It certainly is.
If you hit a pedestrian while driving at 20 mph, the pedestrian has a 95% chance of survival.
At 30mph the survival chance drops to 80%
90% of those hit by a car at 40mph will die.
Almost 70% of all road accidents that result in death or injury happen on roads that have a 40mph speed limit or less.
Drivers tend to have an exaggerated idea about their ability to drive fast, and are somewhat indifferent to exceeding speed limits. One study showed that driving at 40mph in a 30mph zone was seen as less serious than dropping litter.