San Antonio Area Freeway System
Why the I-10/Loop 1604 Yield Signs are Reversed |
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This page last updated August 28, 2017 |
One curiosity about the local freeways are the yield signs at the I-10/Loop 1604 cloverleaf on the
northwest side, specifically that traffic entering the interchange must
yield, which is the opposite of how most cloverleaf interchanges are signed. Originally, the
yield signs in this interchange were at the typical locations. But
as traffic increased substantially in the 1990s, the signs were moved to their
current positions, leading many motorists to wonder why. One
reason the signs were changed was as a safety measure. A large
part of the interchange is elevated and those upper roadways have
concrete barriers on both sides.
When two low-profile vehicles approach the merge point of the upper
cloverleaf ramps, the drivers may have difficulty seeing each other
over the barriers, resulting in a surprise at the merge point and the
increased possibility of a crash. After studying the issue, it was
determined that the driver with the better opportunity to see the other
one and to yield in time was the one going straight, and so the signs
were changed accordingly. A
consequence of this arrangement is that traffic entering the
interchange during peak periods frequently backs-up, exasperating many
a motorist. But, believe it or not, this configuration
actually prevents
even worse congestion, which is why the signs at the lower locations
not affected by the visibility issue mentioned above were also switched.
In a typical cloverleaf, the yield signs are
placed where the ramps merge into the intersecting highway or collector
road, as shown here:

Especially note
that traffic coming from the inner loop ramps must yield. However, in the
I-10/Loop 1604 interchange, traffic on those inner loop ramps does not
have to yield. Instead, the yield signs are placed on the
collector roads, as shown here:

(Note that
there are also two changes from the typical yield placement on the outer right-turn
ramps, specifically in the northeastern and southeastern quadrants. However this anomaly is incidental to this discussion.)
So
how does this actually prevent worse congestion? Because during peak traffic periods,
requiring traffic on those inner loop ramps to yield could cause those
inner ramps to lock-up. Just like congestion on a grid of one-way
streets can eventually lead to "gridlock" because each street's backup
blocks the intersecting streets' egresses, congestion in a cloverleaf can lead to
what I call "cloverlock". If that happened, then the
result would be monumental congestion.
Here's an
example of how "cloverlock" could occur and the ramifications if it did:
1. Heavy traffic
coming into the interchange from the south causes traffic in SE loop
ramp to start backing-up:

2. As that
traffic backs-up, it wraps back around the ramp and eventually reaches the point to where it blocks egress from
the SW loop ramp, thus causing traffic to backup on it:

3. Now traffic
backs-up along that ramp and eventually
reaches the point where it blocks the NW loop ramp:

4. Before long, the
resulting backup
blocks the NE loop ramp:

5. Finally, the backup
reaches all the way back to where it started and blocks egress from the
SE loop ramp:

At this point,
all the loop ramps are blocked by the backup from the next downstream
ramp, creating a self-perpetuating lock-up of those ramps. No
traffic can get out of the interchange, which means no additional
traffic can get into the interchange. This would cause massive
backups on both I-10 and Loop 1604.
So by
switching the yield signs so that traffic exiting those inner loop
ramps always has the right-of-way, a possible lock-up of the
interchange is prevented. Obviously, this isn't the perfect solution,
but it's a far better scenario than "cloverlock"!
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