S UPPRESSIO VERI , suggestio falsi. Over the course of many years, without making any great fuss about it, the authorities in New York disabled most of the control buttons that once operated pedestrian-crossing lights in the city. Computerised timers, they had decided, almost always worked better. By 2004, fewer than 750 of 3,250 such buttons remained functional. The city government did not, however, take the disabled buttons away—beckoning countless fingers to futile pressing.

Initially, the buttons survived because of the cost of removing them. But it turned out that even inoperative buttons serve a purpose. Pedestrians who press a button are less likely to cross before the green man appears, says Tal Oron-Gilad of Ben-Gurion University of the Negev, in Israel. Having studied behaviour at crossings, she notes that people more readily obey a system which purports to heed their input.

Inoperative buttons produce placebo effects of this sort (the word placebo is Latin for “I shall be pleasing”) because people like an impression of control over systems they are using, says Eytan Adar, an expert on human-computer interaction at the University of Michigan, Ann Arbor. Dr Adar notes that his students commonly design software with a clickable “save” button that has no role other than to reassure those users who are unaware that their keystrokes are saved automatically anyway. Think of it, he says, as a touch of benevolent deception to counter the inherent coldness of the machine world.

That is one view. But, at road crossings at least, placebo buttons may also have a darker side. Ralf Risser, head of FACTUM , a Viennese institute that studies psychological factors in traffic systems, reckons that pedestrians’ awareness of their existence, and consequent resentment at the deception, now outweighs the benefits.

Something which happened in Lebanon supports that view. Crossing buttons introduced in Beirut between 2005 and 2009 proved a flop. Pedestrians wanted them to summon a “walk” signal immediately, rather than at the next appropriate phase in the traffic-light cycle, as is normal. The authorities therefore disabled them, putting walk signals on a preset schedule instead. Word spread that button-pressing had become pointless. The consequent frustration increased the amount of jaywalking, says Zaher Massaad, formerly a senior traffic engineer for the Lebanese government.

Beirut’s disabled buttons are, says Mr Massaad, now being removed. They should all be gone within three years. New York has similarly stripped crossings of non-functioning buttons, says Josh Benson, the city’s deputy commissioner for traffic operations, though it does retain about 100 working ones. These are in places where pedestrians are sufficiently rare that stopping the traffic automatically is unjustified. However, internet chatter about placebo buttons has become so common that doubt, albeit misguided, seems to be growing about even these functioning buttons’ functionality. This suspicion, says Mr Benson, has spread beyond New York, to include places such as Los Angeles, where almost all the crossing buttons have always worked, at least during off-peak hours.

Truth be told, though, the end may be nigh for all road-crossing buttons, placebo or real. At an increasing number of junctions, those waiting to cross can be detected, and even counted, using cameras or infrared and microwave detectors. Dynniq, a Dutch firm, recently equipped an intersection in Tilburg with a system that recognises special apps on the smartphones of the elderly or disabled, and provides those people with 5 to 12 extra seconds to cross. That really will be pleasing.