I would like to add motion blur to a scene in photo mode, but can't

The "process motion blur" is assigned by defaul to my accelerator pedal.  When I press the pedal the screen goes black until I change a value (e.g. shutter speed).

Nothing seems to change when I do this.

The game has no option to assign "process motion blur" to the keyboard.

Is this feature bugged or am I doing something wrong?