffic volumes or
justify flyovers and the additional right-of-way they would
How is this different than the
new intersection at Bandera and Loop 1604?
The DDI is similar to the Displaced Left Turn (DLT)
intersection completed in 2019 at Bandera
and 1604 in
that they are both considered to be types
of "innovative" or "alternative" intersection
that make signalized intersections function better by removing the
inefficiencies caused by protected left turns. Both intersection
designs do this by moving the left turns over to the other side of the
road and out of the way of oncoming traffic. However, at the Bandera
DLT, it is
just the left turns that are shifted to the other side of the road,
whereas a DDI moves both left-turning and through
traffic over. A DLT
has its crossovers further away from the main intersection and requires
more signals and right-of-way than a DDI. Also, the DLT does not block
through-traffic on the access road like a DDI does.
Finally, a DLT does not also eliminate the conflicting left turns from
access roads to the cross street like a DDI does.
Why not build a DLT here instead
of a DDI so that the through traffic on the access roads could be
was not sufficient right-of-way at this location for a DLT and
acquisition of that right-of-way would have been cost-prohibitive.
Are any other DDIs planned in San
two other DDIs are currently planned: one at SE Military Dr. and I-37
another at Jones-Maltsberger Rd. (South) and US 281. Other alternative
intersections are also being planned for other locations; see
intersections page for more info.
Who came up with this cockamamie
The DDI design has been used in France since the 1970s and made its
debut in the United States in 2009. It is now in use in over 100 locations in the US and internationally and has a proven
track record of improving traffic and safety wherever it has been
is one of several types of "innovative" or "alternative" intersection
designs being implemented across the country at intersections
not warrant more expensive conventional upgrades (e.g.
flyovers.) TxDOT selected this design for this intersection
after studying multiple other options. Computer modeling showed this
design provided significant improvement in operations with substantial
reduction in delays and imprved safety. The DDI concept was hailed by Popular
one of the top 100 innovations in 2009 and has been featured on
Public Radio (twice)
and in Time,
many other publications.
the images below to open the detailed schematics for this
project from TxDOT with my own annotations added to help clarify and
explain the various elements. Each schematic will
open in a new window that you can scroll and zoom. These
are the schematics presented to the public in March 2019, but match the
final schematics for construction.
(Click the images below to see the
Collector/distributor road and ramps
Video I put together a video that discusses the Blanco Rd. DDI project, how it will work, and how it will improve traffic:
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