Does halad alway imply forward motion? Is it the verb most often used to describe movement of cars?
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".
Why isn't it the plural version of car? autok? (my laptop keyboard doesn't have the accents available so I know that is typed wrong)
The word négy already shows that there is more than one car, so you don't need a plural ending on the noun to say that again.
Oh OK. That makes more sense. I always get confused when to use the plural and not to... thanks! :D
Because autó is in the singular form (because there is a number in front of it).
again: much better in English. Finally somebody actually has a great ear for English. "beside the river" might be OK for stating the location of a stationary object not a moving one.