In Chapter 4 of The Great Gatsby, who rides a motorcycle alongside Gatsby's car?

In Chapter 4, while Gatsby and Nick are taking a drive to the city in Gatsby's car, an unnamed policeman rides alongside on a motorcycle, and Gatsby must pull over.  Gatsby takes a card from his wallet and waves it at the officer, who immediately lets him go, saying, "Know you next time, Mr. Gatsby.  Excuse me!"  When the policeman leaves, Nick asks about the card, and Gatsby responds that he "was able to do the commissioner a favor once, and he sends me a Christmas card every year".

The incident is significant because it supports the suspicion that Gatsby is involved in some undertaking, most likely illegal, which gives him a tremendous amount of power and prestige.  This adds to the atmosphere of mystery and dangerous allure which surrounds him in the community, and to which he has contributed with the cryptic story of his life as revealed to Nick earlier in the chapter.