You need to choose a slower shutter speed and a wider f stop for panning, Jenene. If you used shutter priority mode and selected 1/20th of a second instead of 1/160th of a second, you could have slowed down the shutter by three stops. Your aperture would open up by three stops to compensate for exposure. Panning at 1/120th would give you a completely blurred background and a partially blurred car, creating a tremendous impression of speed. Practice it.
