Almost every additional cyclist is one less car. And since bicycle infrastructure is a lot cheaper than vehicle infrastructure, spending a dollar on bicycle infrastructure can improve conditions for car drivers more than spending that dollar on car infrastructure.
When I bike and have to wait at a busy crossing alongside other cyclists and cars nearby, I take note of how vastly different the two situations are.
Each car (often an SUV or a pickup truck) typically only has one passenger. Each bike also typically only has one passenger. But in the space of one car at a stoplight, you can fit 3 to 5 cyclists.
