In message , at 10:33:12 on
Mon, 30 Jan 2012, Basil Jet remarked:
There's a tunnel under the Terminal 5 Roundabout. It seems that hardly
anyone going to or from any of the terminals would use it, so I'm
wondering why it's there - but then you could also say the same about
the less substantial underpass beneath the Terminal 4 roundabout. Why
were they built?

It's the perimeter road. Likely to be used mainly by airport workers.

Maybe there's a tradition that it isn't interrupted by major routes into
a terminal. (For T1-3 it goes above the tunnel mouth).
--
Roland Perry