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 convert US 281 into a "super-street" from Encino Rio to Marshall.  Under the plan, the intersections of Encino Rio, Evans, Stone Oak, and Marshall are being reconstructed to eliminate some straight-through and left-turn movements and instead use downstream turnarounds on 281 to either turn back the other way on 281 or get back to the intersecting road and then turn right to continue through on it.  While the plan does add additional signals and forces some drivers to travel out of their way, the plan is expected to reduce overall congestion in the area because it will reduce signal cycles at each intersection from five or six phases to just two, thus allowing up to 50% additional green time for 281 traffic.  It is estimated that by doing so, traffic throughput on 281 could increase by at least 30%, average rush hour speeds could increase by about 10 mph in both directions, and travel time could be reduced by about eight minutes between Loop 1604 and Marshall Rd.  Additionally, the project improves safety by reducing conflict points (the point where vehicle paths cross) from 32 to 14.

This proposal is considered a short-term "band-aid" until the environmental studies and associated legal wrangling for a more significant upgrade of 281 are resolved over the next several years.  The project will cost $5.2 to build, nearly 20% below the final estimates and nearly 50% below initial projections.

This will be the first super-street in Texas.  The second will be built on Loop 1604 West.

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 281.  They are intended instead to illustrate the concepts being discussed.  For schematics of the actual proposed super-street improvements for 281, see the "Project schematics" section.  Thanks to "SPUI" for the base super-street illustration used in the intersection diagrams.

On this page:

• Current configuration
• Super-street plan
  • Traffic flow through intersections
  • Encinio Rio intersection
  • Traffic signals
  • Project schematics
  • Super-street concerns
• Project status
• Other sites of interest

Current configuration

The number of lanes and other minor differences aside, this is how the intersections on 281 look today (except for Encino Rio, which is a "T" intersection):

The split-phase signal configuration necessitated by traffic patterns at these intersections requires five signal phases per cycle to allow for each movement through the intersection (not counting the brief all-way red clearance intervals):

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The number of phases and required length of each phase (due to heavy traffic volumes on all the approaches) causes 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.

"Super-street" plan

The super-street proposal would reconfigure the intersections at Evans, Stone Oak, and Marshall like this:

At first glance, I know it looks pretty scary, but it's actually not very complicated once you understand how it works and why.  The proposed 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, then use a downstream turnaround on 281 to either go the opposite direction or to return to the cross-street.  Those turnarounds would be located approximately 1000 feet (about 2/10th of a mile) from the intersection they serve to allow sufficient room for merging and storage.  Essentially, this creates an elongated signalized roundabout with two left-turn cut-throughs.  Roundabouts are generally more efficient at moving traffic than conventional signalized intersections.

Traffic flow through the intersections
So how does traffic flow through the super-street intersections?

Straight-through traffic on US 281 and traffic turning right from 281 to the cross-street will see no change:

Traffic turning left from 281 to the cross-street also will see virtually no change:

And traffic turning right onto 281 from the cross-street will also see no change:

However, traffic wanting to turn left from the cross-street onto 281 will instead turn right, then use the turnaround downstream on 281 to turn-back the other direction on 281 (for simplicity, this diagram just shows the maneuver for one direction):

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

Encino Rio intersection
Note that the Encino Rio intersection will be different than those described above.  Traffic coming from Encino Rio will be allowed to turn left onto 281 southbound.  In fact, an additional left turn lane will be added (for a total of three).  To still allow for operational improvement, traffic from southbound 281 will not be allowed to turn left onto Encino Rio.  Those wishing to do so will instead continue south of 281 for about 1000 ft. and then turn around and head north, where they will turn right onto Encino Rio.

Traffic signals
The proposal does not eliminate the signalized intersections; in fact, it doubles the number of signals in each direction on 281.  I know it's counterintuitive that adding signals will help improve traffic flow; however, by eliminating the straight-through and left turn movements on the cross-street, engineers can make some dramatic changes to the signal phasing.  Specifically, the number of signal phases can be reduced from today's five phases to just two:

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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 time required for those individual phases.  In addition, significant additional time savings will be 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 will allow the green time for 281 through traffic to be increased without having to correspondingly reduce the green time for the cross-street or lengthen the total cycle time.

If that's still unclear, here's another way of looking at it.  Imagine in the diagrams above that you're on the main road headed from left to right.  In the "current configuration", you reach the intersection just as the light turns red (phase 3).  You then have to wait through phase 3 for 30 seconds, phase 4 for 60 seconds, and phase 5 also for 60 seconds before you get a green light again on phase 1.  This means you've waited 2½ minutes.  With the super-street, all of the movements that took 2½ minutes before can now be accomplished in a single 60 second phase.  That means through traffic on 281 only has to stop for 60 seconds; the balance of the 2½ minutes required for the conventional intersection (90 seconds) can then be added to the green time for US 281 through traffic.  So if that traffic today gets 1½ minutes of green time, it would get 3 minutes with the super-street, an obvious and substantial improvement.  (Note that the numbers I provide here are for illustrative purposes only and do not necessarily reflect the actual current or proposed phase and cycle times for 281.)
 

In addition to reducing signal phases, super-streets allow for much better synchronization of the signals.  With conventional intersections, signals for all directions are inherently interdependent-- in other words, changes can not be made to one direction without adversely affecting the others.  These dependencies make synchronization-- or "progression"-- tricky, especially two-way progression.  But with a super-street, the signals for traffic going in each direction on the major roadway (US 281 in this case) are completely independent of each other.  In effect, both sides of US 281 will function like an independent one-way street.  This provides the ability to synchronize 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 synchronization.

Project schematics

Click on one of the links below to see an annotated engineering schematic for each intersection.  These schematics are courtesy of ARMA and are subject to change.  Annotations are my own.

• Encino Rio
• Evans Rd.
• Stone Oak Pkwy./TPC Pkwy.
• Marshall Rd./Northwind Blvd.

Full schematics can been downloaded from ARMA's website (see link below.)

Super-street concerns

• Ability to safely cross over to turnarounds: Some members of the public have questioned how traffic from the cross-streets can safely merge onto 281, then move across the traffic lanes to the turnaround, then merge back onto 281 headed the other direction.  The answer is that there will be signals that stop the 281 through traffic to allow that cross-street traffic out onto 281 at both the main intersection and also at the turnarounds.  As discussed above, even though the signals at the 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: Another concern expressed by the public that these changes might be too confusing to many drivers.  This being the first super-street in Texas, it will naturally take drivers a little time to get used to.  However, because all traffic on the intersecting street is forced to turn right, any confusion should be quickly overcome instinctively once the driver has turned.  Additionally, because all traffic is flowing in the same direction and protected by signals, and because there are fewer conflict points, the likelihood of collisions is substantially reduced, even during the adjustment period.
   
• Requires traveling out of the way: Some folks are bothered that to turn left or go straight on the cross-street would require going out of one's way to accomplish.  This is true, but the amount of through traffic on Evans, Stone Oak, and Marshall is fairly trivial, especially compared with traffic that turns onto 281.  The biggest constituency that would be inconvenienced under this plan would be motorists coming from east of 281 wanting to turn south onto 281.  However, because overall congestion in the area should be reduced, they should still see benefits from this plan.  Also keep in mind that a plan similar to this is already in place in that area-- traffic coming from Red Land Rd. wanting to turn south onto 281 must travel north to the turnaround near Encino Rio first.  This has prevented an additional signal from needing to be installed at Red Land.  Also, a similar plan was implemented during the construction of the 281 freeway from Bitters to Loop 1604 back in the late '80s.  As a result of that, traffic actually moved better during construction than it did before.
   
• Pedestrian crossings: Finally, some people have wondered how pedestrians would be able to cross these intersections.  Here is a diagram of the typical pedestrian crosswalks and pathways in a super-street intersection:

The MPO approved funding for this project on March 23rd, 2009.  Funding is coming from a combination of funds from the federal stimulus, Advanced Transportation District, and City of San Antonio.  On January 14th, ARMA's board selected Ballenger Construction's bid of $5.2 million to build the project.  Ground was broken on the project on March 11th and should be completed by late September.  Nearly all work is being done at night to minimize disruptions to traffic.  As of early July, the work was about 40% complete, with some delays due to unusually wet weather this Spring.

	Alamo Regional Mobility Authority - US 281 Super Street project http://www.alamorma.org/SuperStreet.html
	Pape-Dawson super-street 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