i) Road vehicle speedometers to show the speed as a duodecimal fraction of c.
Most digits in this would simply be painted on due to the inability of most cars to
travel even a tiny fraction of that speed. Standard form not used.

ii) Speed limit signs to be similarly
labelled and consequently quite wide to
express the distance precisely. Warning signs the same. In order to simplify this,
speed limits should be changed somewhat to move them in line with fairly round
numbers.

iii) Odometers to measure distance travelled in light seconds, again with
appropriate digits after the point down to about a light-microsecond. This would
actually stand a chance to go above one in a few vehicles. Distances on road
signs to be expressed the same way.

This would emphasise how small and slow we are compared to the ultimate
speed limit and size of the Universe and provide a universal standard for
expressing speed and distance without controversy regarding metric and imperial
units.

In duodecimal partly because everything should be in duodecimal, partly because
it lends itself well to fractions and partly because it enables speed limits to be set
more closely without confusing the driver.

Oh yes. Twelve divisions between zero and one including eight which recur in
the decimal system along with the ability to express the same fraction as in
decimal using fewer places.

Given the tenets of Special and General Relativity, and Heisenberg's Uncertainty Principle, it will be impossible to determine the true "speed" of the vehicle with respect to C.

// Given the tenets of Special and General Relativity, and Heisenberg's
Uncertainty Principle, it will be impossible to determine the true "speed" of the
vehicle with respect to C. //

Yes, that was in my mind when i posted this. However, you realise that means
that miles per hour, miles in themselves, kilometres per hour, knots, nautical
miles and all the rest are therefore at fault because of the length contraction?
It's my system which is accurate: the others are at fault because they depend
on Newtonian physics.

[hippo], there is no " moving car", nor "stationary observer". Both are "impossible" concepts within the framework of Relativity (the clue's in the name).

[neutral] I like the idea of speed as a fraction of c, but it certainly seems a waste of space and ink, to print all those zeros.

How about using scientific notation? Thus, 60mph becomes 3.2 x 12^-7 c. Well, actually you'd write it as "3.2 x 10^-7 c," because in base twelve, the number twelve is written as "10".

Hippo, the least problematic answer to your question is that since we want to provide information to the driver in such a way that it doesn't appear to be incorrect from his point of view, the *vehicle* must be the "stationary" observer, and the road is what is observed to be moving.

I did think about standard form, but the thing is, it doesn't give one as concrete
an idea of one's minuteness as lots of leading zeros.

I also think the vehicle's meters should take special relativity effects into
consideration and display them, so the distance to one's destination varies
according to the speed one's doing. However, that would only show up a long
way along the display, so that's another argument against standard form.

Interesting question which suggests using gyroscopes in a mad form of FTL. You can go as far as you like, provided it's in a circle smaller than the size of an atom or something.

Is dodecimal more apt for a 12-based system? (Dodecagon, dodecahedron, etc.) Duodecimal made me think of 100; maybe what [coprocephalous] was alluding to.

The cool thing about this idea, is that you would also know your increase in mass, and your decrease in longitudinal length, relative to your direction of travel.

Well, if you wanted to be all Newtonian about it, what you could do is take the direction of the planet's rotation, orbit, the Sun's proper motion and the movement towards the Great Attractor into consideration, and the road signs could be updated constantly to take these things into account, but that would be scientifically naive.

I've also just realised there needs to be a HUD on the windscreen superimposing images of road signs which display the distance to destinations taking into account one's velocity.

Clearly needs revision of traffic laws and maybe also the driving test, as in "a road vehicle is a device used to move the landscape around with rollers", and setting a maximum speed for those rollers, possibly in Hertz or some kind of function of their radius perpendicular to the axis of rotation.

You have no way of determining if what you think of as a "constant" has the same value throughout the Universe. You haven't even worked out where all the Dark Matter is yet.

(Hint: West of England, East of Ireland, rains a lot ... )