As an option, many areas now have those digital warning/traffic info signs on the freeway. Why not have a “variable speed limit” I have seen them with “prepare to slow down” why not put “reduce speed to 50” for a few minutes if a jam occurs. True it dosen’t mean people will obey but laws can be adjusted, highway patrol could cite people for failing to obey “the big flashing yellow sign” 
Of course, smart highways (http://www.memagazine.org/backissues/may98/features/smarter/smarter.html ) will solve all of these problems! 