Increasing and decreasing speeds have different results. A speed tgat seems too slow is not treated the same as one that is too fast.

You are correct that reducing speeds under the speed people drive doesn’t tend to reduce speeds actually driven. Speed limits are not as effective as things thst encourage driving slower.

Increasing speed limits doesn’t mean people ignore the limit and go under the new limit. They do scale up, including those who tend to drive a certain amount over the limit.