Construction operations of construction equipment generate significant greenhouse gas emissions including carbon emissions and diesel exhaust emissions. While improving operational efficiency of construction activities implemented by construction equipment could result in significant emission reduction, there is still a lack of practical knowledge on the impact of operational adjustments applicable in real-world operations. The objectives of this study are to identify common inefficiency of real-world construction equipment activities and quantify emission saving potentials from operation-level adjustments of construction activities. In order to achieve the goals, three construction sites were used as case studies to identify common sources of excessive emissions on construction sites as well as possible opportunities for their reduction. As the means of quantifying potential gas emission reduction, a simplistic method that estimates equipment cycle time and can be easily replicated in practice is introduced. It was found from the case studies that a typical problem on a construction site is unnecessary idling of equipment. In addition, findings of this study indicate that the integration of careful planning on construction equipment activities with appropriate equipment selection could contribute to the reduction of carbon gas emissions as well as savings of construction cost. It is envisaged that this study contributes to the body of knowledge by 1) providing new findings in common inefficiency of construction operation in construction sites through field studies, and 2) quantifying the potential gas emission reduction with the simplistic method.