The city is about to begin construction of a controversial two-way bike lane on Prospect Park West. Supporters, like Paul Steely White of Transportation Alternatives, say that the lane will finally give bikers a way to legally travel north in Park Slope, but also slow down car traffic on Prospect Park West because the bike path will require the elimination of one of the road’s three car lanes. Opponents, including many residents of the strip, say that squeezing Prospect Park West from three lanes to two will make traffic worse — and besides, Prospect Park already provides ample routes for cyclists. It’s a debate that has riven Park Slope — and served as a microcosm for bike lane debates citywide. Here’s how two experts — pro and con — see it.