On Sat, 11 Mar 2006, Roland Perry wrote:

Originally, weren't they exclusively for bicycles (and essential
maintenance/emergency use).

I don't know about "originally", but, quite some years back, as I
recall, they weren't well signposted (possibly as a deliberate
policy), and the route to them was a bit of a dog-leg, but some of the
drivers who took us from Rutherford Lab to LHR would know the way, and
would use the side tunnel when the traffic in the main tunnel looked
heavy. It appeared to me that any car driver who could find the way
there was free to use them (provided they gave way to cyclists...).

More recently they did seem to be better signposted, and quite busy.

The picture in the cited PDF file suggests that there's a double-deck
side tunnel. If that's so, then it comes as a surprise to me, I have
to admit.