Thanks for the feature request. This is something we've discussed internally and it's on our wishlist for a future release. As a possible solution you could try combining the styles as you suggest and then introduce a toggle that swaps between the road striping and the markers. In that way you could set up the roads, turn on the car markers, place the cars using FP, and finally turn the markers off again. See the video and the attached file for a demo.
Alternatively, unless you are using Forest Colour to randomise the car models textures you could always use RailClone to distribute the cars. It has the same instancing power as Forest Pack and you could easily make changes to the traffic without having to reapply the Forest. You can see an example of a style like this here
I hope that helps, please let me know if I can help further.