As an illustration: here's the results of a similar poll of Democratic voting intentions in April 2006, i.e. a similar amount of time before the 2008 elections:

Hillary Clinton 38%, John Kerry 14%, John Edwards 13%, Joe Biden 5%, Wesley Clark 3%, Russ Feingold 3%, Bill Richardson 2%, Mark Warner 2%, Evan Bayh 1%

That's right; Barack Obama is not even in there. In fact, in Wikipedia's whole list of polls for that election, until October 2006 he didn't even show up in any of the polls except one; though in fairness once he did start to show up during that month, he immediately grabbed hold of the second place and didn't relinquish it until he drew level and then moved past Hillary in early 2008.

Admittedly, this is the best example I could find among a number of recent elections, most did feature more obvious front-runners from an early stage (I was surprised to see how crushingly dominant GW Bush was in the Republican field, for three full years before the 2000 elections). But I do think comparing 2016 to 2008 makes the most sense, since it looks likely to be one of those wide open elections again, with no incumbent president or VP running (at least, I doubt Biden will run, and if he does I doubt he will win even the nomination), and even no prominent candidate of earlier elections likely to run, other than possibly Hillary.

But I agree that the bridge thing is likely to die down and not cause Christie much damage, until possibly during the actual campaign when his opponents may try to make it the focus of attack ads. Whether that will work very well - who knows, it'll depend on what he and the others do between now and then.