Helsinki records zero traffic deaths for full year

"More than half of Helsinki’s streets now have speed limits of 30 km/h."

This is the only secret.

People over speeding is what kills.

Drivers are actually calm in Helsinki, not constantly honking and slowly rolling into you in the pedestrian crossing either.

Other places have introduced the same limit and haven't seen the same results.

People who are likely to have crashes are likely to be able who ignore the limit. One of the biggest problems in modern policy-making is the introduction of wide-ranging, global policies to tackle a local problem (one place that introduced this limit was Wales, they introduced this limit impacting everyone...but don't do anything about the significant and visible increase in the numbers of people driving without a licence which is causing more accidents...and, ironically, making their speed limit changes look worse than they probably are).

> People who are likely to have crashes are likely to be able who ignore the limit.

... which is why you have to do actual road design. You can't just put up a speed sign and hope people will magically abide by it. Roads need to be designed for the speed you want people to drive. When done properly the vast majority of drivers will follow the speed limit without ever having to look at the signs, because it'll be the speed they will feel comfortable driving.

It isn't road design, it is behavioural/cultural. People will drive recklessly when they do not care, for whatever reason, about the people they may injure by doing so. That is it. If you look at comparisons between countries, it is clear that means are different.