It's true, the math works out that on average public transit pollutes more per passenger mile than private automobiles. Think about it: an 18-ton bus has to accelerate and stop to pick up and drop off each and every passenger, and often carries only a handful of passengers during off-peak hours.

It's also works out that bicycling is also worst for the environment! It takes far more resources to produce food calories than other fuels... for example, raising cattle uses 15,000 liters of fresh water per kilogram of beef produced, and produces 27 kilograms of CO2 for each kilogram of beef. When all the math is done, converting calories of food intake to miles cycled, and assuming a typical diet of foods, cycling is worst than driving!

However, there are other reasons to cycle and use buses, such as health, cost, congestion, etc.