Try C4Ds native projection shader
That did the trick, thanks!
Have you tried the volumetric material?
I tried that first, but this particular scene has some quirks that prevented it from working as expected. Ideally, the headlights would be a light that matches the shape, and then that would illuminate the volumetric shader and fade as it leaves the light. However, these headlights are made up of a bunch of individual lights to get the reflector look correct, plus separate beams for the road illumination and an IES for the light bleed. The resulting illumination was quite unnatural looking.
When it became evident I couldn't use "realistic" in-scene lighting for the fog, I tried a slight bit of emissive light on the volumetric shader. That had the problem of not fading as it left the light, and I couldn't find a way to fade it out with distance. The emissive light also started shining back on the car, which I probably could have fixed with a corona compositing tag had I continued down that road.
In the end, the subtle faked effect is very adjustable and I can use gradients to fade it at whatever angles I need. The fresnel properties also fade it out automatically as the camera moves in front of the car, to prevent visible edges.
Thanks!