This is something especially important in automotive retouching. Anybody who ever tried to replicate a motion blur taken in-camera knows this problem: The blur in camera starts out thin and then fans out... almost impossible to replicate, right? Or worse... in a rig shot, it follows the curve of the road...ick! How about these two options: You start by drawing a path with your pen tool and then the motion blur gives you the option to select "follow path" and will curve your blur to match your drawing. Same applies for drawing a shape. Use your pen tool to draw a rough outline of your curved street and the motion blur gets calculated to follow the movement and fanning-out of the shape.