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San Antonio Area Roads & More
Superstreets

This page last updated January 10, 2014
   
A "superstreet" is a roadway where intersections have been modified to eliminate left turns and straight-through traffic on the intersecting roadway.  Doing this reduces the number of traffic signal phases required to move traffic through the intersection thereby allowing for longer green times on the major roadway and thus reducing congestion caused by the signals.

The San Antonio area currently has two sections of superstreets: one on US 281 North (four intersections) and one on Loop 1604 West (two intersections).  In both cases, the superstreet provides a transition between the freeway and non-freeway sections of those roads and both are considered to be "short-term" improvements while planning is completed and funding acquired for more substantial upgrades.  A superstreet has also been proposed along Bandera Rd. between Eckhert and Loop 1604.

These were the first two superstreets in Texas.  A third is now operational in Austin.


Please note that the intersection diagrams below are generalized for the types of intersections involved and not intended to reflect specific existing or future conditions on either 281 or 1604.  They are intended instead to illustrate the concepts being discussed.  Thanks to "SPUI" for the base superstreet illustration used in the intersection diagrams.

On this page:


Local superstreet history

US 281
In February 2009, with the proposed US 281 tollway stalled due to ongoing litigation that resulted in a new environmental study being required, engineers with Pape-Dawson Engineering made public an unsolicited proposal to the Alamo Regional Mobility Authority (ARMA) to convert US 281 into a "superstreet" from Encino Rio to Marshall.  The plan was expected to increase traffic throughput on 281 by at least 30%, increase average rush hour speeds by about 10 mph in both directions, and reduce travel time by about eight minutes between Loop 1604 and Marshall Rd.  The MPO approved funding for this project on March 23rd, 2009.  Funding came from a combination of funds from the federal stimulus, Advanced Transportation District, and City of San Antonio.  On January 14th, 2010, ARMA's board selected Ballenger Construction's bid of $5.2 million to build the project, which was nearly 20% below the final estimates and nearly 50% below initial projections.  Ground was broken on the project on March 11th. The first intersection (Encino Rio) was completed on August 29th.  Weather delayed work on the Marshall Rd. intersection; it was finally completed on September 26th.  The remaining intersections were completed on the weekends of October 2nd and 9th.

Loop 1604
In October 2009, with plans for the US 281 superstreet nearing completion, the Bexar County Commissioners Court approved funding to study a possible superstreet in the Loop 1604 West corridor, specifically between Braun Rd. and SH 151, as a possible interim solution to ease congestion while ARMA completes an environmental study on possible long-term improvements.  The superstreet study, completed in early 2010, showed that a combination of improvements along that section of 1604 would provide substantial improvements.  In May, ARMA previewed the initial designs and expected benefits.  Under the plan, the intersections of New Guilbeau and Shaenfield would be reconstructed as superstreet intersections.  It is estimated that by doing so, travel speeds could be increased by 36% and delays reduced by 69% during the morning peak and that average travel speeds could be increased by 65% and delays reduced by 79% during the evening peak period.  The superstreet improvements at New Guilbeau and at Shaenfield began construction in March 2011.  The New Guilbeau intersection was completed August 18th, followed on September 15th by the Shaenfield intersection.

Bandera Road
TxDOT has proposed a superstreet along Bandera Rd. between Eckhert and Loop 1604.  However, funding is currently unavailable.  To date, Leon Valley has not endorsed extending that south to Loop 410.


Conventional intersection operation

Before I explain how a superstreet works, it's important to understand how the conventional intersections on 281 and on 1604 functioned, especially the traffic signal operation.  Understanding the traffic signal phasing is vital to grasping the improvements provided by a superstreet.

The number of lanes and other minor differences aside, this is how the intersections on 281 and on 1604 were configured prior to the superstreet (make the obvious adjustments for the "T" intersections at Encino Rio, New Guilbeau, and Shaenfield):

 

The split-phase signal configuration necessitated by traffic patterns at these intersections required five signal phases per cycle (three for the "T" intersections) to allow for each movement through the intersection:

Phase 1


Phase 2


Phase 3


Phase 4


Phase 5


The number of phases and required length of each phase (due to heavy traffic volumes on all the approaches) caused signal cycles to be long, thus causing long queues and congestion, especially during peak periods.  Additionally, each signal phase adds up to 15 seconds of "lost" time due to yellow and all-way red clearance intervals as well as the time it takes for stopped motorists to react to a green and to start moving.


How a superstreet works

The superstreet reconfigured the intersections like this:

At first glance, I know it looks quite complicated, but it actually makes sense once you understand how it works and why.  The changes eliminate the ability of traffic on the cross-street to continue straight through the intersection or to turn left.  Instead, all traffic on the cross-street must turn right onto 281 or 1604, then use a downstream turnaround to either go the opposite direction or to return to the cross-street to continue on it.  Those turnarounds are located approximately 1000 feet (about 2/10th of a mile) from the intersection they serve to allow sufficient room for merging and storage.  By eliminating the through-traffic and left-turns from the cross-streets, it eliminates the signal phases required for those movements.  It is from eliminating those signal phases that the benefit is derived (see below.)

(Note that the Encino Rio intersection on US 281 is different than described above.  Traffic coming from Encino Rio is allowed to turn left onto 281 southbound.  In fact, an additional left turn lane was added for a total of three.  To still allow for operational improvement, traffic from southbound 281 is not allowed to turn left onto Encino Rio and northbound traffic is not allowed to turn left into the quarry.  Those wishing to do so instead continue along 281 for about 1000 ft. and then turn around, where they can then turn right onto Encino Rio or the quarry.)

The local superstreets don't eliminate the signalized intersections; in fact, it essentially doubles the number of signals in each direction.  Yes, it's counterintuitive that adding signals can help improve traffic flow.  However, by eliminating the straight-through and left turn movements on the cross-street, engineers are able to make some dramatic changes to the signal phasing.  Specifically, the number of signal phases is reduced from the previous five phases to just two (or from three to two at the "T" intersections):

Phase 1


Phase 2

 

Notice how many movements are accomplished in the second phase.  This essentially takes phases 4 and 5 and the left turn movements of phases 1 and 3 of the conventional intersection and combines them all into one, eliminating all the additional time required for those individual phases.  Furthermore, significant additional time savings is realized because every signal phase inherently includes some "wasted" time in the form of yellow and all-way red clearance intervals as well as the time it takes for drivers to start moving when they get a green light.  All of that can add up to 15 seconds of lost time per phase.  These time savings allow the green time for 281 and 1604 through traffic to be increased without having to correspondingly reduce the green time for the cross-street or lengthen the total cycle time.

Superstreet "secret sauce" explained
If the explanation above is still a bit muddy, here's another way of looking at it.  Imagine in the diagrams above that you're on 281 or 1604 headed from left to right.  In the conventional intersection, you reach the intersection just as the light turns red (phase 3).  You then have to wait through phase 3 for 20 seconds, phase 4 for 30 seconds, and phase 5 also for 30 seconds before you get a green light again on phase 1.  This means you've waited for 80 seconds
.  With the superstreet, all of the movements that took 80 seconds before can now be accomplished in a single 30 second phase, which means that through traffic on 281 or 1604 now only has to stop for 30 seconds.  The 50 seconds of time that's been freed-up by eliminating two phases can be added to the green time for 281 or 1604.  So if through traffic previously got 40 seconds of green time, it can get 90 seconds with the superstreet all without having to increase the overall two minute cycle time.  This an obvious and substantial improvement and is where the benefit of a superstreet is derived.  See the graphs below for a visulization of these timings.  (Note that the timings I provide here are for illustrative purposes only and do not necessarily reflect the actual current or previous phase and cycle times on either 281 or 1604.)
 

Example timings

Illustrative red and green signal phase times and associated movements for through traffic
on US 281 or Loop 1604 moving left to right in the diagrams above


Conventional intersection




Superstreet intersection



In addition to reducing signal phases, superstreets allow for much better synchronization of the signals.  With conventional intersections, signals along a corridor in both directions are inherently interdependent-- in other words, changes cannot be made to one direction without adversely affecting the other.  This dependency makes signal coordination and "progression" tricky, especially two-way progression.  But with a superstreet, the signals for traffic going in one direction on the major roadway (US 281 or Loop 1604 in this case) are completely independent of those for the other direction; in fact, separate signal controllers are used for each side of a superstreet intersection.  In effect, both sides of US 281 or Loop 1604 function like an independent one-way street.  This provides the ability to coordinate the signals much more easily and reliably without regards to the number of signals or their spacing, and the signals can react better to various traffic conditions.  Essentially, it provides the perfect environment for signal coordination.


Traffic flow through the intersections
So how does traffic flow through the superstreet intersections?

Straight-through traffic on US 281 or Loop 1604 and traffic turning right from 281 or 1604 to the cross-street works just like a conventional intersection:

 

Traffic turning left from 281 or 1604 to the cross-street also works just like a conventional intersection:

 

And traffic turning right onto 281 or 1604 from the cross-street works just like a conventional intersection:

 

However, traffic wanting to turn left from the cross-street onto 281 or 1604 instead turns right, then uses the downstream turnaround to head the other direction (for simplicity, this diagram just shows the maneuver for one direction):

 

Similarly, traffic wanting to continue straight on the cross-street across 281 or 1604 instead has to turn right, use the downstream turnaround, then turn right to get back onto the cross-street (again, this diagram just shows the maneuver for one direction):

 

Right on red prohibition
Although not required to make a superstreet function, the superstreets in San Antonio also employee a right-on-red prohibition from the cross street at most of the intersections.  This is done because a vehicle that is turning onto a roadway will be traveling more slowly than the through traffic.  In dense, high-speed traffic situations, such as those on 281 and 1604, a slow vehicle injecting into the traffic stream can cause a ripple of slowdowns behind it.  While light to moderate traffic flows can generally tolerate this, heavier traffic volumes simply cannot absorb the impacts of more than a couple of these events before it starts to cause intractable congestion.  Also, traffic darting from the intersecting roadway across to the turnaround lane presents a significant safety hazard in high-speed conditions.  Therefore, it was decided to require traffic coming from the cross street to wait for a green signal, i.e. no turn on red is allowed.  (An exception was made for the far right lane at Stone Oak because an extra lane is added onto southbound 281 at that location; an additional exception was made for the far right lane at Evans.)


Superstreet concerns and drawbacks

  • Ability to safely cross over to turnarounds: Some members of the public questioned how traffic from the cross-streets can safely merge onto 281 or 1604, then move across the traffic lanes to the turnaround, then merge back into the other direction.  The answer is that there are signals that stop the through traffic on 281 or 1604 to allow that cross-street traffic out at both the main intersection and also at the turnarounds.  As discussed above, even though the signals at the main intersections remain and extra signals are added at the turnarounds, the number of signal phases is reduced substantially, thus allowing significantly more green time for 281 through traffic, and signal synchronization is much easier.
     
  • Too confusing for drivers, more accidents: Another concern expressed by the public was these changes might be too confusing to many drivers and would result in more accidents.  These being the first superstreets in Texas, it naturally took drivers a little time to get used to.  However, because all traffic on the intersecting street is forced to turn right, most confusion is quickly overcome instinctively once the driver has turned and as the driver follows other more experienced drivers through the intersections.  Additionally, because all traffic is flowing in the same direction and is protected by signals, the likelihood of collisions is substantially reduced, even during the adjustment period.  Furthermore, superstreets inherently improve safety by reducing conflict points (the point where vehicle paths cross) by roughly 50%.  A study for the North Carolina DOT showed that superstreets reduced traffic collisions by 46% and decreased accidents with injuries by 63%.
     
  • Requires traveling out of the way: Some folks are bothered that to turn left or go straight on the cross-street requires going out of one's way to accomplish.  This is true, but because overall congestion in the area is reduced, everyone still derives some benefit.  Also keep in mind that there are many other cases around the city where traffic wanting to make a left turn is prohibited from doing so due to a median or expressway and must turn right first, then make a downstream U-turn, so this is not an unprecedented concept.
     
  • Pedestrian crossings: Finally, some people have wondered how pedestrians are able to cross these intersections.  Here is a diagram of the typical pedestrian crosswalks and pathways in a superstreet intersection:


How a superstreet differs from a "Michigan Left"

Folks who have lived in Michigan and other parts of the country may be familiar with an intersection layout known as a "Michigan Left" and some have proffered that a superstreet is the same thing just with a different name.  However, while a Michigan Left is similar to a superstreet, there are some significant differences between the two:

  • In a Michigan Left, there are no left turns at all.  All left turns from both the major street and cross-street are made using the turnarounds.  In a superstreet, traffic can make left turns from the major street onto the cross-street; only left turns from the cross-street are made using the turnarounds.
  • In a Michigan Left, traffic on the cross-street can go straight through.  In a superstreet, all traffic on the cross-street must turn right onto the major street.  Straight-through traffic on the cross-street must use the turnaround to return to the intersection, then make a right turn at the intersection to continue in their original direction.

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

Superstreet traffic flow Michigan Left traffic flow
   

US 281 superstreet results

To objectively measure whether or not the US 281 superstreet had the intended results, ARMA did before-and-after measurements of travel times, average speeds, and traffic volumes.  The study found that after the completion of the superstreet, travel times were notably reduced and average speeds increased even though overall traffic volumes increased in the corridor.  The table below summarizes the findings.  To ensure an "apples-to-apples" comparison, data was collected for Tuesday-Thursday periods when school was in session and when there were no abnormal incidents in the corridor (e.g. accidents, signal malfunctions, bad weather.)

Metric Before Superstreet After Superstreet
Southbound travel time
(Bulverde to Loop 1604, morning rush hour)
23.3 minutes 18.9 minutes
Southbound average speed
(morning rush hour)
16 mph 20 mph
Northbound travel time
(Loop 1604 to Bulverde, evening rush hour)
19.2 minutes 12.7 minutes
Northbound average speed
(evening rush hour)
19 mph 29 mph
Traffic count (north of Evans) 60,100 vehicles/day 63,552 vehicles/day
Traffic count (south of Evans) 74,000 vehicles/day 81,526 vehicles/day

Anecdotal reports from motorists in the corridor on Facebook, from media reports, and comments on various websites indicate that most of the time, the superstreet has provided some relief.  Recurring traffic signal malfunctions for the first month or so after completion of the project did cause some intial problems; those were later attributed to the fact that the signal controllers used for the project were a new model and the issues were resolved with assistance from the traffic signal equipment vendor.

Over on Loop 1604, the before-and-after study showed a reduction in peak period travel time of 35% southbound and 14% northbound.  The primary reason for the significant discrepancy between northbound and southbound benefits was determined to be caused by an increase in traffic drawn by the improved traffic conditions brought about by the superstreet exceding the capacity of the improvements previously made at the Braun intersection.


Other sites of interest

Alamo Regional Mobility Authority - US 281 Super Street project
http://www.alamorma.org/SuperStreet.html
US 281 Super Street Facebook page
http://www.facebook.com/US281SuperStreet
Pape-Dawson superstreet presentation
http://www.sanantonio.gov/council/D9/pdf/Superstreet%20Presentation.pdf
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.ncdot.org/doh/preconstruct/tpb/shc/pdf/Superstreet_Presentation.pdf


This page and all its contents are Copyright 2013 by Brian Purcell

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