This achievement description: "King of the Road! Awarded for getting ahead of all 5 rivals in an online lobby!"
To get the achievement you need to be in a full lobby (6 players). I does not matter if the race is ranked or unranked. You must have at least one takedown on all the 5 players in the lobby (green triangle). If you get this during a race you will only get the achievement when you return to the lobby after all the races have completed. If someone quits out on the way back to the lobby you will not get the achievement.
One important thing to note is that you can get this achievement without racing at all. If you are in a lobby with 4 other players that you have takedowns on you will get the achievement if another player joins (the 5th) and you have takedowns against that player.