F EW THINGS delight Britons more than watching a miscreant brought to book. So the dashcam footage released by the Metropolitan Police, London’s force, of officers ramming suspected criminals off their mopeds was a guaranteed crowd-pleaser. In one clip a rider hits the bonnet of a police car before bouncing onto the road. One delighted viewer tweeted that cinemas should screen a feature-length version. Before detailing her reservations, a columnist wrote of her initial “fist-pumping glee”.

Police chases are nothing new, and officers often bring suspects who flee arrest to a “hard stop”. But until recently British cops hardly ever rammed two-wheeled vehicles, especially if a rider removed their helmet. In recent years, though, the number of criminals using motorbikes and scooters for sprees of purse-snatchings, muggings and even acid attacks has jumped. In 2015 there were fewer than 400 such attacks every month in London, but the incidence soon soared, peaking at over 2,600 in July 2017. The Met changed its tactics to fight back. So far this year, its officers have given chase 480 times and knocked a suspect flying on 60-odd occasions.

The police claim the tactic is working: moped crime is down by 36% on last year. They credit the fall to “tactical contact”, a euphemism for ramming, as well as innovations like slim motorbikes for chases on narrow streets and spray-tagging bikes.

Even if it works, is it right? As Lawrence Sherman of Cambridge University puts it, “the videos show what could be either heroic police work or attempted homicide, depending on the facts that led to the decision.” Proponents of the tactic claim it is proportionate to the threat. Snatches from mopeds amount to more than a humdrum property crime, they say: some ruffians threaten their victims with long-bladed “zombie knives”; others carry guns. Since they often commit several offences in a single trip, Mr Sherman reckons there is “sound reason” to believe that if riders are not apprehended, they may go on to commit more crimes. British cops are also pretty good at using force responsibly. Data are patchy, but police chases in America appear far more likely to kill somebody than those in Britain. Amanda Pearson of the Met says officers will ram only if the risk to the public of not doing so outweighs the risk of hitting the rider. The “over-riding objective” is not to begin a pursuit at all.

But Geoffrey Alpert, an American expert on police chases, thinks the ramming tactic risks “pretty vicious falls” or worse, and so is not widely used in America (though fleeing suspects there run the risk of being shot instead). And lawyers predict the policy will soon be tested in court. “There’s quite a high risk that it will be found unlawful, and that won’t help community relations,” says Hugh Southey, a barrister. The government is likely to change the law to make it harder to prosecute cops for dangerous driving. But until it does so they might face trial, even if a jury would probably clear them. There will also be misconduct probes.