You've thrown out quite a number of different topics, and some of the questions you're asking might also lead anyone who can contribute to a discussion about sp

You've thrown out quite a number of different topics, and some of the questions you're asking might also lead anyone who can contribute to a discussion about speed limits being set at safe speeds to admit to having committed what is currently a crime because of an incorrectly calibrated "limit". With respect to the engineering safety of the sudden 30mph sign (which I assume is in a rural area on an otherwise small highway); yes that sounds like you're describing what is typically called a "speed trap". A real traffic safety engineering standard SHOULD call for a clearly posted and graduated reduction in speed as approaching the town at a /bare/ minimum. However, a true engineering solution would isolate the town and the highway entirely. The point of the highway is to be a high speed transit corridor and it is inefficient and unsafe to route such a thing through a dense residential or commercial area. Instead there should be an exit system that leads off to a side path for such a town. I would prefer an actual barrier that encloses the typical route and possibly caps or bridges over it (maybe with a park) for existing situations that have developed in days with less good planning. In any such cases a designated detour route should also be constructed such that it is possible for "oversized loads", emergency response teams, and post disaster travel to route around failure in the now enclosed sections.