One of these works. One of these handicapps your infrastructure's capacity, reduces safety in certain conditions and has a handful of other negative externalities.
I would wager that were it not for someone following the speed limit the woman in OP's story would not have been cruising along in a line of traffic with her situational awareness diminished by limited visibility and temptation to follow the leader without giving her full attention to driving.
Traffic is safer when segregated by type (e.g. you don't ride a bike on the highway) and/or speed.
You can design roads to reduce speed but just posting a different sign has minimal positive effect so better limiting/controlling interaction between different types/speeds of traffic on roads you already have (e.g. adding sidewalks or bike lanes) works better there.