Features:

  1. The vacuum light speed (in unconstrained space) is the highest physical speed.

  2. The vacuum light speed is also invariant in all inertial frames of reference.

  3. How do we know that the vacuum light speed is the highest physical speed and is invariant.

    1. All observations are consistent with these statements.
    2. Special relativity (which as far as we know is exactly correct in the special-relativity limit tells us the statements are exactly true.

  4. The fact that the vacuum light speed is invariant implies other special-relativity effects most amazingly that time flows differently in different frames of reference.

    The different rates of time flow is a well verified effect and amazing. We never notice this effect in everyday life because in everyday life relative velocities are too low to make it noticeable.

  5. The vacuum light speed is given the symbol c and is defined by c = 2.99792458*10**8 m/s .

    Nature has given us an invariant speed c, and so modern metrology makes use of that by making c an absolute standard.

    Since nature has NOT given us an invariant-length macroscopic object, the modern meter definition has been chose to be the distance traveled at the vacuum light speed in 1/299792458 s.

  6. For reference: 
      c = 2.99792458*10**8 m/s 
    ≅ 2.998 * 10**8 m/s
    ≅ 2.998 * 10**10 cm/s
    ≅ 3 * 10**5 km/s
    ≅ 1 ft/ns (which is well known to people building precision circuitry
                    and optical systems).