meaning of "cars have been backed up" ?

English as a Second Language English Vocabulary, Grammar and Idioms
1

Hello,
I’m new here and I wanted to ask you about something I heard (and read in the subtitles!) and apparently misunderstood on a DVD.

There’s been an accident on a bridge in San Francisco and it is now closed to traffic.Then we can hear the following sentence on the radio :
“Cars and trucks have been backed up for three miles on the Marin County approaches.”

My first idea was that it meant that the cars’drivers had been asked to move backward from as far as three miles from the bridge. (Probably because of this meaning of “back” that I remembered: “back: tr.v. To cause to move backward or in a reverse direction: Back the car up and then make the turn.”)

Then I turned on the French subtitles and they read something along the lines of “cars forming a line of 5 kilometers…”. So I look it up in my favorite on-line dictionary and found this :
“back up : To cause to accumulate or undergo accumulation: The accident backed the traffic up for blocks. Traffic backed up in the tunnel.”

Since I already had experiences with bad subtitles/translations I was wondering if this was yet another instance…
But writing this I realize that the first explanation makes little sense and that the latter must be the right one. It looks like I’m making progress just writing to you !

But I still have some questions :
a. What would the sentence be if I wanted it to have the meaning I first thought it had (altering it as little as possible)?
b. Was there a clue? How could I know my first inkling wasn’t right?
c. Or am I completely wrong?

Thank you for being there.

2

The second interpretation (cars forming a long line of stationary or slow-moving traffic) is correct. I think this is just a case of asking which meaning of “back up” is more likely. It seems more likely that traffic formed a queue than that three miles of traffic was made to reverse (quite a thing to organise, I would imagine).

3

Thank you for your answer.

So if I understand correctly, there is no other way to know the right meaning than choosing the more logical.

And if someone said “Cars have been backed up for 100 yards” without being more specific the two meanings could be equally valid?