SAN ANTONIO FREEWAY HISTORY
Read my essay on San Antonio's freeway history for the Express-News' Tricentennial series
The full history of the freeway system is available on this site
Antonio Area Freeway System
Loop 1604 at Bandera
page last updated November 1, 2017
1604 at Bandera Rd. (surface intersections)
This project will convert 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 will also
add 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 will shift ("displace") 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 will then cross
under Loop 1604 in the same space as the existing turnarounds and will
then make a free left turn onto the access road. (The turnarounds will
still be available; however, entry will be controlled by a traffic
signal to prevent collisions with the displaced cross-traffic.) Traffic
longer be allowed to make a left turn from Bandera to Loop 1604 at the
locations they do today. (See
the diagrams below for a depiction of these changes.)
will be the first DLT in San Antonio. There are two DLTs in operation
each in San Marcos and Austin and one in Cedar Park. The design
has been used successfully in several other states as well.
TRAFFIC FLOW FOR BANDERA RD.
DISPLACED LEFT TURN INTERSECTION
TRAFFIC FLOW FOR BANDERA RD.
project will help
DLT 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 but are not
sufficient to justify flyovers now or during the next 20 years. Thus,
this intersection is considered to be in the "sweet spot" for an
"outside-the-box" (no pun intended) intersection design. Computer
modeling shows a dramatic improvement in intersection performance with
a DLT and
the additional turn lanes with perhaps as much as a 75% reduction in
How a DLT
improves traffic flow: A
DLT is a relatively low-cost improvement (this project will cost $7.6
million versus over $20 million for flyovers) that increases the
throughput of an intersection by allowing
traffic headed in both directions (including left turns) to
all move simultaneously (in this case, traffic on Bandera.) This is
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 traffic. The
displacement also provides a short overlap period that permits
left-turning traffic to be released even while the Loop 1604 access
road still has a green light.
"continuous flow" of traffic in both directions means
that only one
is needed for both
directions on Bandera instead
of the two phases required at the conventional intersections. The time
having only one phase can then be distributed to extend the green time
for all approaches, thus moving more traffic through the intersection
on each cycle without having to increase the overall cycle length. For
example, if the DLT allows one of the 45 second phases for Bandera to
be dropped, that would allow 15 seconds of green time to be added to
the two Loop 1604 phases and the remaining Bandera phase. (See example timings illustration
below. Signal timings are for
illustrative purposes only and may not represent actual timings.
Timings are typically adjusted in response to traffic demand.) The
additional green time actually has a
slightly greater benefit because it is continuing the flow of
traffic rather than an added phase where some time and throughput is
lost during the signal changes. The net gain from this is
providing a greater overall benefit during any given time
Simplified timeline showing example signal phase timings for each
movement in the
Loop 1604/Bandera intersection. These timings are for illustrative
purposes only and may not
represent actual timings. Be sure to read the explanation in the
Signs, markings, and signals: To
properly control traffic and minimize potential
signals will be located at the crossover locations, the entrance to the
turnarounds, and at the end of the right turn lanes onto Bandera in
addition to the existing signals at the main intersections. To ensure
proper flow throughout the intersection system, all of the signals
in a DLT intersection are tightly coordinated and operated from the
controller. The signals at this intersection will have a battery backup
lane markings and signage (including large overhead signs) will help
traffic into the correct lanes to turn left from Bandera to Loop 1604.
through a DLT is actually far easier than it may seem from the diagrams-- see
the "how-to" guide at the bottom of this page.
TRAFFIC SIGNAL LOCATIONS IN A DLT
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 Baton
Rouge showed that 87% felt that traffic congestion was improved with
48% reporting their travel time "extremely decreased." Computer modeling of this intersection showed as much as a 75%
reduction in delay with a DLT.
Safety has also
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
main drawbacks of this intersection are that the right turns from Loop
1604 will now be controlled by a signal as will the entries to the
turnarounds. And traffic on Bandera will
encounter an additional signal 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 will be that U-turns will be prohibited at all signals in the
intersection system; this means motorists who are used to making a
U-turn to head toward Helotes after leaving the shopping center on the
SW corner will have to find an alternate (and likely circuitous) route.
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 who will perpetually denigrate anything different, innovative, or
outside their comfort zone or understanding.
is set to begin in late Summer 2017 and take about a year to complete.
- 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. Statistics from most DLT
implementations show no increase in crashes and a reduction in many
- How does adding signals improve
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 a single green phase (instead of two separate phases) thus
allowing one signal phase to be removed from the
cycle. The time formerly used by that phase can be added to the other
phases, thus moving more traffic through the intersection in the same
amount of time. The extra signals are needed to regulate flow through
the extra conflict 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 will 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
overflow back into this intersection due to congestion on the mainlanes
(a problem that will be resolved with a future widening of Loop 1604),
the intersection itself does experience congestion for other reasons
and at other times of the day (especially the afternoon peak), issues
that will get worse as growth in this area increases. The DLT was
designed to solve those issues. TxDOT is aware of the morning backups
from 1604 and how that will affect the DLT and, as a result, they may
implement an interim "quick-fix" at the Bandera on-ramp. However, that
area is still under a long-term study for expansion, so any
improvements there are constrained by that. Even if no improvements
are made to alleviate that backup, the new DLT will be able to store
most of that backed-up left-turning traffic off of the Bandera through
will benefit that through traffic.
- Are the new left turns flyovers?
the new turning lanes are at the same level as Bandera Road and will
cross under Loop 1604 in the area where the turnarounds are now.
- Will there still be turnarounds?
Yes, the turnarounds in both directions will remain and the WB to
EB turnaround will even be expanded to two lanes.
However, the entrance to the turnarounds will be controlled by a
signal as the same roadway will also be used by the displaced
left-turning traffic from Bandera. A signal is necessary to prevent
collisions due to that unexpected traffic.
- Why not build flyovers instead
like the ones at Bandera and Loop 410?
This intersection does not have current or projected traffic volumes to
justify the cost of flyovers. This project will cost less than half of
what flyovers would cost and be completed in half the time.
- 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 turn from the access roads to the cross street like a DDI does.
- Who came up with this cockamamie
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
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 as much as a
75% reduction in delays.
is the detailed project schematic with annotations. The
project schematic is courtesy of TxDOT and is
change (and indeed already has.) Annotations are my own. Below that is
a "how-to" on making a
left turn at a DLT .
DETAILED PROJECT SCHEMATIC
Click above to see the detailed
annotated schematic for
(Base image courtesy of Ohio
Department of Transportation)
Note: This is the typical
operation of a DLT; actual operation of the 1604/Bandera DLT
may vary but should be similar.
the left-turn-only lanes as you approach the Loop 1604 intersection.
traffic signal will control the crossover intersection. Wait until you
have a green signal to cross over.
1604 access road traffic will get a green. Shortly
thereafter, you will get a green to cross over.
approach the Loop 1604 access road, their signal will turn red and you
will get a green.
left onto Loop 1604 without stopping.
video of how to drive through a DLT, see
driver's view visualization of the Bandera/Loop 1604 DLT, see
sites of interest