This is the second essay I have written on prohibition. My first was completed 50 years ago for a school history project.
As a consequence of my grandfather having been the American federal judge who heard Al Capone's final appeal in 1932, I became a juvenile expert on prohibition.
Growing up with a proprietorial interest in the world's most notorious gangster, I learnt all I could about him to defend that interest and impress my friends.
I have lost the essay. But from memory, it began by noting the connection between the passage in 1919 of the Volstead Act, which established prohibition in the US, and the subsequent rise of organised crime.
This essay, my second, has been inspired by a rather different document: a report by an independent think-tank, the Transform Drug Policy Foundation, titled After the War on Drugs: Options for Control .
My interest was recently reawakened by the fact that Bloomsbury, in London, where I have worked for 35 years, has become one of the nation's hottest drugs markets.
Dealers and users have congregated in an area of the capital dominated by large educational institutions including University College London and the School of Oriental and African Studies, in part due to the "success" of police operations in neighbouring Tottenham Court Road and Kings Cross.
The problem has simply been displaced to Bloomsbury, where hostels for the homeless accommodate about 500 people, most of whom are addicts - an embarrassing advertisement for the failure of the present regime of "treatment".
One observer put it this way: "Those on treatment have methadone for breakfast, heroin for lunch and crack for tea."
A local needle service provides a clean way of injecting substances that can be acquired only from criminals. The majority of these problematic drug users are estimated to spend up to £500 a week on drugs, virtually all of it gained through aggressive begging or acquisitive crime.
Last year, the mobile needle "exchange" handed out 80,000 more needles than it received back - leaving behind a significant public health problem.
The police are encouragingly active. But under the current state of the law, the most that they can do with the problem is to displace it, sending it somewhere else.
This strikes me as the worst sort of nimbyism. Hence my revived interest, this time guilt-inspired, in the futility of prohibition, and my enthusiasm for the controlled-legalisation agenda promoted by Transform.
Its report advocates putting drugs under the same form of regulatory control as alcohol. It does not espouse drug taking, and it does not deny that the abuse of drugs is harmful.
It spells out the cost of the failure of the war on drugs and emphasises the advantages of duplicating the regime currently applied to alcohol: control over purity, clear information about dose and, above all, the removal of the trade from organised crime.
The repeal of prohibition in Thirties America drove the bootleggers out of the alcohol business - and into drugs. This second prohibition replicates the problems of the first, but on a far larger scale.
What I have not been able to figure out over the past 50 years is how societies decide which voluntary risks to ban, and why they tolerate the enormous costs of whichever form of prohibition they select.
The Transform report contains the essence of the essay I wish I had written 50 years ago, and its logic is well within the grasp of the average teenager.
The latter-day Capones of the illicit drugs trade hope its challenge will be ducked.
Emeritus professor of geography at University College London
A full version of this article is on the Social Affairs Unit weblog at www.socialaffairsunit.org.uk/blog/