Bus transport is the most widely used form of inter-urban passenger transportation in Colombia and in many other developing countries in which long-distance passenger rail travel is not an option. It is estimated that Colombia currently transports over half a million passengers per day. In many developing countries, private operators provide inter-urban bus systems, with limited control being exercised by governments. In these countries, the contribution that bus travel makes to the volume of inter-urban transport can usually only be calculated by slow and costly forms of data collection. Thus, governments often have a limited capacity to determine levels of oversupply or unmet demand. This investigation develops a gravity transport model for inter-urban trips, the aim of which is to help decision-making in situations where the available information or resources do not support the use of a traditional four-step model. Variables such as population, distance and travel time are used to determine levels of travel between cities. The model is applied to, and balanced and calibrated for, 24 cities in Colombia. It demonstrates the importance of the relationship between population and infrastructure, which can be considered to be a key variable in explaining inter-urban travel. The model emphasises that transport corridors that are likely to increase inter-urban demands as a result of the need for new roads and identifies the growth of areas in which a possible oversupply could occur. The model could be developed as an effective planning and management tool for inter-urban travel in Colombia and other countries