Recent empirical studies have found that travellers route choices deviate from perfect rationality, showing that urban trips do not necessary follow the shortest-time routes. However, there is no consent on how much the travellers' route choice behaviour deviates from the perfect rational assumption. The objective of this study is to contribute to the understanding on how travellers process travel time when making route choices, and to quantify to what extent users are strict travel time minimisers or if bounded rationality is observed. The question of whether travellers evaluate travel time differences in absolute or relative terms is also addressed, and the heterogeneity in the route choice behaviour of travellers investigated. The results of route choice experiments, focused on the choices in diverse OD pairs and traffic conditions, are analysed. In total, 496 participants recorded 5535 choices over 41 OD pairs. It was found that travellers evaluate relative rather than absolute differences in travel time. In 60.5% of the trips participants chose the fastest route, but this percentage is 80% when the travel time between the fastest and the rest of the alternatives is at least 30% higher. Only 10% of the individuals chose the fastest route in all trips, confirming the hypothesis of bounded rationality. Participants exhibited heterogeneous travel time indifference bands: the average participant was indifferent to relative travel time differences of less than 31%. A mixed logit model (MXL), considering heterogeneous indifference bands is proposed. The model shows a similar predictive accuracy compared to the classical MXL model.