The Texas
Highway Man

Home Miscellany What's new About me Contact
Other San Antonio Area Roads
Innovative Intersection Designs

This page last updated February 20, 2024

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

The Problem
One of the biggest problems in a conventional intersection are left turns. Left turns are the most inefficient movements at an intersection. This is especially true at signalized intersections because they require extra signal phases that add delay to the other movements. At intersections with heavy left turn volumes, that delay can be significant. This then causes cumulative delays resulting in poor intersection throughput and chronic congestion.

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.

Left turns are also considered to be the most dangerous maneuver at most intersections, especially those with no signals or with permissive left turn signals.

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:

To date, all of the above except the "Michigan Left" have been implemented in San Antonio or are planned; these 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 simple traffic signals) whereas the other designs are intended for high-volume intersections. SPUIs are typically only used at intersections with freeways, while the others can be used at both freeway and arterial intersections (although RCUTs and Michigan Lefts are typically not used at freeway intersections.)

Each of these intersection designs deal with left turns in different and unconventional ways in order to either eliminate the green time required for them or combine those phases with other phases. Doing so means fewer phases per cycle, which allows more green time for the remaining phases. This results in more throughput and less waiting, and therefore reduced congestion. They also are generally safer.

Cynics abound
As with anything new and unconventional, there will always be a cadre of naysayers. Some have legitimate gripes — after all, every big change has trade-offs. 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. And frankly, many detractors just don't like change, don't fully understand how the intersection functions, or, perhaps unknowingly, just don't like something that forces them to actually pay attention while they drive (say what??), so they 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.

ADVERTISEMENT




Roundabouts

A roundabout works by providing a continuous, one-way circular flow of traffic through an intersection around a central island. Because traffic entering a roundabout is not required to stop if there is no conflicting traffic, the roundabout can move more traffic over the same period of time than a four-way stop or signal. Furthermore, unlike a conventional intersection, the 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%, and typically a 50% reduction of vehicle stops at intersections that have been 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 38% reduction in overall collisions and an astonishing 90% reduction in fatal collisions at intersections converted to roundabouts.

While at one point roundabouts were novel, most drivers by now are familiar with them.

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 downtown at Romana Plaza near the Central Library. Double roundabouts (sometimes referred to as "dumbbells" or "dog bones") are now also being considered for a few access road intersections in the New Braunfels area.

As mentioned earlier, roundabouts are typically limited to low-volume intersections such as in neighborhoods or on minor arterials, usually in lieu of four-way stops or two-phase traffic signals. Roundabouts lose their effectiveness under heavy traffic loads and therefore are inappropriate on most major roads.

Roundabout vs. traffic circle
While some people use the terms interchangeably, a roundabout and a traffic circle are technically two different things. A traffic circle is larger than a roundabout and may have different right-of-way control at each entrance, including possibly traffic signals.

Roundabout schematic

(Source: Federal Highway Administration)

Tips for driving through a roundabout
Reduce speed and start looking for traffic from your left as you approach. If nobody is already in the roundabout immediately to your left, and there are no pedestrians trying to cross, then you can enter the roundabout without stopping. If someone is approaching the roundabout on your left but has not already entered it, you don't need to wait for them — you have plenty of time to enter the roundabout and move on before they're in a position to be a hazard to you or vice-versa. You do not need to signal when entering the roundabout and should not unless you will be taking the first exit out of the roundabout.

Once in the roundabout, keep moving — do not try to be "helpful" and stop to let a driver approaching from the right to enter. Doing so is the equivalent of stopping at a green light; drivers behind you won't expect it and may rear-end you, not to mention that stopping unnecessarily gums-up the works.

To leave the roundabout, watch for your exit. Just before you exit, signal right. Then veer right to leave the roundabout keeping an eye out for any pedestrians that may be crossing or preparing to do so. If you miss your exit, just go around again.

Roundabout

Roundabout tips
A: This driver is yielding properly. If they enter now, they will cause a collision.
B: This driver should continue moving and NOT stop to let driver A enter the roundabout.
C: This driver is entering properly as there is nobody nearby to yield to.
D: This driver is yielding unnecessarily because they think they have to stop if a vehicle is anywhere in the roundabout or even just approaching it.
E: This driver is pretty annoyed with the driver of vehicle D and nearly rear-ended them because driver D stopped unexpectedly and unnecessarily.
(Source: Modified from reddit)

More information:

Wikipedia - Roundabout
https://en.wikipedia.org/wiki/Roundabout
Cheddar - Why the US Hates Roundabouts
https://www.youtube.com/watch?v=AqcyRxZJCXc
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 a 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 of the 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 that 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 multiple directions means that only one green phase is needed for both directions on the major road instead of the two or three 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.

DLTs can be used at both regular intersections (i.e. the intersection of two arterials), or at intersections of an arterial with a freeway. In San Antonio, the existing and planned DLTs are the latter, but there is an arterial-to-arterial DLT in Austin.

DLT schematic

Arterial/freeway DLT intersection schematic
The arrows show the traffic flow for traffic on the arterial.
(Graphic by Brian Purcell)

Improvement statistics
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%. Crashes at the Bandera/1604 intersection decreased 44% during the first nine months of operation. A further review in 2022 shows that, despite traffic having returned to pre-COVID levels, crashes at Bandera and Loop 1604 are less than half of what they were before the DLT.

DLT locations in San Antonio:

For a deeper dive on how a DLT works, as well as before and after data on the Bandera/1604 DLT, see the Bandera and Loop 1604 DLT page.

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



ADVERTISEMENT




Diverging Diamond Interchange (DDI)

A DDI increases the throughput of an intersection by shifting all traffic on the cross street at an interchange over to the left side as it passes 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 one access road can proceed at the same time that traffic coming on the cross street from its left is also moving. This overlapping eliminates one of the two signal phases required for those movements 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 on the freeway.

The major drawback of a DDI is that it 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.

DDI schematic

DDI intersection schematic
The arrows show the traffic flow for traffic on the cross-street.
(Graphic by Brian Purcell)

Improvement statistics
A study of a 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 intersection. 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 intersection.

DDI locations in San Antonio:

For a deeper dive on how a DDI works, see the Blanco and Loop 1604 DDI project page.

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
https://safety.fhwa.dot.gov/intersection/crossover/fhwasa14039.pdf


Single-Point Urban Interchange (SPUI)

A SPUI elongates a standard freeway-arterial 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 between the access roads. To do this, the access road approaches are angled 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 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 schematic

SPUI intersection schematic
The arrows show the traffic flow for the off-ramps.
(Graphic by Brian Purcell)

SPUI locations in San Antonio:

For a deeper dive on how a SPUI works, see the Austin Highway and Harry Wurzbach SPUI page.

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/


Restricted Crossing U‑turn (RCUT) and Median U‑turn (MUT) 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 MUT, there are no left turns at all on either roadway at the main intersection, while in the RCUT, there are left turns from the primary road but no left turns or straight-through traffic on the cross street.

RCUT intersections are also known as "Superstreets", and MUT intersections are also known as "Michigan Lefts".

Below are simplified schematics that show the allowed maneuvers in each type of intersection:

RCUT ("Superstreet") traffic flow
MUT ("Michigan Left") traffic flow
When both roadways have heavy traffic volumes, U‑turns can also be provided on the cross street.
(Graphics by Brian Purcell)


RCUT ("Superstreet") traffic flow
 
 

MUT ("Michigan Left") traffic flow
When both roadways have heavy traffic volumes, U‑turns can also be provided on the cross street.
(Graphics by Brian Purcell)

In both cases, the left turns from the cross street are moved to the U‑turn crossovers, thereby eliminating that time from the signal cycle.

In the Michigan Left, the left turns from the primary road are also moved to the U‑turns, so that time is also eliminated, resulting in a simple and very efficient two-phase signal at the main intersection.

In an RCUT, straight-through traffic on the cross street is also moved to the U‑turns. This has the benefit of allowing the large single intersection to be broken into two smaller independent intersections (one on each direction of the primary road), each with their own two-phase signals. This allows each direction of the primary road to be timed and coordinated independently from each other, resulting in even more efficiencies.

RCUT and MUT locations in San Antonio:

There are no Michigan Left intersections in San Antonio, although one was evaluated for the NW Military Hwy./Wurzbach Pkwy. intersection.

For a deeper dive on how RCUT intersections work and the history of their use locally, see the Restricted Crossing U‑turn intersections page.

More information:

Wikipedia - Superstreet
https://en.wikipedia.org/wiki/Superstreet
Pape-Dawson "Superstreets in Texas" presentation
https://drive.google.com/file/d/16b1w_qy_D8A2U_ZWhkG8jtkPZFqcNXlm/view
Utah DOT - ThrU-Turn Follow-Up
(Excellent video showing outcome of a Michigan Left project in Draper, Utah)
https://youtu.be/UDbP7NiAMHE
Federal Highway Administration - RCUT/MUT intersections
https://highways.dot.gov/safety/proven-safety-countermeasures/reduced-left-turn-conflict-intersections
An Update on Superstreet Implementation and Research (Report)
http://www.teachamerica.com/accessmanagement.info/AM08/AM0807Hummer/AM0807Hummer.pdf
An Update on Superstreet Implementation and Research (Presentation)
https://pdfs.semanticscholar.org/f879/03735f7e186859353e32b9280086a4908270.pdf
NC Department of Transportation presentation on superstreets
(Excellent explanation of all aspects of superstreets)
https://www.partnc.org/DocumentCenter/View/331/NCDOT-Superstreet-Presentation-PDF


Other sites of interest

TxDOT - Innovative Intersections
https://www.txdot.gov/about/programs/innovative-transportation/innovative-intersections.html
Federal Highway Administration - Alternative Intersections/Interchanges Informational Report
https://www.fhwa.dot.gov/publications/research/safety/09060/
Federal Highway Administration - Alternative Intersections/Interchanges: Informational Report
https://rosap.ntl.bts.gov/view/dot/37466
Virginia Department of Transportation - Innovative Intersections and Interchanges
https://virginiadot.org/innovativeintersections/
University of Maryland ATTAP - Unconventional Arterial Intersection Design
http://attap.umd.edu/2015/09/30/unconventional-arterial-intersection-design/
CityLab - Could These Crazy Intersections Make Us Safer?
https://www.citylab.com/solutions/2013/01/could-these-crazy-intersections-make-us-safer/4467/