To make informed environmental choices, individuals must first understand the potential environmental impacts of the modes of transport available and be able to relate this information to their own internal reference points. This study examines the results of an on-line survey conducted to assess the ability of individuals in the Greater Dublin Area to estimate their potential carbon footprint for a variety of modes of transport. The results indicate that nearly one third of those surveyed stated that they simply did not know the carbon footprint of the modes in question, while those who provided emissions estimates showed a wide range of variance. Comparison with existing emissions factors indicates that respondents overestimate the environmental impact of bus journeys and underestimate the impact of small car and tram trips. The results of this study indicate the need for more specific emissions information to allow individuals to make informed and sustainable mode choices.