Recently, an increased number of roundabouts have been implemented across the United States to improve intersection efficiency and safety. However, few studies have evaluated their energy and environmental impacts. Consequently, this study quantifies the energy and environmental impact of an isolated roundabout on a high-speed road by using second-by-second speed profiles derived from traffic simulation models in conjunction with microscopic energy and emission models. The study demonstrates that, at the intersection of a high-speed road with a low-speed road, an isolated roundabout does not necessarily reduce vehicle fuel consumption and emissions compared with other forms of intersection control (stop sign and traffic signal control). This case study found that the roundabout reduces the delay and queue lengths on the intersection approaches. However, the roundabout results in a significant increase in vehicle fuel consumption and emission levels compared with a two-way stop. The study demonstrates, for this case study, that the roundabout provides efficient movement of vehicles when the approach traffic volumes are relatively low. However, as demand increases, traffic at the roundabout experiences substantial increases in unnecessary delay in comparison with a strategy that uses signalized intersection control.