Others complain about bikes just generally slowing the flow of traffic. And it's true, it does happen sometimes, although, on shared roadways, bicyclists have as much legal right to be in the lane as motor vehicles. Bike lanes can help in this situation but obviously there aren't bike lanes on every street.
A study recently released looks at the delay caused by bicycles on streets without bike lanes, and it's more significant than you'd expect in some instances. So while bicycling takes people out of cars and lessens traffic congestion in one respect, it can add congestion when bicyclists ride on busy roads. A catch 22? That's what this Streetsblog article says could be the case.