Other San
Antonio Area Roads
Alternative Intersection Designs |
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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.
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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 gripes— 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.
Roundabouts
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.
(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:
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.
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.
DLT
locations in San Antonio:
More
information:
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
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.
DDI
locations in San Antonio:
More
information:
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 traffic
flow from off-ramps
SPUI
locations in San Antonio:
More
information:
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:
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| 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.
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:
Additional
information
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Freeway system
Alt intersections
