Thanks for the question! So the connection between NR (new residents) and NRB (new roads are built) comes from the sentence that starts with “these buildings.” The second part of this sentence tells us that “…the increased population would probably result in overcrowded schools and would certainly result in roads so congested that new roads would be built.
So the new residents lead to congested roads which lead to new roads being built. That’s how we get
NR —> NRB
Hope this helps! Feel free to ask any other questions that you might have.