The 1997 Vehicle Inventory and Use Survey reports that 2.54 million trucks had a
manufacturer's gross vehicle weight (GVW) rating of 26,001 lb or greater, and these trucks are
classified as class 8 trucks (VIUS 2000). They may be single-unit trucks or combination trucks
(single-unit trucks with a trailer or tractor-trucks with single, double, or triple trailers). Seventy
percent of these trucks are operated
within 200 mi from the home base, and 83% are operated
within 500 mi of home base. However, about 458,000 combination trucks travel more than
500 mi from their home base each day. Most, but not all, of these trucks are equipped with
sleepers. These are the trucks that are likely to be idling overnight during stopovers on long trips.
Drivers idle their truck engines to keep the cab and sleeper area warm when they are
sleeping or resting in the truck (or away but likely to return soon); in extremely cold weather,
drivers idle their truck engines to keep fuel from freezing and to keep the engine block warm to
prevent difficult starting and smoke. They also idle the engines to run air-conditioning during hot
weather and to provide power for appliances and other electrical devices. Sleepers can be quite
comfortable and well equipped with modern conveniences, such as personal computers, stereos,
and televisions. Figure 2 shows a typical sleeper compartment.
Detailed data on idling are not available. However, industry sources have provided rough
estimates. One source reports that a long-distance, freight-hauling, heavy-duty truck idles about
6 h/d on the average (TMC 1995). The actual extent of idling varies with the season and with the
type of operation. A truck may idle for 10 h/d during winter and less than 5 h/d the rest of the
year; the baseline entry in Table 1 reflects a 6-h/d average,
with a seasonal peak in the winter.
Another source estimates that the average is higher
40% of the time is spent idling
basis of information provided by J.B. Hunt (Whiteside 1996). Most trucking organizations now
offer incentives to reduce idling, and some large firms have succeeded in reducing truck idling
time to 20% (Whiteside 1996). However, nearly 40% of the long-haul trucks are in fleets of less
than 25 vehicles, and these small fleets are
less likely to have such incentive programs.
We interviewed several truck operators and fleet owners and found a wide variation in
actual behavior. At one extreme, one owner-operator of an older truck reported that he leaves his
truck running all the time, even at home over the weekend, to make absolutely sure that it will
start. On an annual basis, he idles his truck for more than 5,000 h. At the other extreme is another
owner-operator, who lives in Minnesota, makes two round trips to Chicago each week, and only
runs his truck when he is in it. He plugs in a small electric heater to keep the engine warm at
home in the winter, but he still does occasionally have trouble starting the truck. He idles his
truck for fewer than 1,000 h/yr. In between these extremes, an Iowa contract fleet owner reports
that his drivers run their trucks all the time during the week, but the trucks are turned off for the
Eighty-five winter days at 10 h/d and 218 nonwinter days at 4.5 h/d.