It is tough gaining more usable detail at night, especially at a distance like that, on a target moving as fast as they were. I think you would need a lot of extra infrared to sufficiently light up just a small portion of the road and of course a higher zoom camera like that varifocal you were talking about in the other thread. Then remember, you will probably still get blurry pics with the camera's default settings. Increase the shutter speed in the camera's image options to combat that, then review footage of cars passing each night to fine tune your exposure time and lighting and camera position.
If you can afford it, a hikvision darkfighter or dahua starlight camera may also help, as they have better low light sensitivity right out of the box.