It seems it will be marginally better than now, and there will be a bike lane guiding cyclists form the park to the safety of park lane.

However, you will Still have 4 lanes of cars coming from the South East (through the Park and along Flatbush) being squished into 2 lanes in Flatbush. If both routes are really used at capacity, this can only mean traffic jam. The bottleneck is not the park, it is Flatbush. To eliminate this jam there are a few options: reducing Flatbush East and the Park both to one lane, or closing one of both.