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San Antonio Area Freeway System
PROJECT INFO: Loop 1604 at Blanco Road

This page last updated June 12, 2024

  • Loop 1604 at Blanco Road

  • Under construction (64% complete)

  • Note: The completion percentage is for the entire I‑10 to US 281 expansion project, of which this is a part, as progress metrics for this specific element are not available.

  • Construction stared in November 2021 and is expected be completed in mid 2025.

  • This project was originally planned as a standalone project, but was subsequently rolled into the larger Loop 1604 North expansion project.


On this page:


Here is a brief "PowerPoint" summary of this project:

Project description

This project will convert the access road intersections at Loop 1604 and Blanco Rd. (FM 2696) from conventional intersections to a "Diverging Diamond Interchange" (DDI).

The DDI will shift traffic in both directions on Blanco to the opposite side of the roadway just before crossing Loop 1604. Traffic will then travel over Loop 1604 to the left of opposing traffic (i.e. on the "wrong" side.) Traffic turning left onto Loop 1604 will make a free left turn (i.e. no opposing traffic) while through traffic on Blanco will then cross back over to the right side of the roadway. The two crossover locations will be controlled by a traffic signal. (See the diagrams below for a depiction of these changes.) Shifting traffic over to the left side allows for two left turn movements that would normally be conflicting to occur simultaneously, thus allowing the intersection to move more traffic at one time. (More on that below.)

A DDI inherently blocks straight-through traffic on the access roads. To accommodate this through traffic, an access road bypass (known as a "collector/distributor road") will be built below Blanco next to but separate from the Loop 1604 mainlanes. Through traffic on the access road will descend to this road before reaching Blanco, pass under Blanco, then ascend back to the access road. This bypass roadway will also serve Loop 1604 traffic headed to and from Huebner and Stone Oak Parkway. This will take all that through traffic on the access roads out of the Blanco Road intersections. All of the entrance and exit ramps between Huebner and Stone Oak will be adjusted to accommodate the new collector/distributor roads.

In addition to the above, the project also will add the following:

This will be the first DDI in San Antonio and the eighth in Texas. The others are in El Paso, The Colony, Round Rock (next to the IKEA), Georgetown, College Station, and two in Austin. The design has been used successfully in about 100 other locations around the US as well. Two other DDIs are planned for San Antonio: one at US 281 and Jones-Maltsberger and another at I‑37 and SE Military Dr.


Conventional intersection traffic flow

Conventional intersection traffic flow


Conventional intersection traffic flow

Conventional intersection traffic flow

How this project will help

The DDI 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 now and into the future. (You can learn more about levels of service here.) However, flyovers are not considered to be a good option for this location. Thus, this intersection is considered to be ideal for an "outside-the-box" (no pun intended) intersection design.

Computer modeling shows a dramatic improvement in this intersection's operation with this project. For example, it is projected that it would take through traffic on Blanco (either direction) 16 minutes to get through this intersection during the afternoon rush hour in 2037 with no improvements; with the DDI, it is projected to only take about four minutes. The left turn from eastbound Loop 1604 to northbound Blanco is projected to take 29 minutes during the afternoon rush hour in 2037 with no improvements, but also just four minutes with the DDI — an 86% improvement!

Projected 2037 afternoon peak period travel times
  No improvements With DDI
Blanco through traffic (NB and SB) 16 min 4 min
EB Loop 1604 left turn to NB Blanco 29 min 4 min

How a DDI improves traffic flow
A DDI increases the throughput of an intersection by eliminating the conflicts caused by left turns by shifting all traffic on the cross street (in this case, Blanco) to the left side of the road through the interchange so that left turns from that street to the access road no longer have to cross oncoming traffic. This also creates space on the opposite side of the road that allows the left turns from the adjacent access road to also turn at the same time. This overlapping reduces the number of signal phases from the four required in a conventional intersection to just two, thus allowing more traffic to move through the intersection in the same amount 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.

The traffic turning left from the access road will encounter a red signal at the crossover point on Blanco where they will wait for opposing through traffic on Blanco to finish up. However, because both movements started at about the same time, the Blanco through traffic will be nearing the completion of their green by the time the left-turners reach that location, so the wait will be brief and more than offset by the overall reduction in delay that this intersection provides.

The diagrams below should help to illustrate the description above.

Only one direction of traffic can go through the intersection at a time. This requires four signal phases to move traffic from all four approaches. Right turns not shown.

Phase 1

Phase 2

Phase 3

Phase 4

One direction of Blanco and one access road left turn can proceed at the same time. This requires only two signal phases to move traffic from all four approaches. Right turns from access roads are shown as they will be signalized. Right turns from Blanco will have a yield sign and are not shown.

Phase 1

Phase 1

Phase 2

Phase 2

If each phase in the conventional intersection is 30 seconds, that's a cycle time of two minutes for all four phases. If each phase in the DDI is also 30 seconds, each cycle is just one minute, so two signal cycles can be completed in the same two minute period a conventional intersection would require for one cycle. In other words, each approach would get twice the amount of green time every two minutes than they would in a conventional intersection, meaning the intersection can move about twice as much traffic. (See example timings illustration below. Signal timings are for illustrative purposes only and do not represent actual timings. Timings are typically adjusted in response to traffic demand.)

Note that the description above is simplified to help explain the concept; the actual phasing and timing of signals is a bit more complex and may vary somewhat from what is described.

Example signal timings

Below are simplified timelines showing example signal phase timings for each movement in the Loop 1604/Blanco 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 30 seconds of green time, that's a total cycle time of 120 seconds.

Conventional phasing

DDI intersection
The DDI allows one direction of Blanco and one access road left turn to move simultaneously, so it only needs two phases to move all directions. If each phase remains 30 seconds long, two complete cycles can be completed in the same 120 seconds required for one cycle in the conventional intersection. (The blue phase corresponds to "Phase 1" and the red to "Phase 2" in the DDI diagrams above.)

DDI phasing

Here is an animation that puts it all together. Note that this animation is greatly simplified and represents the "typical" DDI trafficflow. The phasing at Blanco and Loop 1604 could vary from this.

DDI phasing animation

Signs, markings, and signals
To properly control traffic and minimize potential conflicts, traffic signals will be located at the two crossover locations as well as at the left turns from the access roads. The right turns from the access roads to Blanco will also be controlled by a signal.

Additional lane markings and signage (including large overhead signs on Blanco) will help channel traffic into the correct lanes.

The crossover intersections will be angled sufficiently to make the crossover transition much more intuitive; engineers have learned the optimal angles for this from previous projects. Additionally, barriers will be placed between the opposing traffic lanes in the "wrong-side" area to obscure the oncoming traffic from view and thus reduce possible confusion or panic that might be caused by seeing oncoming traffic to your right.

While it probably looks and sounds confusing, driving through a DDI is actually far easier than it may seem from the diagrams and descriptions. See the video at the bottom of this page for a drive through.

Improvement statistics
A study of the DDI built in Round Rock in 2016 showed that despite a 50% increase in afternoon peak period traffic volumes after the DDI was completed, travel times improved 44-58% over the previous conventional intersections.

Safety is also improved at DDI intersections. A DDI reduces the number of potential crash points from 26 to 14, with the worst type (side-angle collisions) reduced from 10 to just two. Additionally, a DDI physically prevents drivers from turning the wrong way onto the access roads, thus helping to prevent head-on collisions. A study at DDI intersections in Colorado showed a 36% reduction in crashes, and a 60% reduction was reported at a DDI in Springfield, Missouri where 97% of drivers surveyed reported they felt the DDI was safer than the previous conventional intersections.

The main negative of this intersection is that straight-through traffic on the access roads will not be possible. While there will be a bypass road for through traffic below the intersection, it will not be accessible for traffic with origins or destinations in close proximity to the intersection (e.g. Starbucks, Jim's, Luby's, etc.) This means many drivers headed to or from those locations via the access roads may have to use an alternate and possibly circuitous route. The impact of this should be fairly limited and is a small trade-off for the significant overall improvements this change will bring.

Another possible drawback is that the DDI may be so efficient, it will cause increased congestion at other downstream intersections on Blanco as the additional traffic that gets through this intersection exceeds the capacity at those intersections.

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. It's worth noting that there was much consternation on social media before the Displaced Left Turn intersection opened at Loop 1604 and Bandera in 2019, but widespread acclaim after it opened both for the congestion relief it brought as well as for it not being nearly as confusing as many had previously feared.



Click on the images below to open the detailed schematics for this project from TxDOT with my own annotations added to help clarify and explain the various elements. Each schematic will open in a new window that you can scroll and zoom. These are the schematics presented to the public in March 2019, but match the final schematics for construction.

Blanco/1604 DDI intersection close-up

Schematic thumbnail
Collector/distributor road and ramps

Schematic thumbnail

Click an image above for a full-sized schematic of this project


I put together a video that discusses the Blanco Rd. DDI project, how it will work, and how it will improve traffic:

Other sites of interest

TxDOT - Loop 1604 at FM 2996/Blanco Rd public hearing
TxDOT - Loop 1604 at FM 2996/Blanco Rd visualization
Wikipedia - Diverging Diamond Interchange
FHWA - Diverging Diamond Interchange Informational Guide
YouTube - How Diverging Diamonds Keep You From Dying
Alternative Intersections - Diverging Diamond Interchanges
The Diverging Diamond Interchange Website
TXDOT - DDI Fact Sheet
(created for RM 1431 project but applies generally)
TxDOT - I‑35/RM 1431 Diverging Diamond Interchange Visualization