Yard Rule

Units in Transport
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An Explanation of the Current Status of Units on the UK's Roads, Railways, Ships and Airlines

Although the British Government initially promised the European Union it would fully metricate its transport systems (and everything else) by the year 1979, all of our roads are still [fortunately] labeled in miles and yards. The complete-by date was extended by negotiation between the UK and EU back to 1989, then 1999, then 2009 before being dropped altogether. It was announced in March 2006 by the then-transport secretary that the signs on our roads would not be changed in the foreseeable future, simply due to the amount of money that such a program would be a waste of (something the order of £500,000,000).
Nevertheless, despite it being national law to show imperial units on all signs, exclusively on some whilst metric equivalents are optional on others, in many places up and down the country metric-only signs have been installed. This is a potential source of trouble as has been demonstrated in the past in incidents involving high vehicles and low bridges. Of note here is the fact that 1 inch, the smallest factor allowed on bridge heights, is four times more precise than 0.1m, the smallest metric graduation allowed. In Milton Keynes alone I have observed no less than five metric-only height restrictions, without even intending to find any. Thanks to the work of the Active Resistance to Metrication, many such signs across the country have since been corrected.
Oddly enough, despite all of the signs being imperial, road planning is done exclusively in metric, and such daft regulations exist as that advising workers to put 800yd warning signs 900m from the start of the works in question. Why the imperial system isn't used here is unfathomable, as metric's supposed advantages do not even come into play.
On the railways, all speed limits on the national rail network (besides the Channel Tunnel Rail Link) have to be displayed in MPH. However, light rail systems the like of the Tyne and Wear metro use km/h, necessitating dual signs on shared routes such as that in Sunderland, where the speed limit for metro trains is actually slightly higher than that for the mainline trains. The dual limits are confusing and once again there is no real need for the use of metric here - light rail systems in the United States use miles to no extra cost.
Despite the spread of metric across the Earth, aircraft across the world continue to use feet for altitude. This is an American practice originally (it being them who flew the first airplane) and has spread across the globe, along with the use of the knot (nautical mile / hour) for speed in both aircraft and ships. throughout the last five decades there has been no attempt by aircraft operators to change these facts. The foot is a more precise measure of distance than the metre (through its smaller size).

All in all, this does beg the question; if the use of imperial units is still so widespread (not all examples are included above), why on Earth are they not taught in schools?

Interestingly, the problem of metrication was demonstrated in Canada with the famous Gimli Glider, which ran out of fuel mid-flight due to confusion between pounds and kilograms. The aircraft had to make an emergency landing at a disused airbase, which was being used as a go-cart track. Parts of Canada returned to teaching feet and inches in schools in 2005.

Teach imperial, use imperial, save imperial