Just for a change, let me tell you a German joke, but I have to explain a bit before telling:
(1) In German, 'one' and 'a' are expressed by the same word, 'ein'. (2) I don't know how you call a driver driving on the wrong side of the motorway, we call him a 'ghost driver'. Now the joke:

Urgent traffic warning in the radio: "On the motorway M1 northbound between Exits 37 and 39 there's a/one ghost driver coming ahead. Please do not overtake and drive carefully in the left lane. We'll announce it if the incident is over."
One driver muttering to himself: "One??? Hundreds!"

Wolfgang