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

San Antonio
Freeway system
Other roads
  Alt intersections
  Bandera Rd proposals
  Cibolo Parkway
  The Circuit
  Hry Wz/Aus Hwy SPUI
  I-10 Boerne Projects
  I-35/SA-Aus Corridor
  LP 1604 South Projects
  SH 130
  SH 211
  Superstreets
  Loop addressing
  Which Military?
  Which Wurzbach?
History
FAQs


Twitter

Facebook

Blog

ADVERTISEMENT

Other San Antonio Area Roads
Alternative Intersection Designs

This page last updated August 14, 2018

ADVERTISEMENT

In recent years, many states and local jurisdictions have started implementing a variety of "alternative" or "innovative" intersection designs to help improve traffic flow and safety at a relatively low cost. That trend has made its way to San Antonio.

One of the biggest problems in a conventional intersection is the handling of left turns. Doing so means introducing extra signal phases that add delay to the through movements. At intersections with heavy left turn volumes, that delay can be signficiant. This then causes cumulative delays resulting in poor intersection throughput and chronic congestion.

To solve this problem, a number of novel intersection designs have been developed over the years both in the US and abroad. These include the following:

  • Roundabouts
  • Displaced Left-Turn (DLT) intersections
  • Diverging Diamond Intersections (DDI)
  • Single-Point Urban Interchanges (SPUI)
  • Restricted crossing/Median U-turn intersections (aka "Superstreet" and "Michigan Left")

To date, all of the above except the "Michigan Left" have been implemented in San Antonio or are planned and are described in more detail below. There are several more designs that aren't in use or planned for San Antonio including the Jughandle, Center Turn Overpass, Michigan Urban Diamond (similar to but different than the Michigan Left), Echelon, Raindrop, and Parallel Flow; you can read all about these at the "Alternative Intersections/Interchanges Informational Report" link at the bottom of this page.

In San Antonio and most of the US, roundabouts have been typically limited to low-volume intersections (usually in lieu of four-way stops or short-cycle traffic signals) whereas the other designs are geared for high-volume intersections. SPUIs are typically only used at grade-separated (i.e. freeway) intersections while the others can be used at both freeway and arterial intersections although Superstreets and Michigan Lefts are typically not used at freeway intersections.

Each of the intersection designs above deals with left turns in different and unconventional ways in order to either eliminate the signal phases required for them or combine those phases with other phases. Doing so means fewer phases per cycle, which allows more green time for the through movements. This results in more throughput and therefore less congestion.

As with anything new and unconventional, there will always be a cadre of naysayers. Some have legitimate gripesó after all, every big change has pros and cons. But oftentimes those folks don't grasp that that the overall benefits of these "outside-the-box" designs outweigh the drawbacks (not to mention that the conventional approach obviously isn't working.) But frankly many detractors just don't like change, can't wrap their minds around it, or, perhaps unknowingly, don't like something that forces them to actually pay attention while they drive (say what??) and so will find every shortcoming in a plan as proof that it's a bad idea. But there were naysayers back when the first cars started roaming the roads, so I guess some things never change. </soapbox>

So without further pontificating, here are some basic descriptions of the alternative intersection designs currently in use or planned for San Antonio, how they work and improve traffic, and where they are or will be used.


Roundabouts

A roundabout works by providing a continuous, one-way circular flow of traffic. Because traffic entering a roundabout is not required to stop if there is no conflicting traffic, it can move more traffic over the same period of time than a four-way stop or signal. Furthermore, unlike a conventional intersection, the extra space in a roundabout and its flow allows a vehicle on all four approaches to enter and traverse the intersection simultaneously, again helping to move more traffic. Studies have consistently shown a reduction in delays ranging from 13% to 89% at intersections converted to roundabouts. Additionally, the curved roads in a roundabout eliminate the possibility for T-bone and head-on collisions and reduce the speed of traffic through the intersection, thus improving safety. Indeed, studies have shown a 37% reduction in overall collisions and an astonishing 90% reduction in fatal collisions at intersections converted to roundabouts.

Typical roundabout

Typical roundabout features

One of the first roundabout retrofits in San Antonio was at Blanco and Fulton where it replaced a traffic signal. It was met with much skepticism, criticism and predictions of mayhem and carnage because of the adjacent elementary school, none of which materialized. 

There are now far too many roundabouts in the San Antonio area to list them all. Dozens of neighborhoods and shopping centers have them. Notable locations include the McCullough/Olmos/Ed Prado intersection in Olmos Park, Ewing Halsell and Charles Katz in the Medical Center (which includes separate right-turn lanes on a couple of the approaches), the main entrance to UTSA, and a new one downtown at Romana Plaza. Double roundabouts (sometimes referred to as "dumbbells") are now also being considered for a few access road intersections in fringe areas.

More information:

Wikipedia - Roundabout
https://en.wikipedia.org/wiki/Roundabout
Insurance Institute for Highway Safety - Roundabout Q&A
http://www.iihs.org/iihs/topics/t/roundabouts/qanda
Washington State Department of Transportation - Roundabout benefits
https://www.wsdot.wa.gov/Safety/roundabouts/benefits.htm
Federal Highway Administration - Roundabouts and Mini Roundabouts Safety
https://safety.fhwa.dot.gov/intersection/innovative/roundabouts/


Displaced Left-Turn (DLT) Intersection 

A DLT increases the throughput of an intersection by allowing traffic headed in both directions on the major road (including left turns) to all move simultaneously (or nearly so.) 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. The displacement also provides a short overlap period that permits left-turning traffic to be released even while the intersecting road still has a green light. This "continuous flow" of traffic in both directions means that only one green phase is needed for both directions on the major road instead of the two phases required at the conventional intersections. The time saved from having only one phase can then be redistributed to extend the green time for all approaches, thus moving more traffic through the intersection on each cycle without having to increase the overall cycle length. 

Conventional flow

DLT traffic flow at Bandera and Loop 1604


A study of four DLT intersections in other states showed a 10-30% increase in throughput and a 30-80% reduction in delays. A DLT in Baton Rouge 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 in Baton Rouge showed that 87% felt that traffic congestion was improved with 48% reporting their travel time "extremely decreased." Computer modeling of the Bandera/1604 intersection showed 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 or near a DLT in Utah were reported to have decreased a whopping 60%.

For a more extensive description of how a DLT works, see the Bandera and Loop 1604 DLT project page.

DLT locations in San Antonio:

  • Bandera Rd and Loop 1604 (under construction)
  • Culebra Rd and Loop 1604 (proposed)
  • Loop 410 at WW White Rd/Cornerway Blvd (proposed)

More information:

Wikipedia - Continuous-flow Intersection
https://en.wikipedia.org/wiki/Continuous-flow_intersection
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
DLT Case Study - Bangerter Highway in Salt Lake County (Utah)
https://www.youtube.com/watch?v=o5-U_TgEtJA


Diverging Diamond Intersection (DDI)

A DDI increases the throughput of an intersection by shifting all traffic on the major street to the left side of the road through the interchange. Doing so then allows two movements that would normally be conflicting to go at the same time. For example, the traffic from an access road can proceed at the same time that traffic coming on the cross street to its left is also moving. 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. 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.

DDI flow

DDI traffic flow at Blanco and Loop 1604

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 on the freeway. 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.

One of the drawbacks is that a DDI does not allow straight-through traffic on the access roads/ramps. If this is needed, it can be facilitated with a bypass roadway either above or below the intersection.

For a more extensive description of how a DDI works, see the Blanco and Loop 1604 DDI project page.

DDI locations in San Antonio:

  • Blanco Rd and Loop 1604 (proposed)
  • SE Military Dr and I-37 (proposed)
  • Jones-Maltsberger Rd (South) and US 281 (proposed)

More information:


Wikipedia - Diverging Diamond Interchange
https://en.wikipedia.org/wiki/Diverging_diamond_interchange
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


Single-Point Urban Interchange (SPUI)

A SPUI elongates a standard access road intersection so that the two intersections typically formed by the access roads on each side of the freeway are instead compressed into a single intersection located in the middle of the overpass/underpass. To do this, the access road approaches are bent inward so that the left turn movements can pass-by each other like they would at a typical surface street intersection. This allows the opposing left turn movements on each axis of the intersection to proceed simultaneously like they do in a typical surface street intersection, thus reducing the number of signal phases required from four to three. Right-turns still take place at the same location as they do in a conventional intersection. However, with a SPUI, there is no straight-through traffic on the access roads. If this is needed, it can be facilitated with a bypass roadway either above or below the intersection.

SPUI flow

SPUI left turn flow at Austin Highway and Harry Wurzbach

For a more extensive description of how a SPUI works, see the Austin Highway and Harry Wurzbach SPUI project page.

SPUI locations in San Antonio:


More information:

Wikipedia - Single-point urban interchange
https://en.wikipedia.org/wiki/Single-point_urban_interchange
TXDOT - Single Point Urban Interchange Fact Sheet
http://ftp.dot.state.tx.us/pub/txdot/my35/capital/concepts/spui-factsheet.pdf
FHWA - Alternative Intersections/Interchanges Informational Report
https://www.fhwa.dot.gov/publications/research/safety/09060/


"Superstreet" and "Michigan Left" intersections

These are essentially two versions of the same concept. Both use U-turns on either side of the main intersection to facilitate some or all left turns. In the "Michigan Left", there is only straight-through traffic and no left turns at all on both roadways at the main intersection, while in the "superstreet", there are left turns from the major street but no left turns or straight-through traffic on the cross street. Below are simplified schematics that show the allowed maneuvers in each type of intersection:

Superstreet traffic flow Michigan Left traffic flow

In both cases, the left turns from the cross street are instead shifted to the U-turn crossovers, thereby eliminating that phase from the signal cycle. In the Michigan Left, the left turns from the major street are also moved to the U-turns, so that phase is also eliminated, resulting in a simple two-phase signal at the main intersection. In a superstreet, because the through traffic on the cross street is also eliminated, it makes each half of the main intersection two smaller independent intersections instead, each with their own two-phase signal. These simplified signals are then able to function more efficiently.

For more details on how a superstreet works and its local history, see the Superstreets page.

There are no Michigan Left intersections in San Antonio.

Superstreet locations in San Antonio:

  • US 281 between Encino Rio and Marshall Rd
  • Loop 1604 between New Guilbeau Rd and Shaenfield Rd (replaced with freeway)
  • Bandera Rd at FM 1560 (under construction)
  • Bandera Rd from Cedar Trail to Triana Parkway (proposed)
  • Bandera Rd from Loop 1604 to Loop 410 (studied but not currently planned)


More information:

Wikipedia- Superstreet
https://en.wikipedia.org/wiki/Superstreet
An Update on Superstreet Implementation and Research
http://www.accessmanagement.info/AM08/AM0807Hummer/AM0807Hummer.pdf
NC Department of Transportation presentation on superstreets
(excellent explanation of all aspects of superstreets)

http://www.partnc.org/wp-content/uploads/2014/09/NCDOT_SuperstreetPresentation.pdf


Additional information

Federal Highway Administration - Alternative Intersections/Interchanges Informational Report
https://www.fhwa.dot.gov/publications/research/safety/09060/
Federal Highway Administration - Alternative Intersection Design
https://safety.fhwa.dot.gov/intersection/alter_design/
CityLab - Could These Crazy Intersections Make Us Safer?
https://www.citylab.com/solutions/2013/01/could-these-crazy-intersections-make-us-safer/4467/




If you found this informative, please consider making a small donation to help support it. Thanks!
This page and all its contents are Copyright © 2018 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.