I
imagine that the reason that
Ice forms first on bridges is that the
bridge is
isolated from the ground.
It can cool off
faster, and hence the ice
can form first.
The ground does not cool down as quickly as the air
surrounding the bridge, however water cools down
more slowly than land. Thats why Moscow
is much colder than, Dublin say, during the winter
even though they are as far north as each other.