It is difficult to assess the effects of engine idling on air pollutant emissions from highway construction equipment because of a lack of combined activity and emissions data. A methodology is presented for quantifying the impact of idling on National Ambient Air Quality Standards criteria pollutant emissions, including nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM). The methodology and results are based on field data collected from 35 items of nonroad diesel construction equipment. Engine idle time was quantified in terms of equipment operational efficiency (), which was defined as the ratio of nonidle time to total equipment use time (nonidle time plus idle time). With and the ratio of idle to nonidle emission rates (re) for each pollutant, the percentage increase in the total quantities of each pollutant emitted (NE) was calculated for each item of equipment for the observed values of and re. Results showed that as increased (or idle time decreased), NE decreased. Mathematical models with as a predictor variable were developed to estimate values of NE for each pollutant. Regression equations in the form of y = ax3 + bx2 + cx + d were developed for NOx and HC, with R2 values of 97% and 87%, respectively. Regression equations in the form of y = mx + b were developed for CO and PM, with R2 values of .50 and .65, respectively. Recommendations include using the models as performance measurement tools to assess idle restriction regulations and operational strategies of highway construction equipment.