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> For most people, a scale that starts at 0="civilization shuts down because you can't ice the roads" and goes up to 100="heatstroke territory" is a fine representation of humanity's day to day temperature. Why would we bother changing it?

I thought you were talking about Celsius at first...

The devil is in the details.

In the US, all speed limits are in 5 mph increments.

So, let's say you convert your 60mph limit to 100kph (Really 96.5). 70mph is 112.65kph. Do you round down to 110, or up to 115 or 120?

If you round down, congratulations, you just cut the speed difference between a rural highway and interstate essentially in half, as a "true" 17kph difference is only 10.

>not too hard on people but a lot of signage needs to change. Feasible.

Very expensive though. Signs normally have a 20 years expected lifetime. Now you need to replace them all at once. Plus a lot of the sign locations need to be changed, unless you want all those "Exit 29 1 mile" signs to now say "Exit 29 1.6 km".

You do not need to replace them all at once. For example the new signs could be European style, red ringed white circles with the number inside. Virtually every car has both readings already. This would make new speed limit signs immediately recognizable to more visitors in the future while avoiding ambiguity of units during the transition. They could even say km/h during the first generation.

Road signs need to be very clear and instantly understandable. Adding a tiny delay to comprehension; a tiny bit of lack of attention to the road; could, when multiplied over the number of kilometres driven and number of drivers and time mean many deaths.

Whether that's acceptable or not is another matter, but it'd suck if "km/h KILLED MY FATHER" became a meme.

It is odd how some countries make massive national changes overnight. eg, Sweden:

(http://users.telenet.be/worldstandards/driving%20on%20the%20...)

> In 1955, the Swedish government held a referendum on the introduction of right-hand driving. Although no less than 82.9% voted “no” to the plebiscite, the Swedish parliament passed a law on the conversion to right-hand driving in 1963. Finally, the change took place on Sunday, the 3rd of September 1967, at 5 o’clock in the morning.

Leaving it until now has meant the UK would find it very hard to change.

Why not phase things in, by having both measurements put on, for all new signs?

Exit 14 - 1 Kilometer (.6Miles)

Then, as the old signs wear out, eventually all signs are replaced.

Rounding to the likes of 1.5km is good enough for a highway sign.

Anyone in a country that uses the celsius scale knows that the temperature argument is rubbish... It's totally and completely what you're used to. I doubt people really think of temperature in more than about eight or ten increments anyway (bloody freezing, cold, pleasant, etc.) so having a range of forty or fifty integers really gives you more than enough precision.

I suppose it doesn't really matter what people use to express the temperature in common use, but having a unified scale across ambient temperature, cooking, science (1K = 1°C), manufacturing, etc. that people in almost every other country uses really does make sense, especially when the conversion is so annoying.

My point is more "why should people have to change what they're used to for a small subset of the scientific class?" - of all the things that people do math with, and would benefit from decimalization and metrifying, temperature is pretty much the last.

Mecanical parts are the most difficult thing to switch because there are many things defined in relation to one another. For example the american UNC threads have different angles than the metric ones do and therefore it is impossible to replace one with another. For this reason alone it will take very long time to do a full switch from one unit system to another. However, many "metric" units are actually slightly adapted imperial ones. For instance you can get a lot of 25 mm stuff in Europe that is almost equivalent to the 1 inch stuff you get in the US (1 inch = 25.4 mm) and similarly there is a lot of 30 cm and 60 cm stuff in stead of 1 foot and 2 feet stuff.