This restriction has a merit. A faster shutter speed is required to freeze a motion. This is the basic concept of using the shutter-priority mode. Sometimes, a tripod will be very useful when shooting with a slow shutter speed. The vehicle is frozen as if it parks in the middle of the road.
Motion blur shows a moving vehicle.
The resulting image is an underexposed image and has to be corrected with PhotoShop. But, the motion blur effect is much better than that of the middle one. Note that the background is sharp, indicating that the camera was fixed at a location without any movement. That is, the camera moves in the direction of the subject. Thus, the subject will be sharp in the image, but the background and foreground become blurred.