its a fine line between to quiet and then too busy.
http://cavemancircus.com/2011/05/12/14-awesome-long-exposure-shots-of-airplane-takeoffs/
From that page, I think this
And this
have got it just right.
You have to remember it's not just about the trails, it's also about the setting. Lots of streaks of light on their own are not necessarily that interesting but put them in a good scene and you'll do much better.
But this takes some experimentation which you can only do once you're in place.
You also can't guarantee that from the moment you start your long exposure that you'll get the right number of vehicles through the shot to make the trails look good.
From my experience, you need to take anywhere between 5 and 10 shots from one position to get a good shot.
You'll definitely need to experiment with the exposure times too, those plane trails will probably need 20seconds and upward.
Have fun.
