Motorized modes of transportation, whether car or plane, bus or train, play a major role in developed societies. As they essentially trade energy against distance and speed of travel, their share in the overall consumption of energy and materials is high, making the reduction of that share an important objective for sustainability. This calls for both the systematic evaluation of environmental performance and the eco-design of transit modes. This twofold objective is addressed in the paper, which applies a Life Cycle Assessment (LCA) methodology to a transit mode, encompassing both infrastructure and vehicle fleets. The methodology is applied to a case study of a Bus Rapid Transit (BRT) system in Martinique, a French Caribbean island. The results show the contributions of the project's subsystems and life cycle phases to its environmental impacts, as captured by a set of 13 indicators. Impacts are normalized per passenger-kilometer. Comparison with other modes demonstrates the influence of vehicle occupancy rates and therefore the relevance of demand estimates in ex-ante evaluation. Finally, a sensitivity analysis is conducted to underline the respective impacts of input parameters on environmental impacts and to assess the potential of eco-design for BRT modes.