Today the massive use of the car in the cities generates a wide range of problems such as traffic congestion, mobility constraints to pedestrians due to illegal parking and high speed traffic, air pollution and noise. Departing from the idea that it is possible to influence modal switchers, studies are needed to access the motives that lead people to chose the car instead of public transport, so that programmes can be drawn to change modal choice. The results presented here are supported by two field studies, where over 300 people were interviewed. The interviews were based on a questionnaire, and were performed by trained psychologists. In order to choose respondents, a first step concerned a selection of the areas under study, which included places with apartments and houses both located at less and over 200 meters far from bus stations. A second step required a random choice of respondents inside those areas. The total sample included the same number of car and bus users, with equal distribution of apartment and house residents, and males and females. Both car users and bus users were questioned about their perceptions towards the travel mode they use, and the other mode. This study presents a psychological explanation of modal choice, emphasising the factors that contribute to the preference for the car. The model is based in the results and literature review. Particular attention is paid to the relationship between stress process and traffic congestion for bus and car users, in order to explain why traffic congestion caused by cars induces car use. So, traffic congestion promotes itself. Besides, comparisons are drawn between stress felt by car commuters and bus commuters, as well as stress attributed to commuters using the opposite mode of transportation. Two patterns emerge in favour of car use. The first concerns the evidence that car commuters’ stress is lower than bus commuters’ stress. The second refers to the fact that while bus commuters perceive stress associated to car commuting as similar to car commuters stress levels, car commuters perceive stress levels of bus commuting as higher that bus commuters’ stress levels. A correlation was also found between stress and perceived travel times. People showing higher levels of stress perceive travel times as higher than people showing lower levels. Not only bus users are more affected by this (as they show higher levels of stress than car users), but perceived times overcome the real travel time measures which are already higher when compared with car. Moreover, considering that when stress is higher, the desire for mode switching is also higher; it can be said that there are more bus commuters interested in changing for car than the opposite. Even because car is perceived as less stressful than the bus. This perception arises due to the role played by other factors. For instance, while during traffic periods car commuters only have to deal with traffic jam, bus commuters also have to deal with crowding. So the last are exposed to a stress that the first don’t have to face. Besides, the coping strategies available for each group are different. While car commuters can have a direct control on their surroundings (choose the music they hear, their travel companions, control the odour and the air temperature, and choose the route for their trip); bus commuters can only develop strategies to avoid less desirable situations, over which they don’t have total control (e.g. hold a bag in a way to avoid physical contact with others, or put a bag over the next seat to prevent others to seat there).A factor related with control is risk perception. Again, an imbalance emerges between car and bus commuters. For car drivers risk perception mainly concerns risk of having an accident, which is underestimated considering the behavioural control of driving and the self overvaluation of driving abilities (most drivers consider themselves as having driving abilities over the average). Bus users are in disadvantage, as they can’t apply self illusions to reduce risk of having a traffic accident, even though it’s perceived as low. But this group also has to face risk of being mugged or risk of aggression, which are considered higher for specific areas of the city and times of the day. Finally it is important to focus that car commuters justify their choice as a matter of comfort and travel time, while bus commuters say that the only reason why they use the bus is because it’s cheaper and they can’t afford to have a car. Implicit to these reasons is a perceived status of car and bus users, which contributes to distinguish users socially and keep car users away of buses and makes bus commuters to desire for a car.