Halad doesn't necessarily mean forward motion, it depends on the context. In this sentence it means that the cars move forward, otherwise it would be quite weird. :) But you can say for example "hátrafelé halad" = "it is moving backwards".
Halad also has a more abstract meaning: making progress. "Jól haladok a munkámmal." = "I'm making good progress with my work."
Another often used verb to describe movement of cars is "megy".
Use the report button next time and check "My answer should be accepted". There are better verbs to use than "move". I've suggested "travel" in another similar sentence. For some reason, I didn't think of "proceed" which is an excellent word. We do talk about a "procession" of cars, like in a funeral procession or at some other important event.