[choeger]

Unsafe driving is not characterized by speeding a slight margin over the limit on good roads (see German Autobahn for statistics, these are among the safest roads on the planet). Similarly, I am pretty certain that driving 40 in a 30 zone (when 30 is safely possible) is not the cause of most, if any, deaths. Causes are varied, but generally the situations where one or two meters of road would make a difference are obviously rare.

So to emphasize my point: Permanent and ubiquitous speed control is the wet dream of many people, but it's a proxy. These people have a problem with cars. And there are many places where that is absolutely justified (dense inner cities for example, or streets in front of schools).

[Symbiote]

The increase in stopping distance from 30km/h to 40km/h is significant, more than 1-2 metres, and much of it is thinking/reacting time, i.e. still travelling at 40km/h. People are much more likely to survive an accident at a lower speed.

"The results from one of these studies is presented in figure 1, which shows a fatality risk of 1.5% at 20 mph [32km/h] versus 8% at 30 mph [48km/h]." [1] (in typically British fashion, the actual data is in km/h, but this general-public version of the document is presented with miles.)

The difference is even greater at the next gap (30mph→40mph, or 48km/h→64km/h). That was the subject of a road safety video a few years ago: [2]

[1] https://www.rospa.com/rospaweb/docs/advice-services/road-saf...

[2] https://www.youtube.com/watch?v=HeUX6LABCEA

[choeger]

Your source speaks of impact speed.

To achieve a difference in impact speed of 16kph, you would either have to go much faster, say 60 to 70kph before the accident or not brake at all.

There are of course situations where a driver cannot brake at all, but there the speed limit should be 10kph or less (and then going, say, 12, would again be more or the same).

On a road where 30kph is considered safe, 40kph is only slightly less safe.