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).

"First 20mph year sees 100 fewer killed or badly hurt" https://www.bbc.co.uk/news/articles/c78w1891z03o

So no, what you're saying is bollocks. And no one ever claimed that speed limits are the only solution.