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Bandera Rd. at Loop 1604 Displaced Left Turn (DLT)

This page last updated February 21, 2024

DLT location map
DLT location map

By the mid 2010s, congestion had become a significant problem at the Bandera Rd. (SH 16)/Loop 1604 intersection. So planners began looking for potential remedies. After reviewing multiple options, engineers settled on a Displaced Left Turn (DLT), a type of "innovative" intersection that had shown good results in several implementations nationwide and had recently been implemented in San Marcos. Computer modeling of the Bandera/1604 intersection predicted as much as a 75% reduction in delay with a DLT.

Construction begin in September 2017 and the intersection opened to traffic on April 28, 2019. This is the first (and currently only) DLT in San Antonio, but several more are planned.

How is a DLT different from a regular intersection?
A DLT — sometimes also called a "Continuous Flow Intersection" — shifts ("displaces") traffic turning left from Bandera Rd. to the Loop 1604 access roads to a parallel roadway on the outside of the opposing lanes via a signalized crossover about 100 yards from 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 then 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 Rd. to Loop 1604 where it typically would. (See the diagrams below for a depiction of these changes.)


Conventional intersection diagram


DLT diagram


How a DLT improves traffic flow
A DLT is a relatively low-cost improvement (this project cost $7.6 million versus at least $25 million for flyovers) that increases the throughput of an intersection by allowing traffic headed in opposing directions (including left turns) to all move simultaneously. 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 time instead of each direction needing its own dedicated green. This overlapping of movements 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.

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

Conventional intersection diagram

Both directions of traffic on Bandera can go through the intersection at the same time.

Conventional intersection diagram
Conventional intersection diagram
Conventional intersection diagram

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 remaining three phases. This means that, in this example, roughly 33% more traffic can get through the intersection per cycle. (In reality, the extra time would more likely be allocated in varying 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 period compared to a conventional intersection, again allowing the intersection to move more traffic in the same period of time.

Question mark sign
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 will vary depending on the time of day and traffic demand.)


Below are simplified timelines showing example signal phase timings for each movement in the Loop 1604/Bandera Rd. 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 can vary throughout the day. Be sure to read the explanation above.

Conventional intersection
Only 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.

Conventional phasing

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.

DDI phasing

Note that the example timings above show separate phases for each access road (blue and purple blocks.) 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 would be stopped on Bandera 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 store 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 turning to northbound Bandera) gets 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. The phasing at Bandera and Loop 1604 differs somewhat from what is shown below and is discussed in more detail below.

DLT animation

Simplified animation of typical DLT flow and signal phases

Customized Bandera/1604 signal phasing
The "typical" 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 inherently provides some flexibility so that the signal phases can be re-arranged to allow for different 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 efficiency possible.

After simulating traffic flows, engineers opted to customize the signal phasing at this intersection in order to better synchronize the signals at 1604 with the signals at Quincy Lee/Stonecroft. This customization 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 light at Quincy Lee/Stonecroft, and also permits the synchronization of the green signals for northbound Bandera at Quincy Lee/Stonecroft and 1604. Because of the signal cycle timing required to handle traffic at Quincy Lee/Stonecroft, using the typical DLT phasing at 1604 would not have allowed signal synchronization in both directions. Different options to re-configure the signals at Quincy Lee/Stonecroft to accommodate the typical DLT phasing would have resulted in shorter and disjointed phases there, which would have resulted in increased congestion for those side streets and the left turns from Bandera. Changing the DLT phasing was a win-win.

The main 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 is crossing Bandera. During peak periods, this results in a crowded merge 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.

DLT animation

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


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 there showed that 87% felt that traffic congestion was improved with 48% reporting their travel time "extremely decreased."

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.

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 and continue to be so as of this writing.

With regards to congestion, the author of this website observed that afternoon peak backups on northbound Bandera Rd. between Quincy Lee and Loop 1604 completely disappeared after the DLT opened (an outcome that continues to time of this writing.) 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.


Google Maps traffic screenshot before


Google Maps traffic screenshot after


Google Maps traffic screenshot after

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 the widespread COVID shutdowns) 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.

(Screenshots from Google Maps by Brian Purcell)

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 noted a 44% decrease in crashes.

Crash statistics graph

Bandera/Loop 1604 Crash History (4/28 - 12/31 of each year)
Crash statistics were obtained from the TxDOT Crash Records Information System (CRIS). The DLT opened on April 28, 2019. This is not an official report.
(Data collected, analyzed, and graphed by Brian Purcell)

An updated review at the end of 2021 showed the reduction in crashes 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.

Crash statistics graph

Bandera/Loop 1604 Crash History
Crash statistics were obtained from the TxDOT Crash Records Information System (CRIS). This is not an official report.

Footnotes: (1) [2018] Intersection under construction; (2) [2019] DLT opened on April 28, 2019; (3) [2020] Traffic reduction due to COVID

(Data collected, analyzed, and graphed by Brian Purcell)

Traffic statistics graph

Bandera/Loop 1604 Traffic Volume
Traffic counts were obtained from the TxDOT Traffic Count Database System. This is not an official report.

Footnotes: (1) [2020] Traffic reduction due to COVID

(Data collected, analyzed, and graphed by Brian Purcell)



Here is a drive-through video of the intersection shortly after it was opened to traffic:

Other sites of interest

Wikipedia - Continuous-flow Intersection
TXDOT - Continuous Flow Intersections Fact Sheet
FHWA - Displaced Left Turn Intersection Informational Guide
DLT Case Study - Bangerter Highway in Salt Lake County (Utah)