Restrictive elements are modifiers or appositives that restrict the meaning of the nouns they follow. Because they are essential to the meaning of the sentence, they are not set off with commas. (For a full discussion of restrictive and nonrestrictive elements, see 32e.)

The modifier who think they own the road restricts the meaning of Drivers and is essential to the meaning of the sentence. Putting commas around the who clause falsely suggests that all drivers think they own the road.

Since Mead wrote more than one book, the appositive contains information essential to the meaning of the sentence.