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.

I think you also have to enforce it. Helsinki also has many automatic speeding cameras. I doubt just putting up a 20 mph speed limit sign would make a big difference without more enforcement.

make cars not go faster than 30 mph at the engine control level. Problem solved and no need to put thousands of cameras everywhere.

But muh freedumb.