From the abstract of a new study from the Center for Global Development:

In the experiment, messages designed to lower the costs of speaking up were placed in a random sample of over 1,000 minibuses in Kenya. Analysis of comprehensive insurance data covering a two year period that spanned the intervention shows that insurance claims for treated vehicles decreased by one-half to two-thirds, compared with the control group. In addition, claims involving an injury or death decreased by at least 50 percent. Passenger and driver surveys indicate that passenger heckling contributed to this reduction in accidents

I haven’t read this paper (just the abstract), largely because we haven’t seen any major charities focusing on interventions like this one. Note that the Disease Control Priorities Project sees “increased speeding penalties, enforcement, media campaigns, and speed bumps” as having high potential cost-effectiveness (see this table).