[Vivtek]

You appear to be suffering from the notion that there is a single defined value at which the speed limit will be optimum at all times.

But I entirely agree with your larger point that violation of arbitrary speed limits because of obvious engineering considerations does train people to break the law casually.

[tomkarlo]

The quote I posted explicitly points out that the speed limit is based on worst-case road conditions. Obviously a single-value speed limit can't be optimal in all conditions, for all drivers, in all vehicles. It has to be based on what's safe for most vehicles in most conditions, but there is a single optimal value for those constraints on a particular road.

I'd have no problem with variable speed limits on roads - I think it would make a lot of sense to have speed limit displays that change based on time of day, weather conditions, etc. But until that's practical, speed limits will be set based on worst-case, with a proviso that drivers ALSO have to exercise discretion and further reduce speed in exceptional conditions like fog or snow.