In low-income countries, road traffic accidents account for 3.7 percent of deaths, twice as high as deaths due to malaria. Anyone who has traveled in Kenya won’t be surprised to hear that 20 percent of recorded crashes involve matatus, the private buses that careen around the city. Billy Jack  and James Habyarimana  have a fascinating impact evaluation where they randomly put posters in matatus encouraging passengers to “heckle and chide” the driver if he is driving too fast or recklessly. The idea is that the posters solve a collective action problem: most passengers don’t like being driven dangerously, but individually they’re reluctant to speak up. Their preliminary results are impressive: the frequency of road traffic accidents in a 12-month period was one quarter in the treatment group compared with the control group (those without posters).