Location
Loop 1604 at Bandera Rd. (access road intersections)

• Description
• How this project helps
• How a DLT improves traffic flow
• Customized Bandera/1604 signal phasing
• Why it helps summary
• Signs, markings, and signals
• Drawbacks/cons
• Improvement statistics
• Timeline
• FAQ
• Schematic
• How to drive through a DLT
  
  

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 entrance for westbound to eastbound Loop 1604.

The DLT shifts ("displaces") traffic turning left from Bandera to Loop 1604 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 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 turnarounds are still available for access road traffic; however, entry to them is controlled by a traffic signal to prevent collisions with the unexpected displaced-left cross-traffic. With the new displaced left turn, 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.

NEW DISPLACED LEFT TURN INTERSECTION
TRAFFIC FLOW FOR BANDERA RD.

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 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.

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.

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. 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.

What is a signal phase? A signal "phase" is the green time assigned to a specified movement or collection of 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.

An important note: Observed signal phasing and timings at the Bandera/Loop 1604 intersection vary from what is described above, 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.)

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
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 intersection
The 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 phases 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.

Simplified animation of typical DLT flow and signal phases

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 typical 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 turn 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.

Simplified animation of customized Bandera/1604 DLT flow and signal phases

• 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 a gap 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.

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

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.

Another 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.

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 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 was some skepticism and initial confusion while drivers learned the new traffic patterns and adapted. And there are always some naysayers who will perpetually denigrate anything different, innovative, or outside their comfort zone or understanding.

Improvement statistics
Nationally: 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 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 predicted 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 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, which seems to be the maneuver that many people expect will happen.

Bandera/1604: By all indications, the Bandera/1604 DLT has been successful at reducing congestion and crashes. Anecdotal reports on social media in the weeks after the opening of the Bandera DLT were overwhelmingly positive.

With regards to congestion, the author of this website observed that afternoon peak backups on northbound Bandera between Quincy Lee and Loop 1604 virtually disappeared after the DLT opened. An analysis of Google Maps data showing "typical" traffic at that intersection prior to and after the DLT showed a marked decrease in congestion levels.

BEFORE
AFTER (MARCH 2020)		AFTER (AUGUST 2022)

Google Maps "typical" traffic for Wednesdays at 5:30 pm. The "before" snapshot was taken on April 28, 2019 (the DLT opening day), thus reflecting typical traffic prior to opening. The "after" snapshots were taken March 1, 2020 (just before COVID) and August 20, 2022.

In the 2020 "after" snapshot, note the substantial decrease in congestion (shown as red and orange) on both of the Bandera approaches to Loop 1604 as well as the southbound Loop 1604 access road between the exit ramp and Bandera Rd. The northbound access road shows no change in congestion levels, while the southbound access road south of Bandera shows a moderate increase (from green to orange); this is a result of slowing there due to the merging issue discussed elsewhere on this page.

In the 2022 "after" snapshot, we can see that overall, congestion levels remain substantially improved compared to before. One notable exception is the northbound access road approaching Bandera Rd.-- it shows a marked increase in congestion. (This might require a signal timing adjustment to resolve.) Both left turn bays from Bandera to Loop 1604 also show a slight uptick in congestion. But the other approaches all show no notable increases in congestion.

As for safety, an unofficial review of crash statistics at this intersection by the author 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 compared to the prior year and were down 44% over the preceding five year average. (A preliminary report from TxDOT on the crash data at this intersection also shows a 44% decrease in crashes.)

Crash statistics were obtained from the TxDOT Crash Records Information System (CRIS).
The data was collected by the author of this website and the graph above generated from that data.
The DLT opened on April 28, 2019. This is not an official report.

A further review in 2022 shows the reduction in crashes has continued. Even with traffic volumes in 2021 back to pre-COVID levels, the number of crashes in 2021 was less than half of those in 2016 and 2017, years with comparable traffic volumes prior to construction. Furthermore, intersection-related crashes decreased from 70% of all crashes in 2016/2017 to 56% of all crashes in 2021. Preliminary data for the first half of 2022 shows the trend continuing: 39 total crashes, with 17 of those (43.6%) intersection-related.

(1) Intersection under construction
(2) DLT opened on April 28, 2019
(3) Traffic reduction due to COVID

Crash statistics were obtained from the TxDOT Crash Records Information System (CRIS).
The data was collected by the author of this website and the graph above generated from that data.
This is not an official report.

(1) Traffic reduction due to COVID

Traffic counts were obtained from the TxDOT Traffic Count Database System.
The data was collected by the author of this website and the graph above generated from that data.
This is not an official report.

• 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 reduction in crashes during the first eight months of operation compared to the same period 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 northbound side), but given the traffic patterns here, this phasing makes the intersection work better overall and synchronizes better with the signal at Stonecroft/Quincy Lee. This is explained in the "Customized Bandera/1604 phasing" section above. The rationale gets pretty complicated, but here 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 getting a red at Quincy Lee/Stonecroft just as it was arriving from the DLT. Any other adjustments to the Quincy Lee/Stonecroft 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 until the 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 this project.
• 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.
• To 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.
  

So 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 want to 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 the shopping center as needed. Should the situation there warrant, the merge lanes can be extended.


• 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.
  
  
• 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 traditional grade-separated solutions. This allows the limited transportation dollars to be saved and used for other needed 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 under construction at Blanco and Loop 1604?
  The DLT is similar to the Diverging Diamond (DDI) interchange being built 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 partial DLTs (one direction only) at US 281 and Basse and at I-35 and FM 306 in New Braunfels (now open to traffic.) Other varieties of 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 doesn'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, and the actual data so far from this intersection shows substantial decreases in crashes and congestion levels.
  

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 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 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 the City of Cedar Park)

Additional videos:
	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

	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