London Mayor Sadiq Khan recently announced plans to ban future construction of fast food restaurants within 400 meters of city schools. Khan says the policy is designed to reduce the “ticking time bomb” of childhood obesity.

Khan’s right to be concerned about the health of London’s children. According to various reports, London has the highest rate of overweight children in England. But is banning restaurants that serve high-calorie meals the right answer? Such a move suggests fast food is to blame. Khan certainly thinks so. Consider what he said about the new policy (emphasis mine), “Takeaway restaurants are a vibrant part of London life, but it’s important that they are not encouraging our children to make poor food choices.”

They? Are they—meaning the restaurants—encouraging children to make poor choices simply by existing? Do they have that power? Khan goes on to explain that his plan “will encourage a healthier food environment around our schools so that junk food is no longer the option for children nearest the school gates.”

It’s true that banning the construction of fast food restaurants will limit children’s ability to eat that type of food, but only when they’re at school or right after school is dismissed. What happens when those kids go home or take a bus to an area where fast food restaurants are located?

What Khan and many other politicians who pursue these sorts of anti-obesity policies fail to consider is the important role parents play in a child’s nutritional development. Parents who are involved in feeding their children and who take an active role in explaining healthy food decisions are more likely to have healthy kids. This might seem like common sense, yet encouraging parental involvement is never considered a policy worth trying.

Here’s another reason to give the parent strategy a try: banning fast food restaurants has been tried before and it failed spectacularly.

In 2009, the Los Angeles City Council unanimously passed a regulation to ban new construction of fast-food restaurants in South Los Angelesostensibly, like the London policy, to curb obesity in the city. This policy was widely praised at the time by food writers, politicians and health activists who said a fast food ban was a positive step towards helping people make better nutrition decisions.

Yet, by 2015, a study by the RAND Corporation proved that the policy was a dud. The report stated that the policy “failed to reduce fast-food consumption or reduce obesity rates in the targeted neighborhood.” In fact, obesity rates in that area of Los Angeles had risen at a faster rate than in areas of the city that didn’t ban fast food restaurants. Apparently, those who applauded the plan didn’t know that cars and buses could transport people to other areas of the city—specifically areas that still allowed fast food restaurants to exist.

While obesity didn’t decrease in South Los Angeles, something else did: the employment rate. According to the Los Angeles Times, by 2012, South Los Angeles—where the fast food bans were in place—was economically worse off than it was at the time of the riots two decades earlier. The Los Angeles Times reported at the time:

Median income, when adjusted for inflation, is lower. Many middle-class blacks have fled in search of safer neighborhoods and better schools. And the unemployment rate, which was bad at the time of the riots, has reached even more dire levels. In two areas of South Los Angeles—Florence Graham and Westmont—unemployment is almost 24%. Back in 1992, it was 21% in Florence Graham and 17% in Westmont.

Naturally, the article failed to mention that the fast food ban as at least partially responsible for the dearth of low skilled jobs—the very jobs that often employ workers just entering the workforce and teenagers, who often lack the type of work experience that would help them get higher-paying jobs.

The L.A. fast food ban offers a valuable lesson about the unintended consequences of “good for you” government policies. They might sound good on paper but they rarely lead to improved health outcomes and sometimes, they even end up hurting the very people they are intended to help.

Let’s hope London is listening.