In theory, bus "bulbs" should allow for a faster flow for buses by allowing them to pick up and discharge passengers without having to get out of the traffic flow. In reality, bus "bulbs" tend to create more traffic congestion without really speeding up the service.
The reality is what is happening along Broadway in the Big Apple. The bus "bulbs" are also generating many complaints from residents and businesses in the area. The only ones who seem to like them are the MTA brass, city officials and mass transit advocacy groups who claim the bus "bulbs" reclaim the street from cars and gives it to the buses and pedestrians.
Like many things related to public transit, bus "bulbs" have their place but often are not properly utilized. "Bulbs" technically narrow a street and space that was once used by trucks unloading at businesses will move to the street from the curb when that is all that is available.
Pittsburgh has one as well from the time when South Hills routes still used Oliver Avenue. After placing the bus "bulb" in, the buses faced a much harder right hand turn as well as more problems with vehicles in a legal loading zone because of the narrowing of the street.
Bus "bulbs" do work well in residential areas with lower volumes of traffic but transit planners love to place them in already congested areas rather than where they would work best. The idea is to keep the bus in the traffic flow to speed the ride but as I mentioned, the reality is that is just makes a congested area even more congested which ultimately slows the service down as buses behind are slowed down as well. When all is said and done, everything is slowed down by the incorrect usage of what should be a valuable transit tool.