What is the slowest a human can ride bike?

There is no definitive answer to this, as it depends on a number of factors, including the terrain, the rider's fitness level, and the type of bike. Generally speaking, however, the slowest a human can ride a bike is around 1 mile per hour. This is roughly the speed of a very slow walk, and it would be difficult to sustain this speed for long periods of time without becoming fatigued.