Hi! Number one is actually strange...it should be, "The straggling unmade road was lined on either side by houses." And yes, the road is the grammatical subject. Imagine a long road...and on both the left and right side of the road there are many houses "lining" (following / demarcating) the road. The second example is also a passive. And here, too, the lake side is the grammatical subject. I think one big problem with these sentences is that they seem to come from an English language grammar book. My advice is to NEVER use examples in textbooks as real or accurate samples of English. Instead, read a blog article online. Read a travel site. Read anything that is written to communicate...then find examples in there. Good question, though!

A. "The straggling, unmade road was lined on either side by houses." The subject and doer of the action is "houses" in this passive-voice sentence. To transform to active voice, say "Houses lined the straggling, unmade road on either side". B. In the second sentence, "The side of the lake was lined by trees". Yes, "trees" does the action. It is the subject. In active voice, the sentence would read "Trees lined the side of the lake".