There are several possibilities from somewhat mundane ( just a replay of something you heard/saw in passing during last day or so) to a confirmation of your own ability to see risks others don't) or a more esoteric message to yourself about something you might not being seeing all the risks about which you might not be seeing all risks--and are not ready quite yet to acknowledge them consciously, so your subconscious disguised it with a stranger on a motorcycle.
i wouldn't put too much stock in or fret over it unless similar dreams about dire consequences to risky actions occur over the next week or two. Then pay attention to the details--is there any thing (build, clothing style) that would indicate the person is just a surrogate for yourself? While i don't subscribe to the notion that everyone in our dreams are just substitutes for ourselves--sometimes they are, sometimes we dream ourselves as if we're watching a movie, and are actually aware--'yeah, that's me' it's often the tipoff we're dreaming viewing oneself from the outside like that.
