Aprepositional phrase can be used as a/an
a. adjective.
b. object.
c. subject.
d. conjunction.

object

Ex.  I went to the store.

I = subject

went = verb

store = object 

prepositional phrase = to the store

Either: Going through a red light is illegal

Or the fines are not high enough to discourage drivers to stop going through red lights.

Explanation:

Red lights are illegal and it’s a big deal but your argument is also about fines.