At this week's committee meeting, Roger described the problem he had with ice on the bridge between Eastern Road and Third Avenue. There was no ice on the roads at the time.
Turns out that is a well-established phenomenon and it's quite common to see signs saying "Bridge freezes before road" in the US.
The reasons are explained on several sites - Google "bridges ice before roads" to see a few. This site is particularly graphic.
Black ice is specially dangerous over bridges, so be careful out there!