[GenerocUsername]

Obviously a static sign is not aware of your current state, so it's message can only be interpreted as relevant to your likely state... i.e. the posted speed limit.

[recursive]

If you should slow down relative to the posted speed limit why not change the speed limit to reflect that directly?

[estearum]

Usually the reason is the "slow down" portion is very small, and it's confusing to shift down the actual speed limit for a 200 foot stretch of road then increase it again.