A quick and very underused technique to create blur effects.
The principle is simple: you create a short animation and, instead of rendering it entirely, just render one frame, which will be your final artwork.
Step 1: you need some parameters to animate. I used a simple plastic with only 2 transforms.
Step 2: Open the animation editor (Ctrl+T or Window > Animation Editor). Set total length to some reasonably amount of time. I use 30-60sec currently, since its short enough to not get lost and long enough to have some room to tweak to control the blurring.
Exposure time controls the amount of motion blur. No worry much about getting the "right" value right now, since you can change it at any point.
Since we are only looking for one frame and not a full animation, there is no need to create many keyframes: 2 usually do the job. If, after making the animation, there are no frames that you like, you can just change one of th