One If By Land And Two If By Sea

That was just the back-up plan:

Despite its historical significance, the “One if by Land Two if by Sea” signal was just a backup plan. It was meant to warn patriots in Chalrestown, a borough across the river from Boston in case if the messenger himself could not make it there from Boston to start his ride. With so many British troops present in Boston at that time P.R. could easily be arrested by patrols. But at the end he was able to safely leave Boston by boat and ride himself so the signal was in fact redundant. With this a popular myth was created that the lanterns were intended for Revere himself who was waiting for the signal across the river.

This is how P.R. himself described his plan to use “Lanthorns” in a letter to Jeremy Belknap, Corresponding Secretary of the Massachusetts Historical Society dated in 1798: 

"The Sunday before, by desire of Dr. Warren, I had been to Lexington, to Mess. Hancock and Adams, who were at the Rev. Mr. Clark's. I returned at Night thro Charlestown; there I agreed with a Col. Conant, and some other Gentlemen, that if the British went out by Water, we would shew two Lanthorns in the North Church Steeple; and if by Land, one, as a Signal; for we were aprehensive it would be dificult to Cross the Charles River, or git over Boston neck."