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Other San Antonio Area Roads
Alternative Intersection Designs

This page last updated May 12, 2022


In recent years, many states and local jurisdictions have started implementing a variety of "alternative", "innovative", 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.

One of the biggest problems in a conventional intersection are left turns. Left turns are the most inefficient movement 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.

What is a signal phase?

A signal "phase" is the green time assigned to a specified movement or collection of 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 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 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 the intersection designs above deals 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 therefore less congestion. They also are generally safer.

As with anything new and unconventional, there will always be a cadre of naysayers. Some have legitimate gripesó after all, every big change has tradeoffs. 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??), 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.


A roundabout works by providing a continuous, one-way circular flow of traffic through an intersection. 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 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. 

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. Double roundabouts (sometimes referred to as "dumbbells" or "dog bones") are now also being considered for a few access road intersections in fringe areas.

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.

Typical roundabout

(Source: Federal Highway Administration)

Tips for driving through a roundabout
Reduce speed and start looking for traffic to your left as you approach. If nobody is already in the roundabout to your left and there are no pedestrians trying to cross, then 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.

Once in the roundabout, keep moving-- do not try to be helpful and stop to let a driver approaching from the right 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. Watch for your exit and then veer right to leave the roundabout keeping an eye out for any pedestrians that may want to cross. If you miss your exit, just go around again.

More information:

Wikipedia - Roundabout
Cheddar - Why the US Hates Roundabouts
Insurance Institute for Highway Safety - Roundabout Q&A
Washington State Department of Transportation - Roundabout benefits
Federal Highway Administration - Roundabouts and Mini Roundabouts Safety

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 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 on primary road

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.

DLTs can be used at both freeway and non-freeway locations.

For a more extensive description of how a DLT works, as well as before and after data on the Bandera/1604 DLT, see the Bandera and Loop 1604 DLT project page.

DLT locations in San Antonio:

More information:

Wikipedia - Continuous-flow Intersection
TXDOT - Continuous Flow Intersections Fact Sheet
FHWA - Displaced Left Turn Intersection Informational Guide
DLT Case Study - Bangerter Highway in Salt Lake County (Utah)

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 an 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. 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 intersection.

DDI flow

DDI traffic flow on primary road

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 intersection.

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:

More information:

Wikipedia - Diverging Diamond Interchange
Alternative Intersections - Diverging Diamond Interchanges
The Diverging Diamond Interchange Website
TXDOT - DDI Fact Sheet (created for FM 1431 project but applies generally)
FHWA - Diverging Diamond Interchange Informational Guide

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 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 traffic flow from off-ramps

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
TXDOT - Single Point Urban Interchange Fact Sheet
FHWA - Alternative Intersections/Interchanges Informational Report

Restricted Crossing U-Turn (RCUT) 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 are no left turns at all on both roadways 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. Below are simplified schematics that show the allowed maneuvers in each type of intersection:

RCUT traffic flow Michigan Left traffic flow
(U-turns can also be provided on the cross street.)

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 two-phase signal at the main intersection. 

In an RCUT, through traffic on the cross street is also moved to the U-turns. This allows the large single intersection to be broken into two smaller independent intersections (one on each side of the primary road), each with their own two-phase signal.

In both cases, the simplified signals are then able to function more efficiently, which reduces wait times and congestion.

RCUT intersections are also known as "Superstreets". Michigan Left intersections are also known as a "Median U-Turn" or MUT intersection.

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

For a more extensive description of how RCUT intersections work and history of their use locally, see the Restricted Crossing U-Turn intersections page.

For a more extensive description of how RCUT intersections work and history of their use locally, see the Restricted Crossing U-Turn intersections page.

RCUT locations in San Antonio:

  • Bandera Rd and FM 1560 South (in operation since 2018)
  • US 281 between Encino Rio and Marshall Rd (in use from 2010 until replaced by a freeway in 2020 and 2021)
  • Loop 1604 between New Guilbeau Rd and Shaenfield Rd (in use from 2011 until replaced by a freeway in 2016)
  • Loop 1604 between US 90 and Macdona Lacoste Rd. (construction expected to start in 2024)
  • Bandera Rd, at Hausman Rd./Leslie Rd. and at Cedar Trail (construction to start in 2022)
  • Bandera Rd, multiple intersections from FM 1560 South to Triana Pkwy (proposed)
  • Bandera Rd, multiple intersections from Loop 410 to Loop 1604 (previously studied but shelved, now the recommended concept after further study of other alternatives.)

More information:

Utah DOT - ThrU-Turn Follow-Up
(Excellent video showing outcome of a Michigan Left project in Draper, Utah)
Pape-Dawson "Superstreets in Texas" presentation
Wikipedia - Superstreet
Federal Highway Administration - Restricted Crossing U-Turn Intersection Informational Guide
An Update on Superstreet Implementation and Research
NC Department of Transportation presentation on superstreets
(Excellent explanation of all aspects of superstreets)


Additional information

Federal Highway Administration - Alternative Intersections/Interchanges Informational Report
Federal Highway Administration - Alternative Intersection Design
Virginia Department of Transportation - Innovative Intersections and Interchanges
University of Maryland ATTAP - Unconventional Arterial Intersection Design
CityLab - Could These Crazy Intersections Make Us Safer?

This page and all its contents are Copyright © 2023 by Brian Purcell

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.