Location Loop 1604 at Bandera Rd. (access road intersections)
Description This project converted the access road
intersections at Loop 1604 and Bandera Rd. (SH 16) from conventional
intersections to a
Turn" (DLT) intersection, also known as a "Continuous Flow"
intersection. The project also
added an additional left turn lane (for a total of three) on the
southbound access road, dual right-turn lanes on both Loop 1604
access roads, and a two-lane turnaround entrance for westbound to
DLT shifts ("displaces") traffic turning left from
Bandera to Loop 1604 to a new
parallel roadway on the outside of
lanes via a signalized crossover about 100 to 150 yards upstream of
the near-side intersection with Loop 1604. That parallel roadway then
crosses the access road and joins the turnaround to cross under Loop
1604 where traffic makes a
free left turn onto the access road. The
still available for access road traffic; however, entry to
controlled by a
signal to prevent collisions with the unexpected
displaced-left cross-traffic. With the new displaced left
longer allowed to make a left turn from Bandera to Loop 1604 where it
previously did at the
main intersections. (See
the diagrams below for a depiction of these changes.)
is the first DLT in San Antonio, and several more are planned. There
also two DLTs in operation
each in San Marcos and Austin as well as one in Cedar Park. The design
has been used successfully in several other states as well.
TRAFFIC FLOW FOR BANDERA RD.
NEW DISPLACED LEFT TURN INTERSECTION
TRAFFIC FLOW FOR BANDERA RD.
project helps The
DLT reduces congestion and increases safety at this
volumes were sufficient to cause level of service "F"
(congested) conditions at the previous conventional intersection during
periods. However, the cost for traditional grade-separated
solutions (underpasses, flyovers) is high and new innovative intersections
such as this have been shown to provide good congestion relief and
safety at a fraction of the cost,
thereby saving valuable transportation dollars for other needed
this intersection was determined to be an ideal candidate for an
modeling showed a dramatic improvement in intersection performance with
the DLT with as much as a 75% reduction in
delays and an improvement to level of service "B" (near free-flow) or
"C" (stable flow), which is considered quite good for a major urban
intersection. (You can learn more about levels of service here.)
has also be improved. The
previous intersection was rated by one report as the second most
dangerous in the city. While
the DLT intersection looks more confusing, and many people's first
reaction therefore is that it will cause more
accidents, studies of previous DLTs
have found a notable decrease in collisions, especially the more
serious types, and early data from this intersection shows a
dramatic decrease in crashes.
How a DLT
improves traffic flow: A
DLT is a relatively low-cost improvement (this project cost
million versus over $20 million for flyovers) that increases the
throughput of an intersection by allowing
traffic headed in opposing directions (including left turns) to
all move simultaneously. (At this
location, the traffic on Bandera was reconfigured for this.) This
accomplished by diverting
("displacing") the left-turning traffic to the opposite side
the roadway several hundred feet upstream from the near-side
intersection. This displacement moves most or all of the left-turning
traffic across and out of the way before opposing through traffic
the crossover location, thus minimizing or even eliminating the time
through-traffic has to wait for the opposing left turning
these two opposing movements can travel through the
intersection at the same time, they can share the same
instead of each direction needing its own dedicated green. This is
the "secret sauce" of the DLT; more on that below.
crossover angle for the left turns in a DLT is more gentle than for a
left turn, a bit more left-turning traffic is typically able to get
through in the same
amount of time as compared to a conventional intersection.
diagrams should help to illustrate the descriptions above. Note that
for simplicity, the DLT diagrams below only show the sequence for one
half of the flow on Bandera and are based on a "typical" DLT flow.
INTERSECTION Only one
direction of traffic can
go through the intersection at a time
DISPLACED LEFT TURN
directions of traffic on Bandera can go through the intersection at the
same time (Note
for simplicity, the diagrams below only show the sequence for one
half of the flow on Bandera.)
"continuous flow" of traffic in both directions means
that only one
green signal phase
is needed for both
on Bandera instead
of the two phases required at the conventional intersection. The time
having only one phase can then be distributed to extend the green time
for everyone, thus moving more traffic through the intersection
on each cycle without having to increase the overall cycle length. For
example, if each of the conventional intersection phases for Bandera
was 45 seconds long and the
DLT allowed one of them to be eliminated, that could allow 15
seconds of green time to be added to
the two access road phases as well as the remaining Bandera phase. This
means that, in this example, roughly 33% more traffic can get through
the intersection per cycle. Or, the
extra time could be allocated in unequal amounts as dictated by traffic
volumes. Alternatively, the time
from the eliminated phase could be dropped from the cycle
altogether, which means the signals will cycle more often in a given
time period compared to a conventional intersection, again allowing the
intersection to move more traffic in the same period 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.
important note: Observed
signal phasing and timings at the Bandera/Loop 1604
intersection vary from what is described above, but
the underlying concepts
still the same (See example timings illustration
below. Signal timings are simplified and
illustrative purposes only and do not represent actual timings.
Timings are typically adjusted in response to traffic demand.)
Simplified timeline showing example signal phase timings for each
movement in the
Loop 1604/Bandera 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 45 seconds of green time, that's a total cycle time of 180 seconds.
DLT allows the opposing movements on Bandera Rd. to move simultaneously
(orange block), so it only needs three phases now to move all
directions. The 45 seconds needed for one of the Bandera
above can now be redistributed, meaning each phase can now have 60
seconds of green time and still have a total cycle time of 180 seconds.
example timings above show separate phases for each access
road. However, the DLT configuration allows the access road phases to
overlapped, i.e. both access roads could have a green simultaneously.
In this scenario, traffic turning left from the access roads would be
between the access roads
for a short period of
time. It would
then released at the same time as through traffic on Bandera.
This generates additional efficiency. However, turning volumes at this
location are generally too high for that configuration to work during
most of the day (i.e. there isn't enough room under the overpasses to
that much traffic), so each access
road instead has a
separate phase with a partial overlap, i.e. the tail end of traffic
making the turn in one direction (typically the northbound access road
to northbound Bandera) gets stopped under the overpass. They
will get a green and be gone well before the access
turned from gets a green again.
Here is an animation that puts it all together. Note that this animation is
greatly simplified and represents the "typical" DLT flow. There
is some additional overlapping
of the phases as discussed above. The
phasing at Bandera and
Loop 1604 differs somewhat from what is shown below and is discussed in
more detail below.
of typical DLT flow and signal phases
Customized Bandera/1604 phasing: The
signal sequence, which is what is described above, releases the left
turns on both sides simultaneously (or
nearly so), then releases the through traffic in both directions a
short time later. However, the DLT design inherently provides some
that the signal phases can be re-arranged to allow for
combinations of normally-conflicting movements to go at the same time.
While atypical, this still is advantageous as it customizes the
optimization of the signals based on the intersection's traffic
patterns and allows engineers to wring-out the most
After simulating traffic flows, engineers opted to customize the signal
phasing at this intersection in order to better
synchronize the signals here with the
ones at Quincy Lee/Stonecroft (QLS). This
changes the sequence so that the two Bandera displaced left
turns don't run
concurrently, but instead run independently following the corresponding
near-side access road phase. Rearranging
the signal phases this way allows traffic leaving the DLT headed south
on Bandera to encounter a green signal at QLS and also permits the
synchronization of the green signals for northbound Bandera
at QLS and 1604. Using the typical DLT phasing here would have
resulted in one of those two synchronizations breaking. Different
options to re-configure the signals at QLS to accommodate the
DLT phasing would have resulted in shorter and disjointed phases at
QLS-- this would have reduced the efficiency of the signals there
resulting in increased congestion for those side streets and the left
turns from Bandera.
The drawback of this customization is that the displaced left
traffic from northbound Bandera to southbound Loop 1604 arrives at the
southbound access road at the same time as the southbound through
traffic on the access road. At peak periods, this results in a
crowded merge there and some slowing on the access road as a result.
This is generally only an issue during the afternoon rush hour and was
considered to be an acceptable trade-off for the overall improved
efficiency. Should it become problematic in the future, the merge lanes
on the access road can be extended or the intersection could be
reverted to the typical DLT phasing.
of customized Bandera/1604 DLT flow and signal phases
"Why it helps" summary: If the explanation above was a
bit verbose or confusing, here's a quick summary of how this
DLT intersection allows through and left-turning traffic in both
directions on Bandera-- or a combination of other
normally-conflicting movements-- to go at the same time.
can't be done at a conventional intersection because left turns from
one direction on Bandera block the path of traffic going the other
direction on Bandera.
solve that, the left turns are moved ("displaced") to a new crossover
about 250 yards upstream from their typical location, then
continue on a parallel
roadway and onto the
turnaround roadway under 1604, where
they then turn and merge onto the access road.
moving the left turns this way, it provides a gap on Bandera for
traffic to start moving while the left turn traffic is still
both directions of traffic on Bandera can now overlap, the green time
("phase") that was allotted for one direction of Bandera can now be
eliminated green time can then be reallocated to the remaining phases,
extending those green times, which allows more traffic through the
intersection in the same period of time.
depending on traffic conditions,
the time from the eliminated phase can effectively be dropped from the
altogether, which means the signals will cycle more often in a given
time period compared to a conventional intersection, again allowing the
intersection to move more traffic in the same period of time.
markings, and signals: To
properly control traffic, minimize potential
conflicts, and promote smooth traffic flow, traffic
signals are located at the crossover locations, at the entrance to the
turnarounds, and at the end of the right turn lanes onto Bandera, in
addition to the signals at the main intersections. To maintain
traffic flow throughout the intersection system, all of the signals
in the DLT intersection are sequenced and operated from the
controller. In other words, all the signals at this intersection work
as a single system instead of multiple individual signals. The signals
at this intersection also have a battery backup
lane markings and signage (including large overhead
traffic into the correct lanes to turn left from Bandera to Loop
another first for San Antonio, this intersection will include
illuminated pavement markers (much like runway lights) at the crossover
locations to help guide traffic to the correct lanes. This work is
through a DLT is actually far easier than it may seem from the
the "how-to" guide at the bottom of this page, or just go try it!
DLT TRAFFIC SIGNAL LOCATIONS
study of four DLT intersections showed a 10-30% increase in
throughput and a 30-80% reduction in delays. A DLT in Baton Rouge,
reported a 40% decrease in travel time and average delay of less than
half of that before the DLT. A survey of drivers who regularly use the
DLT there showed that 87% felt that traffic congestion was
48% reporting their travel time "extremely decreased." Computer
modeling of the Bandera/1604 intersection during design showed as much
as a 75%
reduction in delay with a DLT.
at DLT intersections studied with serious crashes decreasing 34% at the
Baton Rouge location (total collisions were down 25%) and crashes at
and near a DLT in Utah were reported to have decreased a whopping
Federal Highway Administration study
took 96 drivers who had never navigated a DLT before and used a
simulator to test how they handled three different DLT signage and
marking treatments. Only five drivers missed the left turn, and only
one of them made an illegal left turn at the main intersection. Only
one driver stopped at the wrong place on the intersecting road at the
displaced left crossing. No drivers went the wrong way at the left turn
reports on social media after the opening of the Bandera DLT have been
unofficial review of crash statistics at this intersection by the
of this website showed that crashes decreased markedly in the eight
months after the DLT
opened as compared to the same period in previous years. There was a
44% decrease in total crashes compared to the period the previous year
and a 36% decrease in total crashes over the average of that period for
the previous five years. Crashes categorized as "intersection-related"
were cut in half during the eight months after opening and were down
44% over the preceding five year average.
statistics were obtained from the TxDOT Crash Records Information
Data was analyzed by the author of this website and the
graph above generated from that data.
This is not an official report.
main drawbacks of this intersection are that the right turns from Loop
1604 are now controlled by a signal as are the entries to the
turnarounds. Also, traffic on Bandera now encounters an additional
just beyond Loop 1604 that may be red
depending on traffic patterns and the point in the cycle motorists
reach that signal. However, any delays caused by those additional
signals should be brief and be more than offset by the overall
improvement in the operation of the intersection.
drawback resulting from the customized signal timing at this
intersection (discussed in more detail above) is the merge conflict
between through traffic and displaced-left traffic on the southbound
access road. This is generally only significant during the afternoon
rush hour and the benefits of the synchronization of the Loop 1604 and
the Quincy Lee/Stonecroft signals that this customized timing allows
offset this drawback.
drawback are that U-turns are prohibited at all signals in the
intersection system; this means motorists who were used to making a
U-turn to head back toward Helotes after leaving the shopping center on
SW corner now have to find an alternate (and likely
(Technically, U-turns are already prohibited at all signalized
intersections in the City of San Antonio anyway even if they're not
marked, although few drivers actually know this and its rarely
as with anything new and unconventional, there was some skepticism
and initial confusion while drivers learned the new traffic patterns
there are always
some naysayers who will perpetually denigrate anything different,
outside their comfort zone or understanding.
began in September 2017 and the intersection was opened to traffic on
April 28, 2019. Final paving was completed in June 2019. Due to
pavement deficiencies, the intersection had to be repaved in October
Why did work stop for so long
before the DLT opened? Construction of most everything needed to open the DLT
was substantially complete in late March 2019.
However, final installation of the new signal system took
longer than expected due to a
fabrication issue with the new cabinet for the signal controller,
which delayed the opening.
Why were the roads in the
intersection and vicinity left rough and unfinished for so
long? The final pavement could not be laid until the
island under the overpasses was complete. But that couldn't be done
until after the DLT went into operation because the island was
expanded to remove the old left turn lanes, but obviously that
couldn't be done before the DLT opened as those lanes were still needed.
Why did they rip up the new
pavement and repave it again in October 2019? Inspections showed that the previously-laid pavement
defective and would soon begin to crack and rut, so the decision was
made to replace it before that occurred. The work was done under
warranty by the contractor.
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. See the "how-to"
diagram and video at the bottom of this page or just go take a drive
through it yourself. Statistics from most DLT
implementations show no increase in crashes and a reduction in many
cases. Preliminary crash counts at this location show a significant
crashes during the first eight months of operation compared to the same
during the previous years.
Why does the displaced left
turning traffic onto southbound 1604 have to merge with through traffic
coming down the access road?
This is not typical for a DLT (and indeed doesn't happen on the
opposite side at this location), but given the traffic patterns here,
makes the intersection work better overall and synchronizes better with
signal at Stonecroft/Quincy Lee. This is explained in the "Customized
Bandera/1604 phasing" section above. The rationale gets
it is in as few words as possible:
The typical DLT
phasing would result in the access
road getting a green as northbound traffic on Bandera was arriving at
Loop 1604 due to the timing of the Quincy Lee/Stonecroft
signal. To prevent that, the Quincy Lee/Stonecroft signal could be
adjusted, but that would then result in southbound Bandera traffic
red at Quincy Lee/Stonecroft just as
it was arriving from the DLT. Any other
adjustments to the Quincy
signal to coordinate it with the typical DLT phasing would require
extra signal changes and odd phasing that would make it much less
efficient, thus increasing congestion on those side streets as well as
the left turns from Bandera.
If the southbound access road traffic was held
displaced left turns
cleared there, it would create wasted time in the intersection, which
would increase congestion and undo much of the improvements provided by
Conversely, holding the displaced left turns until
road traffic cleared would also create wasted time or would require
stopping the traffic that had just
turned left from the access road in order to let the left turns cross
over. The backup from that would then
would interfere the subsequent release of the southbound Bandera
summarize, changing the DLT phasing to the way it is allowed both
directions of Bandera to be coordinated with the Quincy Lee/Stonecroft
intersection, and the southbound access road merge
was considered to be a good trade-off for the overall
efficiencies provided by that.
as you can see, getting those two intersections synchronized is quite
complex! The resulting merge condition on the southbound access road,
while not ideal, is manageable,
especially if motorists coming from northbound Bandera who
turn into the first driveway (near
Taco Cabana) instead merge smoothly over to the right,
then use one of the other driveways further down and backtrack within
center as needed. Should the situation there warrant, the merge lanes
can be extended.
How does this crazy design
traffic? How does adding even more traffic signals help?
This intersection design improves traffic because it moves left turns
out of the way of oncoming traffic prior to the intersection. As a
result, both directions of traffic (straight and left turning) can move
during the same green phase. A conventional intersection requires two
green phases for this-- one for the straight through traffic and
another one for the opposing left turning traffic. Because both of
those directions move at the same time in a DLT, one green phase can
used for both, allowed the other phase to be
removed from the
signal cycle and the time formerly used by that phase reallocated to
phases, thus moving more traffic through the intersection in the same
amount of time. All the extra signals are needed to regulate flow
the extra crossing points created in this intersection type. Those
signals, however, are all tightly coordinated by the same controller so
that traffic flow is synchronized through the intersection.
Any time spent stopped at one of those new signals should be
minimal and more than offset by the overall reduced delay at this
This does not solve the problems
at this intersection as they are caused by backups from 1604.
While traffic headed to Loop 1604 eastbound in the mornings does
sometimes overflow back into this intersection due to congestion on the
mainlanes, the intersection also typically experienced
congestion at other times of the day (especially the afternoon
peak) that was not a result of the access road backup, and those issues
were expected to only increase as growth in the area
The DLT was
designed to ameliorate those issues. TxDOT is aware of the morning
from 1604, how those backups interact with the
DLT and, as a result, is reversing the two ramps
on eastbound Loop 1604 between
Bandera and Hausman to provide additional storage for that morning
backup; that project is expected to be complete by mid 2020. Until
that ramp project is complete, the DLT is able to
backed-up left-turning traffic out of the main
intersections and off of the Bandera through
lanes, which is beneficial, and much of that stored traffic can
continue to turn onto the access road while other traffic moves through
the main intersections.
Why not build flyovers instead
like the ones at Bandera and Loop 410?
Across the country, traffic engineers have
discovered that innovative
intersections like this can produce good congestion relief and safety
at a fraction of the cost and construction time of flyovers and other
grade-separated solutions. This allows the limited
be saved and used for more projects.
Why are there signals to enter
This is because the same roadway is also used by
left-turning traffic from Bandera. A signal is necessary to prevent
collisions due to that unexpected traffic.
How is this different than the
intersection planned at Blanco and Loop 1604?
The DLT is similar to the Diverging Diamond (DDI) interchange being
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, in the DLT, it is
just the left turns that are shifted to the other side of the road,
whereas the 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 eliminate the conflicting left turns from the
access roads to the cross street like a DDI does.
Who came up with this cockamamie
design? This won't work and is just a waste of money.
The DLT design has been around for several decades
and is in use in
several dozen locations 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
being implemented across the country at intersections
where improvements from conventional expansions will be short-lived but
where conditions do
not warrant more expensive traditional upgrades such as
flyovers. TxDOT selected this design for this intersection
after studying multiple other options. Computer modeling showed this
design provided significant improvement in traffic flow with as much as
75% reduction in delays. Initial data from this intersection shows a
substantial decreases in crashes and congestion levels.
the image below to open the detailed schematic for this
project from TxDOT with my own annotations added to help clarify and
explain the various elements. The schematic will
open in a new window that you can scroll and zoom.
DETAILED PROJECT SCHEMATIC
Click above to see the detailed
annotated schematic for
How to drive through a DLT For
driving through a DTL is far easier than it may seem from the diagrams.
Usually, once folks drive through it once or twice, they report that it
was not nearly as confusing as they expected.
a DLT opened in Cedar Park near Austin, the city
government there had an
educational campaign for drivers there with the message "follow the
signs, stay in the lines". That's really all you have to
do to easily navigate
is a step-by-step guide as well as links to videos showing how to drive
important note: you can no longer turn left from Bandera to the access
roads from the usual location after crossing under Loop 1604.
courtesy of the
City of Cedar Park)
(Base image courtesy of Ohio
Department of Transportation)
the left-turn-only lanes as you approach the Loop 1604 intersection.
Overhead signs and pavement markings will guide you into the correct
traffic signal will control the crossover intersection; wait here if
the signal is red.
over when you get a green. Be sure to keep to the left of the
oncoming traffic here. Signs and pavement markings delineate the
correct path through.
approach the Loop 1604 access road, you should have a green signal;
continue straight under the overpass. Be
aware that if you're at the tail end of your group, it may change to
red as you arrive, so be prepared to stop. To avoid this, try to keep
pace with the group as best as you can.
left onto Loop 1604 without stopping and merge onto the access road.
view video of driving through the Bandera/Loop 1604 DLT shortly after
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