Paul Corfield wrote:

As at 27/1/07 there were 7037 buses required to run the scheduled TfL
services on Mondays to Fridays when the schools are open. Note that this
figure does NOT include any spare buses that the operating companies own
to cover for repairs, damage, training or just to provide flexibility
for swapping buses over during the day. Typically there is a 13-15%
margin for spare buses so the 8,000 value is about spot on.

That would explain why I never see fewer than three vehicles standing
idle at TfL layover points. More, incidentally, than you're likely to
see simultaneously at Stalybridge bus station of an evening.
--
Joyce Whitchurch, Stalybridge, UK
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