80 or Not?

I have reservations about the proposed 80 mph limit on motorways. Certainly, on many occasions, 80 mph is perfectly reasonable - but what of the times when it is not?

Today, drivers look at the 70 limit and often do 80 (guilt, guilt, guilt). With the new laws, it will be look at the limit then do 90. Psychologically - that's how it works.

It will lead to more deaths. Is this a price worthy of the paying? - And this from a writer who understands that most accidents are caused by folly rather than by speed. However, speed will frequently and inevitably exacerbate the consequences of folly.
I might suggest that the only way that this can work is by having countless temporary speed limits where high speeds are inappropriate - but these must be better than what we have at the moment. Signs of '40 mph - FOG' in bright daylight with a visibility of five miles must not happen - people need to be able to trust the signs.
I do wish that they were talking about this as a 12 month experiment.