Er. If you got to the end of the traffic jam and lo, at its end there was an accidient accross from you, do you necessarily know that that was the cause of your congestion? shurely (sic) it depends on how fast the compression wave travels relative to its constituent traffic?

On the occasions when I have been on that particular part of Odegra (chiefly, going home from school) some jams will have an apparently obvious cause, but they can also suddenly clear up for no visible reason.