Today marks a year since Transport for London (TfL) introduced a ban on junk food advertising anywhere on its network.

But what impact has it had? And what are the lessons for London’s policymakers?

One year on, there is little evidence that the ban has achieved much beyond generating a few headlines. In fact, it looks like the perfect example of a poorly thought-through government initiative that hijacked an important issue in order to virtue-signal, rather than achieve real results.

The ad ban was aimed at addressing London’s childhood obesity problem — often cited as spiralling out of control. That’s a noble goal.

But as the Royal College of Paediatrics and Child Health has made clear, before we can find a solution to the obesity crisis, we first need better data to adequately understand the problem in the first place. This research should have been commissioned at the outset.

In fact, both experience and research show us what the most proven methods for tackling childhood obesity really are. Amsterdam, for example, achieved a 12 per cent reduction in childhood obesity between 2012–2015 by adopting a whole systems approach that included investment in education campaigns, dietary advice, and extensive exercise and sporting activities for children.

The TfL ad ban, in contrast, resulted in uncertainty, confusion, and complexity. The rules have led to adverts for family staples like olive oil and nuts being banned, while allowing marketing of some chicken burgers, nuggets and fries.

As with all misguided top-down government policy, it ends up costing the taxpayer. The ban cost TfL an estimated £25m a year in lost advertising revenue according to industry experts. TfL’s own figures are more conservative, but still show an estimated £13m loss in revenue, shouldered by Londoners.

The public should have some way of assessing what that £13m price tag achieved — but conveniently enough, they can’t. Because in typical public sector fashion, there are no key performance indicators, no metrics, and no measurables accompanying the ban, meaning that the policy cannot undergo any meaningful future review.

So how many fewer children are obese thanks to the ban? We actually don’t know.

Encouraging organic grassroots action would have paid far greater dividends. Without involving schools, councils, community activists and even private actors, it is almost impossible to ensure that adequate programmes, facilities and nutritional guidance can be made available to those that most need it.

Worse of all, when the mayor is challenged about tackling childhood obesity, he just refers to the TfL policy. Not only has the ban failed to improve childhood health, but it makes further action on this issue unlikely, because the policy gives the false impression that substantive action has already been taken.

This policy is the perfect example of how a well-meaning idea concocted by intelligent officials can have disastrous consequences.

Implemented via top-down diktat, decided without meaningful consultation with the private sector or civil society, and with no real way of measuring success, the TfL ad ban was always doomed to fail.