Alright, here's my script:
It's python, and it may or may not work
Start with a car, and a path, but not connected to it.
Run the script (in the editor or whatever) which will give a simple UI. Input the number of cars and the number of frames within which all the cars should traverse the path.
Then select the car, select the path, and hit execute.
The cars will begin at the start of the path at the current frame (so watch at what frame you hit execute) and randomly start moving down the path. They all go at the same speed (which is the number of frames/number of cars, you can modify the script to go more dense, more sparse, more randomly).
You can also modify axes, path parameters, max values etc in the script if need be, pretty simple stuff.
I don't know if this fits the bill (I didn't get many parameters to start with :p) but I learned a ton with this exercise about scripting, so I don't really care if no one can use it.