Long-haul truck drivers idle their vehicles to operate heating systems and air conditioners, generate electricity, charge the vehicles’ batteries, and warm up the engines. This type of idling, which often occurs over extended periods and could be a significant source of emissions, is expected to increase because of the U.S. Department of Transportation's latest rule mandating that truck drivers rest 10 h for every 14 h of driving. The emissions associated with this type of discretionary idling occurring at the beginning and end of trips are not included in emissions inventories and can be an important contributor to the overall mobile source emissions. There is, therefore, a need to develop a procedure that would yield accurate estimates of extended truck idling emissions. This study developed such a methodology by using a nonattainment area in Texas as a pilot study. On the basis of interviews and observations made at generators such as truck stops, public rest areas, industries, ports, and intermodal facilities, a model was developed to perform the emissions estimates. The procedure was applied to the remaining metropolitan areas in Texas. It was found that extended truck idling emissions were a major source of mobile source emissions, resulting in emissions of more than 30 tons per day of oxides of nitrogen and more than 0.8 ton per day of particulate matter in Texas metropolitan areas. Although these emissions represent less than 4% of the overall on-road mobile source emissions, they are important in assisting nonattainment areas to reach conformity.