Answer 1:
Even if you're traveling at 99.9999999% of the
speed of light, you will still find that light
travels exactly at C, relative to you. The
reason you don't measure light traveling at 2*C is
that your measuring tools are measuring something
different than when you're standing still.
We measure speed as distance divided by time,
like miles per hour, or meters per second. The
astonishing thing about the universe (as
discovered by Einstein) is that both "distance"
and "time" are variable. If I was holding a 1
meter long stick and traveling past you at 1/2 of
the speed of light, my "meter" stick would appear
to be just 0.75 meters long to you. The same
thing happens to time: 1 second to me will be
longer than 1 second to you. So the "speed" of
light is constant because both "meters" and
"seconds" change.
If you could actually travel exactly *at*
the speed of light, then there's no way to measure
anything at all, because time has totally stopped
for you. No time would pass at all. You are
frozen and will not change forever, or at least
not until you slow down. If you could magically
travel along with a photon of light from the
moment it was generated until the moment it was
absorbed, the universe would simply seem to
instantly change for you. It would seem like it
was instantly replaced. You would not noticeor
even be able to measureanything along the way,
even if the photon traveled for millions of
"years" as they would be measured by a stationary
observer.
The old joke asks: What happens if
you're driving at the speed of light and you turn
your headlights on? The answer is: You can't,
because if you're traveling *at* the speed of
light, you won't move, breathe, or think, much
less reach for a switch. You and your car will
simply fly across the universe in a state of
suspended animation. Of course, if you're
traveling at any speed less than C, then it's not
that special. Your headlights will still emit
light that travels exactly at Cboth for you and
for the rest of the universe.
As for your last question, I don't actually
know why the speed of light is a constant in
the universe. I'm not sure anybody does. It's
one of those questions waiting for a bright young
student to come up with a new idea, and test
whether that idea matches experimental
observations.

Answer 2:
One way to get a better insight on this problem
would be to play around with one very useful
formula relating to the relativity of speed:
u = (u'+v)/(1+u' v)
This equation lets you find u, the
velocity of an object relative to a frame of
reference S (for example 'stationary'
Earth), given u', the velocity of the same
object, relative to another frame of reference
S' (for example a flying airplane), and
v, the velocity with whichS' moves
relative to S (in this example the velocity
of the airplane relative to Earth). To keep the
math simple, this equation assumes that motion is
only in one dimension and that all velocities are
given as a fraction of the speed of light so that
if say, an object moves at half the speed of
light, then its velocity would be numerically
equal to 0.5. This means that if you are using
m/s, ft/s mph or (why not?) knots, you need
to first translate that as a fraction of the speed
of light.
Example: Say an airplane is flying at 575mph
relative to Earth (typical airliner). If you
divide this speed by that of light (around 671 000
000 mph!) then v would be equal to 0.000
000 857 (or 8.57^{7} if you are familiar
with computer notation for very large or very
small numbers). Now imagine someone at the back of
the airplane throws a pillow toward the front
with a speed of say, 15mph (that's relative to the
airplane). Then u' would equal 0.000 000
022 4 (you do the math, as they say, bearing in
mind that my results are rounded to three
figures). If you use our magic formula above (you
will need a scientific calculator given such small
numbers) you'll get u = 0.000 000 879 which
translates, after multiplying by the speed of
light, to a pillow moving at 590 mph relative to
Earth. But wait, isn't this simply the sum of
575 mph and 15 mph? It is in indeed; that's
what you would expect and what Galileo already
knew almost 400 years ago! This is what is known
as Galilean relativity; the idea that
motion is relative to the frame of reference you
use to measure velocities or speeds.
Now, the difference between the relativity
theory of Galileo and that of Einstein (where our
magic equation belongs) is only apparent when you
are moving REALLY fast. So... let's run another...
Example: Say you are in a spaceship and you
launch a shuttle at onehalf the speed of light
relative to you (you are reference frame S
and the shuttle is reference frame S') so
that v = 0.5. Now the shuttle launches a probe in
the same direction of travel, at onehalf the
speed of light relative to the shuttle. If you
asked Galileo, he would tell you that the probe
will travel with the speed of light relative to
you ( 0.5 + 0.5 = 1 ), however, if you use the
formula above, Einstein will tell you that the
probe will travel at 'only' 0.8 times the speed of
light relative to the spaceship (again, try to get
that number on your own).
You ask in what sense is the speed of light
absolute, so let's do some algebra with our
equation. Imagine you are traveling with
speedv relative to Earth, say. If you light
a flashlight in front of you, the beam of light
will travel with a speed u' equal to 1 (of
course) so if we now substitute 1 for u' we get:
u = (1+v)/(1+v)
but this simplifies to u = 1 !!! This
means that relative to Earth, that beam of light
you emitted will travel also with the speed of
light. It is in this sense that the speed of light
is absolute, (and yet we call it "Relativity
Theory", right? ). Please note that the
previous simplification is valid for ANY value of
v, no matter how fast, including v = 1 (you
traveling with the speed of light). In this last
case however, other parts of the theory brake down
when you try to calculate things in a reference
frame that moves with the speed of light relative
to some valid reference frame. On the other hand,
from a physical point of view, it would take an
infinite amount of energy for you to acquire the
speed of light so it would be actually impossible
and thus, well, not really a problem, right?
Still, you may imagine traveling at 99% the speed
of light (possible, though expensive (very!!!)).
In such case, v = 0.99 and a beam of light emitted
by your flashlight would still travel with the
speed of light relative to both, you and Earth.
That runs against common sense, yet that's real;
no sciencefiction here. In the end, it
comes down to the relativity of time. That
same time you need to measure speed, for instance.
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