Centralised discussion space for TfL plans and cycling in London

You only get double the traffic if you double the limiting factor of traffic capacity, which is junction capacity. Two-way traffic needs more complex junctions, with therefore lower capacity and therefore less traffic can use them, and because traffic is elastic capacity less traffic does.

In fact the one-way system that was there before was probably introduced to "reduce congestion". The problem in London is that any spare capacity is eaten up by induced demand until congestion returns to the same level.