Driving down a busy street, a bus seems to be about twice the size of another car, and a little bit harder to pass. But that bus is also carrying about the same number of people as all the other cars for several blocks combined. In other words, you could replace all the traffic with just two buses. If the whole lane were replaced with light rail, it could carry 18 times as many people per hour.
Via Smart Growth America, here is a visualization created by the City of Munster, Germany, showing the amount of space on a street taken up by the same number of solo drivers, bus riders, and bicyclists: