In yourchallenge, you discovered that speeds could affect how blurry a moving image was within your scene. You ended up with nice background, but a blurry car, as you used slower speeds.
Now slow speeds can be very effective when photographing moving water (waterfalls etc), but on a car, the vehicle ends up blurry with your background .
This challenge is about reversing that, using a slower to capture the car reasonably sharply and the background blurry. To do this we use a technique called 'panning'. Panning involves following your subject with the camera and taking the shot without stopping the movement of following the vehicle.
A good analogy would be the golf swing, you swing your club, hit the ball and the club continues in the same arc, even though the ball has already been hit.
So for this challenge, you will need to get out on your street again. If your camera has a continuous or 'servo' focus setting, use that. Your and camera should then track your moving object to try and maintain focus on it.
Start, by not taking any photos, but look through the viewfinder and just follow the vehicle as it passes by, try and keep a part of the vehicle in the same part of the frame as you 'pan' with the vehicle as it passes.
Practice this a few times without taking an actual shot.
Now set your camera to mode and chose a of about 1/30 - 1/60th second. Now as you pan with your vehicle try and keep a constant flowing motion as you follow the vehicle and press the button, but keep the panning flow going. Do not or jerk as you take your shot, it should be one continuous fluid pan, following the vehicle, with a nice soft press during it.
Try it with even slower speeds, down to 1/15th second.
If you dont have access to decent regular traffic, try it on the kids riding bikes around, or a pet (dog) running around the yard etc. All you need is a moving object that moves across in front of you.
Panning can take a lot of practice to do well, but come back to this thread, post a couple of your shots and talk about them, what does the background look like, how does it differ from your shots in challenge II.
Panning is a good skill to learn as it is often used in sports photography, and to capture shots of birds in flight, as examples.
most of all, have fun!