2011-08-13 - 1948 London Olympic Ride

http://www.britishcycling.org.uk/cycletraining/article/ct20110110-cycletraining-Bitesize-Bikeability--Part-4--On-Road-Positioning-0

Riding in the primary position is sometimes called taking the lane as the cyclist takes the position normally taken by the motorist, who is thus prevented from attempting to overtake.
When riding in the primary position, trainees should travel at a reasonable speed, as part of the traffic flow. If, however, traffic is building up behind them and the road ahead is clear, they may wish to move to the secondary position to avoid obstructing other road-users unnecessarily.

The secondary road position (roughly 1 metre to the left of the traffic flow and not less than 0.5 metres to the edge of the road) may be appropriate if the road is wide enough to allow safe overtaking, and the rider’s safety is not reduced by riding in this position.
The correct choice of road position can be used to help riders affect driver behaviour, in order to help avoid collisions with other road users, or to avoid them having to brake suddenly. The secondary position is a useful position on most minor roads where there are few parked vehicles on the carriageway