It has been illegal since 1925 in the UK to drive a vehicle while drunk. It wasn't until 1967 however that anyone bothered to say exactly how drunk.
The Road Safety Act of that year introduced the first legal maximum blood alcohol drink driving limit of 80mg of alcohol per 100ml of blood and there was some good science to suggest that, in the absence of something extraordinary, this equated to 35 microgrammes of alcohol in 100 millilitres of breath or 107 milligrams of alcohol per 100 millilitres of urine.
The breathalysers available in 1967 were pretty basic and involved crystals changing colour (see below), so they didn't provide an acceptable quality of evidence. There was also a lot of discussion of civil liberties and the pub landlords went ballistic. To cut a long story short, it wasn't until 1983 and the introduction of the Lion Intoximeter that we ended up with the procedure we know and love today.
As a sop to those who distrusted the breath tests, the Act of 1981 introduced an option that anyone who blew under 50 could ask for an allegedly more accurate blood or urine test. The police choose the method - either get a police doctor to come and take blood or measure the alcohol content of the second of two urine samples given within one hour.
All this is fine - but after a quarter century, surely we trust the breath technology enough by now to dispense with this ?
Can you guess what happens if the doctor can't find a vein and the accused can't wee twice in an hour ?
Let's just say that lawyers get rich and Magistrates lose the will to live and the accused still gets their ban.
1967 breath test - Orange-yellow crystals of a mixture of sulphuric acid and potassium dichromate in a tube turn to blue-green chromium sulphate and colourless potassium sulphate when the mixture reacts with alcohol breath. Check the colour by eye - if blue then naughty; if very blue then very naughty.
Modern breath test - the breathalyser has a platinum anode which acts as a catalyst to cause the alcohol in the person's breath to oxidise into acetic acid. In the process, the alcohol molecules lose electrons, producing an electric current, which is proportional to the amount of alcohol in the breath.
An accurate ammeter will give you a more accurate reading than checking crystals against a colour chart.