Underpasses
There are a few underpasses in the centre of London, some of which are merely for cars, some of which are pedestrianised and some of which cater for both. The pedestrianised ones tend to be collections of subways, although these are moving out of favour; I'm fairly sure there used to be some in Hammersmith which are now closed (although there's still quite a collection to the south and west of the shopping centre). There were several which went under the Shepherd's Bush roundabout, but these were closed and walled off a couple of years ago as they were dank, threatening and horrible
The longest, straightest underpass I'm aware of is that in [Beech Street]?, part of the Barbican development. Open to both pedestrians and traffic (it even has a bus stop), it has some very nice lights.
Others:
Traffic only:
- The ones on the continuation of [Lower Thames Street]? near [Blackfriars]? (there are a couple)
- Hyde Park Corner
- [Euston]?
- [Aldwych]? (northbound only, old tram tunnel; see also disused Kingsway section)
- [Blackwall Tunnel]? (move to tunnels? already listed? (river crossings?))
Mixed:
- [Rotherhithe Tunnel]? (you'd have to be fairly insane to walk through it, though, apparently. Also a river crossing (see Blackwall))
Pedestrian complexes:
- [Blackfriars]? (again)
- Marble Arch (large, grotty)
- Hyde Park Corner (large, relatively clean)
- Hammersmith
- St. Agnes Well - (kind of part of Old Street Station; easy to get lost)
- Edgware Road flyover (?)
- Aldgate Barrs
- Greenwich and [Woolwich]? foot tunnels (again, river crossings)
- Elephant And Castle (network of underpasses. very confusing)