1604 at Bandera Rd. (access road intersections)
Status The new intersection is now open to
traffic. (Still to come: installation
of illuminated pavement markers at the crossover
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 for westbound to eastbound Loop
DLT shifts ("displaces") traffic turning left from
Bandera to Loop 1604 in both directions 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 traffic then
the turnarounds to cross under Loop
1604 and makes a
free left turn onto the access road. The
still available; however, entry to them is controlled by a
signal to prevent collisions with the unexpected displaced-left turn
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.)
should 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
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 green signal
time 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
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 somewhat from what is described above (the
reason is explained in the next paragraph), 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.)
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 does provide flexibility so
that the signal phases can be re-arranged to instead allow a different
combination 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. This "customization" was done at this location. For
example, the two Bandera left turns typically
don't run concurrently, but instead are overlapped with the tail end of
the near-side access road
phase. This was done to better synchronize the signals here with the
ones at Quincy Lee/Stonecroft. The flexibility in signal
that this intersection
design permits allows engineers to wring-out the most efficiency
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
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 is stopped
between the access roads
for a short period of
time. It is
then released at the same time as through traffic on the cross
This generates additional efficiency. However, turning volumes at this
location are generally too high for that configuration to work at many
times, so most of the time, 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) might get 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, and, as mentioned, the observed
phasing at Bandera and
Loop 1604 differs somewhat from what is shown below.
of typical DLT flow and 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 room on Bandera for through
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 in there showed that 87% felt that traffic congestion was improved
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
cursory review of crash statistics at this intersection by this author
showed that crashes decreased markedly in the month after the DLT
opened as compared to the same period one year and two years earlier as
well as compared to the month before opening.
reports on social media after the opening of the Bandera DLT have been
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
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 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 will now have to find an alternate (and likely circuitous)
(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 will be some skepticism
and initial confusion while drivers learn the new traffic patterns and
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.
Why did work stop for so long
before the DLT opened? Now that it's open, why is there still more work
to do? Construction of most everything needed to open the DLT
was substantially complete in late March.
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. Now that the
DLT has opened, work is underway on the final items
including removing the existing left turn lanes on Bandera (they aren't
needed anymore) and placing final pavement and markings throughout the
these items couldn't be done until after
the DLT opened.
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 in use.
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 reduction in
crashes in the first month of operation compared to previous periods.
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 left turn), but given the traffic patterns here, this phasing
makes the intersection work better overall and synchronizes better with
signal at Stonecroft/Quincy Lee. The rationale gets pretty complicated,
it is in as few words as possible:
If the southbound access road traffic was held
displaced left turns
cleared there, it would create wasted time in the intersection, which
defeats the purpose.
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
The typical DLT phasing, which would give
southbound Bandera the green (instead
of the access road)
while the opposing left turns are moving, would result in the access
road getting a green as northbound traffic on Bandera was arriving at
Loop 1604 as a result of the timing at the Quincy Lee/Stonecroft
signal. To prevent that, the Quincy Lee/Stonecroft signal could be
adjusted, but that would then result in southbound Bandera getting a
red just as it was arriving from the DLT. Any other adjustments to that
signal would make it much less efficient.
as you can see, getting those two intersections synchronized is quite
complex! The resulting merge condition, 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 and backtrack within the shopping
center as needed.
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), a problem 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
planning to reverse the two ramps
eastbound Loop 1604 between
Bandera and Hausman to provide additional storage for that morning
backup; that project is expected to start shortly after completion of
the DLT (in fact, the same contractor has been retained for that
that ramp project is complete, the new DLT should be able to store
most of that backed-up left-turning traffic out of the main
intersections and off of the Bandera through
lanes, which is beneficial, and that stored traffic can continue to
turn while traffic moves through the main intersections.
Why not build flyovers instead
like the ones at Bandera and Loop 410?
Across the country, traffic agencies 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 scarce transportation
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 interchange being planned
at Blanco 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, 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
designs 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.
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, Texas (near Austin), the city
government 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 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 deliniate
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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