The conclusions are based on the idea that higher speeds create external costs to third parties which aren't required to be internalized by the driver, making the driver more likely to choose a higher speed than would otherwise be optimal.

Which means the conclusion is wrong. The solution is not to have a lower speed limit, it's to require the internalization of externalized costs: Impose a fuel tax that accounts for the full cost of the pollution created to society. Require drivers whose speed has caused a collision to pay higher insurance premiums. Then drivers will have to weigh the cost to others in their decisions and the identified problem goes away.

> It's part of the reason why "law" isn't the answer to every societal problem.

I sure wish more people like you had stronger control over the legislative process.

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.

No, they're deployed as regular speed cameras too. The M25 cameras are linked to a variable speed limit system to dynamically manage traffic flow; not sure if that fits in your definition of temporary (the cameras are permanent but the threshold speed changes) but there are also many instances of fixed speed cameras both on the trunk road network (eg the A14 near Cambridge) and even in villages (eg in Nottinghamshire)