They are doing transportation math.  Essentially they don't care if you are in a car or a bus.  A bus can fit 30 ish people at a time and a typical car has 1-2 at any given time.  Their math suggests that it's better for 30 people in bus to travel in one express lane opposed to 30 cars traveling down 2 lanes.  This all depends on people actually using the bus, but the Colfax lines are typically packed so it's reasonable to think improved service will stimulate increased ridership.  There are a lot of assumptions made in their projections but they are all based on transportation engineering and studies.  Denver doesn't have the real estate required to move everyone around in their individual cars (especially if the City adds another 100,00 residents over the next several years).  So we might as figure something out that doesn't clog the entire traffic grid.