When you spot a motorcycle at an intersection, say out loud "Motorcycle!" or "Motorbike!" and claim 3 points! Once a motorcycle is spotted no other player can receive points for spotting it.
If you spot a motorcycle at an intersection but another player says "Motorcycle!" first, that player is awarded the points.
If two players say "Motorcycle!" at the same time for a motorcycle at an intersection they are both awarded three points.
Make sure to check the Intersections FAQ page to see what counts as an intersection.