Mizter T wrote:

Wikipedia tells me that there are two smaller tunnels that did used to
be dedicated for pedestrians and cyclists, though these have now been
modified to take a lane of cars, with cyclists apparently having
priority and pedestrians getting booted out. I'm mildly intrigued -
one day I'll go and investigate this arrangement.

The southbound tunnel is heavily used as a handy way to get into the
airport. I find it mildly scary because the walls and roof are so close to
the vehicle that it feels like playing that kids game where you move a metal
loop along a metal wire and mustn't position it even a centimetre wrong or
the bell will ring. The northbound one seems even narrower and lower, and is
virtually unused because there is a forced left turn at the northern end,
meaning it's no use unless you want the A4 to Colnbrook.

http://maps.live.com/default.aspx?v=...2826&encType=1
shows the red cycle path encouraging cyclists to use the middle rather than
hug the kerb where some fool might try to overtake them. There is a similar
marking on the southbound one.