When I see the title of this thread, I always think someone is reviving the old idea of putting the South Circular underground under FH. My hopes are dashed when I see it's actually all about the subway.

Why don't we follow established British usage, which I think restricts the use of the American import 'underpass' to a road which passes under another and uses 'subway' to refer to a short foot tunnel?

It's as bad as 'power outages' for the good old English 'cuts'!