74% of Europeans live and move every day in cities, and 40% of the total CO2 emissions from transport is caused by urban mobility. The decarbonisation of urban transport can therefore produce a substantial reduction in total CO2 emissions. Cities have proposed various measures to reduce CO2 emissions from mobility, concentrating particularly on reducing private individual transport (car) by offering a range of alternatives. Sustainable Urban Mobility Plans (SUMPs) are not new in Europe, as numerous precedents in France, UK, Italy and Germany have now been underway for a decade. SUMPs propose a strategy to reduce dependence on private cars by imposing a series of measures. As the plans were designed for the long term (ten to fifteen years), it is only natural to question their effectiveness after a certain time has elapsed. However, there is little literature on this issue. To fill this gap this article proposes a methodology to evaluate Sustainable Urban Mobility Plans using indicators to assess the current situation of the city in terms of the effectiveness of the measures implemented, focusing on the specific case of the city of Burgos in Spain, where the Sustainable Urban Mobility Plan was introduced in 2005, and on the results based on the proposed methodology. CO2 savings are always the primary target of a SUMP. Most assessments highlight changes in behaviour or public perception, but contain no in-depth analysis of cost-effectiveness. This paper aims to calculate a cost analysis for each tonnes of CO2 saved. This methodology is used in other fields such as lighting and energy, but not in transport due to its complexity. However, effectiveness ratios can be calculated with the available data on modal shift and investments in the SUMP, taking into account the costs as a whole. Other methodologies to determine external cost savings are included in the proposed methodology to achieve a more accurate ratio and consider more external factors. •Methodology to estimate the cost of CO2 saved through the use of sustainable transport modes.•Identification of variables affecting the assessment of Sustainable Urban Mobility Plans.•Proposal of a new system to assess the measures implemented in a Sustainable Urban Mobility Plan.