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San Antonio Area Freeway System
PROJECT INFO: Loop 1604 at Bandera (DLT)

This page last updated July 10, 2019

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Project locationLocation
Loop 1604 at Bandera Rd. (surface intersections)

Status
The new intersection is now open to traffic. Final "punch-list" items are being completed. 

Description
This project converted the access road intersections at Loop 1604 and Bandera Rd. (SH 16) from conventional intersections to a "Displaced Left 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 1604.

The DLT shifts ("displaces") traffic turning left from Bandera to Loop 1604 in both directions to a new parallel roadway on the outside of the opposing lanes via a signalized crossover about 100 to 150 yards upstream of the near-side intersection with Loop 1604. That traffic then uses the turnarounds to cross under Loop 1604 and makes a free left turn onto the access road. The turnarounds are still available; however, entry to them is controlled by a traffic signal to prevent collisions with the unexpected displaced-left turn cross-traffic. Traffic is no 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.)

This is the first DLT in San Antonio, and several more are planned. There are 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.


CONVENTIONAL (PREVIOUS) INTERSECTION
TRAFFIC FLOW FOR BANDERA RD.

Conventional flow


NEW DISPLACED LEFT TURN INTERSECTION
TRAFFIC FLOW FOR BANDERA RD.

Conventional flow


How this project helps
The DLT reduces congestion and increases safety at this intersection. Traffic volumes were sufficient to cause level of service "F" (congested) conditions at the previous conventional intersection during peak 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 projects. Thus, this intersection was determined to be an ideal candidate for an innovative intersection. Computer 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.)

Safety 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 serious types. 

How a DLT improves traffic flow: A DLT is a relatively low-cost improvement (this project cost $7.6 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 is accomplished by diverting ("displacing") the left-turning traffic to the opposite side of 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 reaches the crossover location, thus minimizing or even eliminating the time through traffic has to wait for the opposing left turning traffic. Because 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.

Also, because the crossover angle for the left turns in a DLT is more gentle than for a regular 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.

These 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.

CONVENTIONAL INTERSECTION
Only one direction of traffic can go through the intersection at a time


DISPLACED LEFT TURN INTERSECTION
Both directions of traffic on Bandera can go through the intersection at the same time
(Note that for simplicity, the diagrams below only show the sequence for one half of the flow on Bandera.)



This "continuous flow" of traffic in both directions means that only one green signal phase is needed for both directions on Bandera instead of the two phases required at the conventional intersection. (A signal "phase" is the green time assigned to a specified movement or collection of movements in a traffic signal cycle.) The time saved from 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.

An 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 are still the same (See example timings illustration below. Signal timings are simplified and for illustrative purposes only and do not represent actual timings. Timings are typically adjusted in response to traffic demand.)

The "standard" DLT 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 timing that this intersection design permits allows engineers to wring-out the most efficiency possible.


EXAMPLE TIMINGS

Simplified timeline showing example signal phase timings for each movement in the Loop 1604/Bandera intersection. Note that these timings are simplified for illustrative purposes only; they do not represent actual timings and the actual phasing is a bit more complex and variable. Be sure to read the explanation in the paragraph above.


Conventional intersection



DLT intersection


Note that the example timings above show separate phases for each access road. However, the DLT configuration allows the access road phases to also be overlapped, i.e. both access roads could have a green simultaneously. In this scenario, traffic turning left from the access roads is stopped on Bandera between the access roads for a short period of time. It is then released at the same time as through traffic on the cross street. 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 road they 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.


Animation 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 intersection helps improve traffic:
  • The 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.
  • This 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.
  • To 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.
  • By moving the left turns this way, it provides room on Bandera for through traffic to start moving while the left turn traffic is still crossing over. 
  • Because 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.
  • That 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.
  • Alternatively, depending on traffic conditions, the time from the eliminated phase can effectively 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.
Got it?  :-)

Signs, 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 same 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 system.

Additional lane markings and signage (including large overhead signs) help guide traffic into the correct lanes to turn left from Bandera to Loop 1604. 

In 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 still pending.

Driving 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, or just go try it!

DLT TRAFFIC SIGNAL LOCATIONS


Improvement statistics:
A study of four DLT intersections showed a 10-30% increase in throughput and a 30-80% reduction in delays. A DLT in Baton Rouge, Louisiana 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 with 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.

Safety has also generally improved 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 60%. 

A 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.

Anecdotal reports on social media after the opening of the Bandera DLT have been overwhelming positive.

A 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 crossover.

Drawbacks/cons: The 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 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.

One other major 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 the SW corner will now have to find an alternate (and likely circuitous) route. (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 enforced.)

Finally, 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 denigrate anything different, innovative, or outside their comfort zone or understanding.

Timeline
Construction began in September 2017 and the intersection was opened to traffic on April 28, 2019. Final paving was completed in June.

FAQ

  • 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 project; 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 expansion of central 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.


  • 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 the signal at Stonecroft/Quincy Lee. The rationale gets pretty complicated, but here it is in as few words as possible:
    • If the southbound access road traffic was held until the displaced left turns cleared there, it would create wasted time in the intersection, which defeats the purpose.
    • Conversely, holding the displaced left turns until the access 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 through traffic.
    • 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.

    So 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 want to 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 improve 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 the other phases, thus moving more traffic through the intersection in the same amount of time. All the extra signals are needed to regulate flow through 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 intersection.


  • 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 continues. The DLT was designed to ameliorate those issues. TxDOT is aware of the morning backups from 1604, how those backups interact with the DLT and, as a result, is planning to reverse the two ramps on 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 project.) Until 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 traditional grade-separated solutions. This allows scarce transportation dollars to be saved and used for more projects. 

  • Why are there signals to enter the turnarounds?
    This is because the same roadway is also used by the displaced 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 designs 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.

  • Are any other DLTs planned in San Antonio?
    Yes, two other DLTs are currently planned: one at Loop 1604 and Culebra and one at Loop 410 and Cornerway Blvd/WW White Rd., as well as a partial DLT (one direction only) at US 281 and Basse. Different alternative intersections are also being planned for other locations; see the 
    alternative intersections page for more info.

  • 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 a 75% reduction in delays.

Schematic
Click on 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 this project


How to drive through a DLT
For most people, 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.

When 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 a DLT.

Below is a step-by-step guide as well as links to videos showing how to drive through a DLT intersection.

An important note: you can no longer turn left from Bandera to the access roads from the usual location after crossing under Loop 1604.
Image courtesy of City of Cedar Park

(Image courtesy of the
City of Cedar Park)


(Base image courtesy of Ohio Department of Transportation)


1
Enter the left-turn-only lanes as you approach the Loop 1604 intersection. Overhead signs and pavement markings will guide you into the correct lane.
2
A traffic signal will control the crossover intersection; wait here if the signal is red.
3
Cross 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.
4
As you 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.
5
Turn left onto Loop 1604 without stopping and merge onto the access road.
 
Driver's view video of driving through the Bandera/Loop 1604 DLT:
https://www.youtube.com/watch?v=Q_tbYwU7wyo.
Driver's view visualization (computer rendering) of the Bandera/Loop 1604 DLT:
https://www.youtube.com/watch?v=wgWpgeS3k74
Video showing the decrease in congestion after the DLT opening:
https://www.youtube.com/watch?v=G9DYPv_toMw



Other sites of interest

SH 16 at Loop 1604 existing and proposed intersection animations
https://www.youtube.com/watch?v=r6Bc0dfvNEU
TXDOT - Continuous Flow Intersections Fact Sheet
http://ftp.dot.state.tx.us/pub/txdot/my35/capital/concepts/cfi-factsheet.pdf
FHWA - Displaced Left Turn Intersection Informational Guide
https://safety.fhwa.dot.gov/intersection/alter_design/pdf/fhwasa14068_dlt_infoguide.pdf
Wikipedia - Continuous-flow Intersection
https://en.wikipedia.org/wiki/Continuous-flow_intersection
DLT Case Study - Bangerter Highway in Salt Lake County (Utah)
https://www.youtube.com/watch?v=o5-U_TgEtJA
‘How To’ drive through a continuous flow intersection
https://www.youtube.com/watch?v=ULyjDcEeHG0




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This page and all its contents are Copyright © 2019 by Brian Purcell

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