When you're rendering your files out with motion blur you'll want to make sure you don't have any banding in your image like the image below.
To correct this we need to increase the samples used when rendering the image. Go to the Edit menu and the select Project Properties.
You'll notice that the default sample is set to 8, to increase the motion blur quality we need to increase the samples. For this example set the samples to 32, you can go higher but be warned the higher you go the longer the rendering times.
There now look at the final image, looks much better than the first one.
