I disagree, it's sadly easy. You can be involved in a serious accident while stationary (broken down on the side of the road). This is an extreme example of h

I disagree, it's sadly easy. You can be involved in a serious accident while stationary (broken down on the side of the road). This is an extreme example of how a lot of accidents occur when mixing speeds (being rear ended at lights etc.). It's that mix that road calming measure often create. A plastic knife is barely a knife at all, it's almost completely useless and so is a 10mph road. The solution to road safety can't be to drive at jogging pace. Roads are still a transport infrastructure and for practical reasons, speed has to remain an evaluation metric (a good road is both safe and fast). There's plenty of examples of safe and performant roads. We wouldn't want to make the 50 mile commute to the shops or work over the course of several days.