John B wrote:

In case I wasn't clear above, I meant "over a motorway without closing
the motorway for the duration of the bridge-building". Obviously you
can build a bridge over a motorway, but you can't do it without
disrupting traffic, which is what the original "why would it require
any at all" question was about.

Although you can minimise disruption by construction off-site (as with the
ECML/CTRL bridge and the new Bishop's Bridge Road bridge at Paddington) and
then rolling the new structure into place using a closure over a matter of
hours, preferably on a Sunday or overnight.