I can't possibly see how 2 cyclists can take up the same as one
Most cyclists I see riding two abreast (& this includes club runs & casual riders alike) take up 2 metres minimum widthways. That's a minimum of 50cm from edge of road which is secondary + 2x55cm rider widths (that is conservative as I take up a minimum 62cm measured) plus another 30-50cm between them(a lot of cyclists cannot ride as close to each other as they do in the pelaton!)
A lot of A roads in the UK are only 6-7m in width,
A width that is almost optimally nasty for cyclists.
It is not wide enough to permit safe overtaking in the face of oncoming traffic, but not narrow enough to make this obvious to some impatient motorists who will aim for a gap a few cm wider than their wing mirrors, rather than wait a few seconds for a safe opportunity to overtake.
(EU min standard is 7m I believe), 2 riders abreast ensure that a car(approx 1.7m wide) giving 1 metre overtaking space would in most cases HAVE to move completely over the centre divide to the other lane, riding single file would mean a greater opportunity to pass cyclists with enough room for both the cyclist and for oncoming cars. Lorries are a differing matter altogether though.
OK - a bit of maths for the situation where you come up behind a single cyclist riding (inadvisably) in secondary with a stream on oncoming traffic on a 7m wide road.- 0.5m
- minimum advisable distance from edge of road+ 1.0m
- dynamic envolope of cyclist (0.75m elbow to elbow + 0.125 either side as a certain amount of side to side motion is inevitable).+ 1.0m
- absolute minimum clearance for overtaking a cyclist at low speed (though I don't consider that adequate and would not drive that close to a cyclist)+ 0.5m
- distance opposing traffic keeps from the kerb (assuming they are all driving very carefully and precicely and have anticipated your stunt)+ 1.7m
- width of opposing vehicles - assuming they are all small cars.+ 1.0m
Add this together and it comes to 5.7m
leaving 1.3m of tarmac for you to attempt to squeeze your 1.7m car through.
Unfortuanately there are a few motorists too impatient to wait for a safe oportunity and will head for any gap that is physically wider than their vehicle (and if they are going to take a chance with safety margins it will be at the expense of the road user who presents least risk to themselves) hence the need to adopt a primary position on such roads.
The vast majority of drivers are
careful and responsible and will change lanes just like the picture in the highway code (we tend to notice the minority of close overtakes rather than the vast majority of uneventful ones). To competent drivers the key issue is not the width of a group of cyclists that they are overtaking, but the length. By riding side by side rather than inline you are actively helping them to overtake safely.