Biologically, human beings are aroused by the amount of stimulation around them and the modern city is full of sights that trigger psychological arousal. When we walk through cities we are bombarded by stimuli of all types. Imagine walking into a busy street with the noise of people, cars and roadworks. The sounds may be unpleasant but still stimulate us. Adrenalin pumps through our body, making us react more quickly than normal; our heart rate increases the blood flow to our muscles, enabling us to break quickly into a run to catch a bus. Our heightened arousal means that we can react quickly. Although this can be useful, there is a limit to how much our senses can take in at once. When we step off a train at a busy station after a relatively quiet journey, our senses are bombarded. There is simply too much noise and too much happening for our brain to process. We're overloaded.

The general level of arousal in the busy station may enable us to attend to more of the stimuli, but we still have to ignore some of what is happening, as we do not have the capacity to deal with everything: it is as if we have tunnel vision. City life consists of a huge number of encounters with potential sensory and information overload. The necessary filtering out of some of these stimuli has been institutionalized into new rules of behaviour for the city. People don't stop and talk, they keep their faces blank and their eyes straight ahead. They are not just protecting themselves, but avoiding overloading others.