Every morning I pass a sign that reads: "No footway 400 yards." Yards? How many decades have passed since every maths scheme in every one of our schools went metric? But the county highways department remains wedded to its "yards", even though everyone under 40 should prefer their roadworks warnings in metres.
Televisions are advertised by the inch, carpets by the square foot. Crimewatch estimates its villains in feet and inches, computer salesmen measure their floppies in inches. We swim, run and jump competitively in metric measures, but for almost every other purpose, linear measurement is becalmed in the 1950s.
My desktop publishing software has helpful guides on screen - in inches. The first time I took netball, I caused confusion and laughter by blowing for an infringement and telling the offender: "You must be a metre away." It confirmed their suspicion that I didn't know what I was doing. I was the wrong gender. "It's three feet, sir," they told me, "not a metre."
So does it matter that society has yet to catch up with what we have been doing in schools for 30 years? Yes.
If our job is to prepare children for the real world they will confront when they leave school, we're failing - by miles. And having everyone outside school measuring in miles, yards, feet and inches makes teaching children metric measurements more difficult.
It would be overstating the case to pretend children are given no knowledge of imperial measures in maths. We have to cover a range of units and ways of measuring and converting. I liken this to my experience of learning temperature.
At school I conducted experiments and performed calculations in degrees Centigrade (which I have since learned to call Celsius). In everyday life, temperatures were given in Fahrenheit. At school I was taught to convert Centigrade to Fahrenheit, and not only can I still remember the formula, I regularly use it. BBC meteorologists predict temperatures exclusively in Celsius, whereupon I divide by five, multiply by nine and add 32: only then do I know whether I will need to take a jumper or an overcoat on my walk.
Linear measure is only a small part of maths. But it is an important area, and it's one in which our pupils are being short-changed. It is more than time the Government made up its mind - either we are going to convert our syllabuses back to working principally in imperial units, or the miles, yards, feet and inches must go. After all, 30 years of preparation is more than we had for the introduction of decimal coinage and metric weights.
And when that's sorted, maybe County Highways could start producing comprehensible signs.
"No footway 400 yards."
Footway? I teach my pupils to call it a pavement.
John Cosgrove is a deputy head in Cornwall.