"Haled" can also be translated as "pass". And therefore, "Fifty cars pass beside the river" is a fine alternate answer.
Wasn't "halad" translated as "proceed" in an earlier lesson? I got this marked wrong.
"Fifty cars are proceeding by the river" is a good answer and should be accepted. You can report it if it comes up again for you.
What about ...are passing by... ?
Fifty cars are moving beside the river. You taught me that there’s no difference!