'Space for cycling' protest ride 2nd September

What is the effect be of differentiating and segregatiing cycling further in a city with limited scope for wholesale infrastructure changes? What might the downsides of this approach be?

The infra for cyclists is not only about segregated lanes. I think we can have both. Some areas could have cycle only areas, other roads need to be re-designed. Having visited many cycle friendly cities i can only see the benefits of supporting the growth of cyclists with a safe infra.

I'm in france at the moment. It is a joy to ride on the cycle paths. They in no way slow down the pace of my journey.