Drivers use various signals to show their driving intentions. Some of these are official such as the turn signal. Other signals are read by interpersonal inference, as when drivers at a four-way stop crossing wave or nod, or merely exchange glances to determine who will go first. Driving in traffic routinely involves events and incidents. Events are normal sequential maneuvers such as stopping for the light, changing lanes, or putting on the brakes.
Incidents are frequent but abnormal events.
Some of these are dangerous and frightening, such as near-accidents, while others are merely annoying or depressing, such as being insulted by a driver or forgetting to make a turn. Driving events and incidents are sources of psychological forces capable of producing powerful feelings and irrational thought sequences. Traffic psychology is the study of the social-psychological forces that act upon drivers in traffic. Situations are analyzed through external as well as internal methods of data gathering. For example, in one study the aggressiveness of drivers was measured in terms of observed rate of speed reduction, or the making of some hostile gesture at pedestrians in a marked crosswalk.