Which is correct, Miles or Kilometers?

When driving on the road, how should distance be displayed on sign posts ?
If you are driving in a european country with a national speed limit of 60 mph (approx 100 kph), the answer is miles.  Unfortunately, the European Union got this one wrong.

The reason ?
Speed (v), distance(d) and time(t) are all related by the following formula:
d = v*t
In most countries, the national speed limit is 60mph (approx 100 kph) and in all countries there are 60 minutes in 1 hour.
This means if you are driving at the national speed limit (60mph) you will do in 1 hour (60 mins) a total distance of 60 miles.

If you see a sign that says distance = 20 miles you can use t = 20/60, which is 1/3 of an hour, which is 20 mins.  Or, if you don't like the maths, the easy way to remember this is:
At 60 mph, you do:
1   mile in 1 minute,
10 miles in 10 minutes,
20 miles in 20 minutes
30 miles in 30 minutes etc.

In other words, there is a one to one correlation between distance and time.  The reason why the maths is so simple is that there are 60 minutes in 1 hour. Miles are not metric, but neither is time.

Now, say you change from using miles to kilometeres.  It's easy to change distance and speed to be metric, but time cannot be changed to metric.  This means when you see a sign saying distance = 20 kilometers, you can't immediately see how long it is going to take to get there at 60mph (or 100 kph). 

Lets metricise our table above:

At 100 kph, you do:
1   km in 0.6 minutes,
10 km in 6 minutes,
20 km in 12 minutes
30 km in 18 minutes etc.


As you can see, by metricising distance and speed (without metricising time) working out how long it will take to reach your destination gets very difficult without a calculator.