(extract from our Traffic Calming Good Practice Guide)
As has been stated, the speed reduction effect of horizontal deflection traffic calming is dependent upon the effective road width and is limited by the need to allow the passage of all non-prohibited vehicles and to provide for the safety of cyclists.
For a cyclist to be safely overtaken, the width required depends upon the width of the overtaking vehicle but in general a lane width of 4m is needed. For widths of between 3 and 4m the cyclist will be 'squeezed'. Road widths less than 3m ensure the overtaking vehicle must wait behind the cyclist.
Separate cycle lane or cycle bypass provision is advised where at all practicable but this may not always be possible.
The above remains guidance but it should be noted that the minimum acceptable lane width that would allow all types of vehicle (apart from agricultural vehicles) to pass is approximately 2.75m.
See also Traffic Calming