The Dutch now believe that the number is 230cm. This is an increase from a previous recommendation of 200cm. The thinking behind this number is roughly as follows. Apparently there are Dutch laws stipulating that bikes can't be wider than 75cm. So this is the starting point.

But since it's impossible to always ride in a perfectly straight line, there seems to be a generally accepted rule that, at an absolute minimum, cyclists need about 100cm of width to themselves.

If you now double this so that two people can ride side-by-side, you're at 200cm. This is an important design criteria because the Dutch also seem to believe that (1) cycling is a social activity and (2) a child should be able to ride beside their parent. (Love this!)

Finally, add in a bit of buffer so there's room to pass slower cyclists and/or nobody feels like they're going to crash into oncoming cyclists, and you get to 230cm as the ideal width of a single bike lane.

I'm not sure I had given this much thought before, so I look forward to scrutinizing (and possibly measuring) every bike lane I ride in going forward.