PURPOSES: In this paper, the effectiveness of speed-maintained standardization in road geometry on environmental impact at a downward slope location, based on greenhouse gas (GHG) emission indicators, was studied. Specifically, the aim of this study was to ascertain whether speed-maintained standardization resulted in decreased CO2 emissions as well as noise pollution, due to reduced vehicle speeds. METHODS : In this study, speed-maintained standardization in road geometry was proposed as a means to reduce vehicle speeds, with a view to reducing CO2 emissions and noise pollution. This technique was applied at a downward slope location. The vehicle speeds, CO2 emissions, and noise levels before and after application of speed-maintained standardization were compared. RESULTS: It was found that speed-maintained standardization was effective as a means to reduce speed, as well as CO2 emissions and noise pollution. By applying speed-maintained standardization, it was confirmed that vehicle speeds were reduced consistently. As a result, CO2 emissions and noise levels were decreased by 9% and 11%, respectively. CONCLUSIONS : This study confirmed that speed-maintained standardization in road geometry is effective in reducing vehicle speeds, CO2 emissions, and noise levels. Moreover, there is further scope for the application of this method in the design of roads in urban and rural areas, as well as in the design of highways.