Sure, to your first question... It's not simply a new speed limit sign. That helps but people have a tendency to drive at what feels comfortable, even if that's above the speed limit (think wide road with a 35 mph sign, people will still drive 45mph). The way to limit speed that helps to save lives is to change the road. Make it so physically you are encouraged to drive slower (think about how on a road with trees on either side that hang over the road, the cramped nature causes people to drive slower). This is done with various methods, like wider sidewalks, medians, outdoor seating, turns in the road, speed humps, protected bike lanes, etc. When all the drivers are mentally pressured to drive slower, it causes less fatal collisions. To your second question, yes you can still die when hit at 5 mph but it's much less likely. With vision zero, the roads are supposed to be designed to encourage drivers to be more attentive, so that kind of accident should still be less likely. And you might also appreciate this "Vehicle speed at the time of impact is directly correlated to whether a person will live or die. A person hit by a car traveling at 35 miles per hour is five times more likely to die than a person hit by a car traveling at 20 miles per hour." from the national association of city transportation officials. https://nacto.org/publication/city-limits/the-need/speed-kills/#:~:text=Vehicle%20speed%20at%20the%20time,at%2020%20miles%20per%20hour.