Home | About me | Contact | What's new | Privacy | Search
 

San Antonio
Freeway system
  Primer
  Fwy system history
  2016 traffic statistics
  The freeways
    I-10 East
    I-10 West
    I-35 North
    I-35 South
    I-37
    Kelly Pkwy
    Loop 410 (I-410)
    Loop 1604
    SH 151
    Spur 371
    US 90 West
    US 281 North
    Wurzbach Pkwy
  Construction projects
    I-10E Expansion
    I-10E/Loop 410
    I-10 Boerne area
    I-10/Dominion
    I-10/Woodlake
    I-10/Wurzbach
    I-35 Comal Projects
    I-35 NEX
    LP 1604 North
    LP 1604/Bandera
    LP 1604/Bndra-Hsmn
    LP 1604/Blanco
    LP 1604/Bulverde
    LP 1604/FM 78-I-10
    LP 1604/I-35-FM 78
    LP 1604/Marbach
    LP 1604/Potranco
    SH 151/LP 1604
    SH 151/Loop 410
    US 90W Expansion
    US 90/Loop 410
    US 281 North Exp
    US 281/Basse
    US 281/Jones-Mltsbrg
    Wurzbach/NW Military
  Etcetera
    HOV lanes
    Media galleries
    Tollway system
    TransGuide
    10/1604 yield signs
    History of 281/410
Other roads
History
FAQs

Search this site


Twitter

Facebook

Blog

ADVERTISEMENT

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

This page last updated April 11, 2019

ADVERTISEMENT

Project locationNOTE: As this project is currently in planning, all information is subject to change.

Location
Loop 1604 at Blanco Rd.

Status
Advanced planning (see timeline below)

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 project also proposes to add the following:

  • One additional through lane on Blanco (for a total of three) between Madera Parkway and Country Club Lane
  • An additional left turn lane (for a total of three) on both access roads at Blanco
  • A second right-turn lane to Blanco on the westbound access road
  • An access road bypass (collector/distributor road) to allow straight-through traffic on the access roads to completely bypass the Blanco/1604 intersections

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. This will allow traffic turning left to Loop 1604 to then make a free left turn onto the access road. 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.)

A DDI blocks straight-through traffic on the access roads. To accommodate this through traffic, a bypass (collector/distributor road) will be built below Blanco next to but separate from the Loop 1604 mainlanes. This bypass roadway will also serve Loop 1604 traffic headed to and from Huebner and Stone Oak Parkway. On the westbound side, this will produce the benefit of removing all that through traffic going to Huebner from the Blanco Road intersection. All of the entrance and exit ramps between Huebner and Stone Oak will need to be adjusted to accomodate the collector/distributor roads.

This will be the first DDI in San Antonio and likely the sixth in Texas; the others are in El Paso, The Colony, Round Rock (next to the IKEA), South Austin, and College Station. 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-Maltberger and another at I-37 and SE Military Dr.


CONVENTIONAL INTERSECTION (EXISTING)
TRAFFIC FLOW FOR BLANCO RD.

Conventional flow


DIVERGING DIAMOND INTERSECTION (PROPOSED)
TRAFFIC FLOW FOR BLANCO RD.

DDI flow


CONVENTIONAL INTERSECTION (EXISTING)
TRAFFIC FLOW FOR ACCESS ROADS

Conventional flow


DIVERGING DIAMOND INTERSECTION (PROPOSED)
TRAFFIC FLOW FOR ACCESS ROADS

DDI 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!

How a DDI improves traffic flow: A DDI is a relatively low-cost improvement that increases the throughput of an intersection by eliminating the conflicts caused by left turns. This is accomplished 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 no longer have to cross oncoming traffic. Since this is done across the interchange from where the left turn takes place, it creates "unused" space on the opposite side of the road to allow for another movement that would normally be conflicting to go at the same time. For example, traffic turning left from the eastbound access road can move at the same time as southbound traffic on Blanco. This overlapping reduces the number of signal phases from the four required in a conventional intersection to just two, making the signals much more efficient by allowing more traffic to move through the intersection in the same amount of time. (A signal "phase" is green time assigned to a specified movement or collection of movements in a traffic signal cycle.)

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

The diagrams below should help to illustrate the description above. 

CONVENTIONAL INTERSECTION
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

Phase 1

Phase 2

Phase 3

Phase 4


DIVERGING DIAMOND INTERSECTION
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

Phase 1


Phase 2


If each phase in the conventional intersection is 30 seconds, that's a cycle time of two minutes. If each phase in the DDI is also 30 seconds, you can complete two signal cycles in the same two minute period; 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 may 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 above.


EXAMPLE TIMINGS

Simplified timeline showing example signal phase timings for each movement in the Loop 1604/Blanco 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. Be sure to read the explanation in the paragraph above.


Conventional intersection



DDI intersection


Signs, markings, and signals: To properly control traffic and minimize potential conflicts, traffic signals will be located at the crossover locations as well as the left turns from the access roads. The dual right turn lanes on the westbound access road will also be controlled by a signal (this is not required for the DDI but is typical for dual right turn lanes.) These are all simple, two-phase signals that will be tightly coordinated and operated from the same signal controller.

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 from previous projects the optimal angles for this. 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.

Driving through a DDI is actually far easier than it may seem from the diagrams and descriptions.

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 reported they felt the DDI was safer than the previous conventional intersections.

Drawbacks/cons: 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.

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 is not yet scheduled. As of this writing, it is anticipated that work could begin in 2021 and take about two years to complete.

FAQ

  • This looks confusing. 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" video at the bottom of this page. Statistics from DDI implementations show a significant reduction in crashes and practical experience from other DDI locations shows drivers quickly adapt to it.

  • How does this improve traffic?
    This intersection design improves traffic because shifting the traffic to the left side of the road through the intersections removes the conflicts between left turns and opposing traffic found in a conventional intersection. As a result, traffic in two of the four legs of the intersection can move during a single green phase instead of needing two separate phases. This reduces the total number of green phases required from four to two, thus moving about twice as much traffic through the intersection in the same amount of time.

  • The new third lane on Blanco needs to extend further or it will create a bottleneck.
    The scope of this project was limited to this intersection and immediate vicinity. The additional lanes that will be added on Blanco are considered to be "auxiliary" lanes intended to help traffic get through the intersection and then smoothly merge back into the through lanes. If additional capacity is needed on Blanco beyond this point, that is a much bigger project requiring separate study and additional funding. That said, TxDOT has said it will monitor the situation after implementation. Keep in mind that due to funding and other constraints, road improvements often need to be incremental, much like unkinking a hose one kink at a time.

  • It looks like there will be no straight-through traffic on the access roads. This will prevent access to nearby businesses, severely inconvenience drivers, and cause extra traffic on Blanco. This will also block westbound through traffic headed to Huebner.
    It is correct that traffic on the access roads will have to turn left or right onto Blanco. Keeping the through movement on the access roads would cancel-out much of the improvement this design provides. However, an access road bypass will be provided below Blanco (next to the Loop 1604 mainlanes) that will allow most of the traffic that would normally go straight through the intersections on the access roads to instead pass below without stopping. This will have the major benefit of taking traffic going to and coming from Huebner out of the Blanco intersections. The ramps to and from this bypass will be located about 1000 feet from the Blanco intersection, so motorists going to or from businesses located between these points may need to turn onto Blanco and make a U-turn to get to or depart from those locations; this will most especially affect the businesses at the NE and SE corners; the shopping centers at the other corners have plenty of access points that will minimize the affect of this change. While this is a bit of an inconvenience, it is no more so than what is required on streets with medians or when accessing businesses on the opposite access road between interchanges and is a small trade-off for the significant improvements this change will bring.

  • Will there still be turnarounds?
    Yes, the turnarounds in both directions will remain
    .

  • Why not just add the extra turning and through lanes?
    While this would help move more vehicles through the intersection on each cycle, the improvements would be short-lived as traffic increases to fill the new lanes. The DDI dramatically improves the underlying efficiency of the traffic signals themselves which provides longer-term relief.

  • Why not build flyovers instead like the ones at Bandera and Loop 410?
    This intersection does not have current or projected traffic volumes or patterns to justify flyovers.

  • How is this different than the intersection under construction at Bandera and Loop 1604?
    The DDI is similar to the Displaced Left Turn (DLT) intersection being built at Bandera 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 DDIs planned in San Antonio?
    Yes, two other DDIs are currently planned: one at Military and I-37 and another at Jones-Maltsberger (South) and US 281. Other alternative intersections are also being planned for other locations; see the 
    alternative intersections page for more info.

  • Who came up with this cockamamie design?
    The DDI design has been used in France since the 1970s and made its debut in the United States in 2009. It is now in use in nearly 100 locations in the US and internationally and has a proven track record of improving traffic and safety wherever it has been implemented. It is one of several types of "innovative" or "alternative" intersection designs being implemented across the country at intersections that do 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 substantial reduction in delays. The DDI concept was hailed by Popular Science magazine as one of the top 100 innovations in 2009 and has been featured on National Public Radio and in Time, Forbes, and many other publications.

Schematics
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.


DETAILED PROJECT SCHEMATICS
(Click the images below to see the detailed annotated schematic)

Blanco/1604 DDI intersection close-up


Collector/distributor road and ramps



How to drive through a DDI

For a video of how to drive through a DDI, see https://www.youtube.com/watch?v=GnukMEsezJU


Other sites of interest

TxDOT - Loop 1604 at FM 2996/Blanco Rd public hearing
https://www.txdot.gov/inside-txdot/get-involved/about/hearings-meetings/san-antonio/032819.html
TxDOT - Loop 1604 at FM 2996/Blanco Rd visualization
https://www.youtube.com/watch?v=TlYuEQXlCbw
Alternative Intersections - Diverging Diamond Interchanges
http://www.divergingdiamondinterchange.org//
The Diverging Diamond Interchange Website
https://www.divergingdiamond.com/
TXDOT - DDI Fact Sheet (created for FM 1431 project but applies generally)
http://ftp.dot.state.tx.us/pub/txdot-info/aus/rm1431-intersection/fact-sheet.pdf
FHWA - Diverging Diamond Interchange Informational Guide
https://safety.fhwa.dot.gov/intersection/alter_design/pdf/fhwasa14067_ddi_infoguide.pdf
Wikipedia - Diverging Diamond Interchange
https://en.wikipedia.org/wiki/Diverging_diamond_interchange
TxDOT - I-35/RM 1431 Diverging Diamond Interchange Visualization
https://www.youtube.com/watch?v=2i6_UYu-tfY




If you found this informative, please consider making a small donation to help support it. Thanks!
This page and all its contents are Copyright 2019 by Brian Purcell

NOTICE
The information provided on this website is provided on an "as-is" basis without warranties of any kind either express or implied.  The author and his agents make no warranties or representations of any kind concerning any information contained in this website.  This website is provided only as general information.  The author expressly disclaims all liability with respect to actions taken or not taken based upon the information contained herein or with respect to any errors or omissions in such information.  All opinions expressed are strictly those of the author.  This site is not affiliated in any way with any official agency.