How to create a STRONG motion blur?

I have footage taken from inside a moving car; the car itself is relatively stationary, and I'd like to blur what's outside the window.

I've tried Pixel Blur, CC Force Motion Blur, experimenting with settings.... can't seem to get anything much more than a gentle suggestion of motion blur. I don't need to re-create a lightspeed effect, but would like to have more control over making a very noticeable blur.

Any help appreciated!