[mjevans]

I want to propose a second more radical suggestion.

I think pacer cars might work very well for phantom traffic jams, but I very much disagree about /how/ they should be used. Instead of encouraging an over-capacity jam, I believe that the pacer cars should expressly communicate to other traffic something along the lines of.

"Temporary" / "Speed Limit" / "Follow at XX"

On a rear message board.

The pacer car would then draw out the stuck traffic in to the space /ahead/ of the jam and encourage the compression wave to expand to the front instead.