FF/RWD a video clip, while part of the frame plays normally

As seen in this music video:

(skip to 3.42 for a good example).

So I want to have, for example, a shot of cars on a motorway skipping between playing forwards (normally) and then playing backwards, in a loop. This should be straight forward to achieve.
I also want to have another part of the frame playing back normally, while the rest of the frame is in a play forwards/ play backwards loop. How can I do this? Any advice much appreciated.

Please don't post the same message multiple times. It clutters up the forum and will delay you getting an answer.

If you had a segment that needed to play forward, then backwards, then forwards... if it were me, I'd just duplicate that clip, put it on an adjacent track, go to Change Speed, and hit the Reverse button and position it where necessary. Do this enough time, you can make the action/reaction fairly smooth. Looping it would simply require cutting-and-pasting, cutting-and-pasting, over and over again.

My way of thinking is, it'd be easier to do this than to try to figure out the keyframes and math necesssary to do it all with the Change Speed keyframe settings... but that could also work.