Helsinki records zero traffic deaths for full year

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.

Maybe not but people tend to not go more than 5-10mph over unless they’re on the interstate/highway. If it leads to overall significantly slower traffic it’s worthwhile.

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

Speed sensors that turn the traffic light red for 10 seconds are also quite effective without making the place dystopian with CCTVs and fines. I've seen it in Portugal. At the other end is Austria, which uses cameras and fines.

But muh freedumb.

"'freedumb' is when you want your car to be capable of going over 30 mph"

Which is fine and good on any road where the speed limit is over 30mph.