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[i] Information SAN ANTONIO FREEWAY HISTORY
Read my essay on San Antonio's freeway history for the Express-News' Tricentennial series here. The full history of the freeway system is available on this site here.

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

This page last updated December 4, 2017

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Project locationLocation: 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 will also 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 WB 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 WB side, this will produce the benefit of taking through traffic going to Huebner out of 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 road.

This will be the first DDI in San Antonio and the fourth in Texas (the others are in El Paso, The Colony, and Round Rock next to the IKEA.) The design has been used successfully in nearly 100 other locations around the US as well.


CONVENTIONAL (EXISTING) INTERSECTION
TRAFFIC FLOW FOR BLANCO RD.

Conventional flow


DIVERGING DIAMOND INTERSECTION
TRAFFIC FLOW FOR BLANCO RD.

Conventional flow


CONVENTIONAL (EXISTING) INTERSECTION
TRAFFIC FLOW FOR ACCESS ROAD LEFT TURNS TO BLANCO RD.

Conventional flow


DIVERGING DIAMOND INTERSECTION
TRAFFIC FLOW FOR ACCESS ROAD LEFT TURNS TO BLANCO RD.

Conventional flow


CONVENTIONAL (EXISTING) INTERSECTION
TRAFFIC FLOW FOR ACCESS ROAD THROUGH TRAFFIC

Conventional flow


DIVERGING DIAMOND INTERSECTION
TRAFFIC FLOW FOR ACCESS ROAD THROUGH TRAFFIC

Conventional 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 but are not sufficient to justify flyovers now or during the next 20 years. Thus, this intersection is considered to be in the "sweet spot" for an "outside-the-box" (no pun intended) intersection design. Computer modeling shows a dramatic improvement in this intersection's operation with a DDI and the additional lanes over additional lanes alone.

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 by shifting all traffic to the left side of the road through the interchange. This then allows two movements that would normally be conflicting to go at the same time. For example, traffic turning left from the EB access road could move at the same time as SB traffic on Blanco. This overlapping eliminates one of the two signal phases required for them in a conventional intersection, making the signals much more efficient by allowing more traffic to move through the intersection in the same amount of time.

DDIs also improve safety by reducing 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.

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 WB access road will also be controlled by a signal (this is typical for dual right turn lanes.) These are simple, two-phase signals and will be tightly coordinated and operated from the same signal controller.

Additional lane markings and signage (including large overhead signs) will help channel traffic into the correct lanes. The crossover intersections will be angled sufficiently to make the crossover transition much more intuitive. Additionally, barriers will be placed between the opposing traffic lanes in the "wrong-side" area to block 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 has also improved at DDI intersections. A study of safety improvements at DDI intersections in Colorado showed a 36% reduction in crashes. 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 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 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 conflict between left turns and opposing traffic. 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, thus moving more traffic through the intersection in the same amount of time.

  • The new third lane on Blanco needs to extend further. It will create a bottleneck.
    The scope of this project was limited to this intersection and immediate vicinity. The additional lanes on Blanco are considered to be "auxiliary" lanes intended to help traffic get through the intersection and then smoothly merge together. If additional capacity is needed on Blanco beyond this point, that is a much bigger project requiring separate study and additional funding. TxDOT 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 WB 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 the 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 access road intersections to instead pass below without stopping. On the WB side, this will have the major benefit of taking the Huebner-bound through traffic out of the Blanco intersection. 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. 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 access roads 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 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 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 turn from the access roads to the cross street like a DDI does.

  • 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 as one of the top 100 innovations in 2009 and has been featured in Time, Forbes, NPR, and many other publications.

Schematics
Below are the detailed project schematics with annotations. The project schematics are courtesy of TxDOT and subject to change. Annotations are my own. Below that is a "how-to" video on navigating a DDI .


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

Blanco/1604 DDI intersection close-up


Collector/distributor road and ramps

 

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 meeting
https://www.txdot.gov/inside-txdot/get-involved/about/hearings-meetings/san-antonio/113017.html
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




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This page and all its contents are Copyright 2017 by Brian Purcell

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