said intersection to stop, triggering the walk signal on the opposite side of the road (green person means walk, red person means don’t walk), thereby allow the pedestrian to navigate the crosswalk in relative safety (assuming they cross before the allotted time runs out). However, vast differences in the relative sizes of pedestrian populations necessitate the existence of several variations to the popular design. 
In the most well-known type, pressing the button when there is no nearby traffic will immediately trigger the walk signal. If there is traffic, it will either allow one to cross at the soonest opportunity or, if sooner, its regular time interval. 
The second type, which is often hard to distinguish from the first, only allows crossings at its fixed time interval. Pressing the button only causes the green person to appear after traffic stops.
The third, and most deceptive type, does absolutely nothing. It is known as a placebo button.