Emissions tend to be highest during acceleration.
Furthermore, the amount of emissions during acceleration appears to be related to the magnitude of the acceleration. Very hard or aggressive accelerations can produce large amounts of several pollutants, but especially carbon monoxide, relative to the rest of your driving trip. Even accelerations at highway speed, such as a rapid 5 mph increase in speed to pass another vehicle, can produce large carbon monoxide emissions relative to emissions during cruising. So far, we have found that emissions for properly operating newer vehicles are very low during idling, and also are typically very low during steady cruising on level roads and with normal loads. Emissions during deceleration also are relatively low compared to acceleration and in fact are comparable to idling or cruising emissions.