Chapter 1: Location
This is perhaps the most important thing to consider when attempting the Invisible Rope Prank. There are two reasons for this, effectiveness and safety.
You'll want to pick out a secondary or side street that is two lanes wide and that averages only one or two cars per minute. It's important to have enough time in between cars to setup effectively without the car seeing you "get ready" or else they'll know something's up. In addition, you DO NOT want to try this on a busy road because the last thing you want to do is to cause an accident. Seriously, it's not a prank when someone gets hurt.
Lastly, when choosing a location, don't pick your own street. This should be self explanatory.
Chapter 2: Camera Location
If you're planning on shooting a video out of this, it's a good idea to pre-plan where to place your camera(s). For us, in our first video, we just had the camera on a tri-pod a little off of the road somewhat hidden in some bushes. For the second video, we planted the camera inside of a parked car on the side of the road.
Wherever you put your camera, make sure it's hidden. Better yet, make sure it's somewhere in a safe location in case if you have to flee the scene from any upset drivers until the coast in clear -- like inside of a locked car.