[jamincan]

Human behaviour can be a confounding thing. There was some debate a while ago [1] about whether bike helmet use may actually lead more head injuries due to factors like drivers passing closer to helmeted riders vs. unhelmeted ones or riders riding more recklessly, among a tonne of other factors. I still prefer to wear a helmet, but its an interesting example of how difficult it can be to engineer human behaviour.

Another good example of this is how civil engineers adding safety factors into design of roads - lane widths, straighter curves, and so on - leading drivers to speed more and decreasing road safety overall.

1. https://bigthink.com/articles/the-bike-helmet-paradox/