Those traffic shots could be referred to as "ghost shots"...All it involves a a delayed/slowed shutter speed for an extended period of time e.g. 6-12 seconds
All this does is make stationary objects seem crisp compared to objects that are in motion during that time...
Now to make a real "ghost" shot you would use that same method, but with people...Have a bunch of stationary people and someone moving in front of them while the shutter is delayed...
Perfect example:
Notice that the person in the white gown seems to be a "ghost", but if you look closer you can see the blue jeans they are wearing...everyone else is still during the shot, but that person...
Easy!