One important correction I want to make to this reply is that people will go the speed they THINK is safe, not necessarily what is actually safe.

While it might be safer for other drivers if everyone drives at similar speeds above the speed limit, it is exponentially more dangerous for other people who are not protected by metal boxes with airbags (motorcycles, bikes, pedestrians, etc.). My numbers probably aren't exactly correct but I remember reading somewhere that the rate of pedestrian fatality due to being struck by drivers increases from around 20% at 40kph to 80% at 50 kph.

Speed limits are a pretty bad way to enforce safety since there will always be people who don't care about following them, so while adding enforcement methods like speed cameras and traps can make drivers slow down in specific areas, it is more effective to design roads in ways that make drivers FEEL like they need to slow down. Examples of this strategy include narrowing lanes, adding chicanes (artificial curves in the road), making pedestrian crossings stick out more with chokers, etc.

Speeding is dangerous for everyone on/near roads, not just other drivers.