The I-66 shoulder usage was originally a rush-hour-only thing intended to provide a lane to compensate for the left lane being HOV at that time of day. Over the years, they've gradually increased the permissibility of using it, to the point where nowadays it seems more often than not the green arrow is on to allow use of the shoulder unless there's an incident blocking it. It's really not a great situation at all because vehicles don't necessarily have the decency to wait for the designated "emergency pull-off" areas before breaking down.
Hopefully the problem will go away with the planned reconstruction over the next five years.
I think one of the biggest problems with part-time shoulder lanes is that there will always be jerks who persist in driving in them when they're closed, often at fairly high speeds because they view it as their special shortcut. If you're driving legally and you want to exit at the designated spot, you have to be extremely careful to be sure there's nobody driving illegally on the shoulder before you start to exit (in my observation, the people who ignore the red "X" are, not surprisingly, the same people who don't turn on their headlights when visibility is poor or the sun is setting or the like).