Trucks and Buses

Based on typical ESAL equivalency factors and the fourth power law it is clear that heavy vehicles cause a majority of pavement structure damage (with the notable exception of studded tire wear).  Therefore, even though trucks make up a minority of motor vehicles (see Figure 1), they are a major consideration in pavement design.  This in-depth section briefly looks at the truck and bus population in the U.S. and some of their typical characteristics.

Figure 1: Truck and Bus Population in the U.S. (data taken from FHWA, 2000)

Truck Categories

Trucks can be divided up into any number of different categories or classes.  The most general truck classification is probably by gross weight.  For instance, a family sports utility vehicle or 3/4 ton pickup is drastically different than a delivery van or an interstate tractor-semi trailer.  Thus, one common practice is to classifying trucks and buses by gross vehicle weight rating.  The three most common categories are shown in Figure 2. 

Figure 2: Common Truck Categories

Vehicle manufacturers use more precise technical definitions and divide trucks into eight classes according to gross vehicle weight rating (GVWR).  Table 1 shows vehicle manufacturer truck classifications.  Figure 3 shows a basic breakdown of the truck and bus population in the U.S. 

Table 1: Vehicle Manufacturer Truck Classification¹

Figure 3: Truck and Bus Population in the U.S. (from FHWA, 2000)

Looking at Figure 3, it is interesting to note that the trucks of primary consideration for pavement performance, the heavy trucks, only make up a small fraction of the U.S. truck population.   In essence, structural pavement design is usually concerned with no more than about 0.6% of the U.S. motor vehicle population (38.4% trucks, of which 2% are heavy trucks/buses).  However, these trucks make multiple trips and typically travel many more miles than the average passenger vehicle.  For example, a typical passenger vehicle may travel between 5,000 and 15,000 miles/year while a typical heavy truck may travel from 30,000 - 80,000 miles/year (USDOT, 2000).  

FHWA Vehicle Classification

The FHWA classifies vehicles in terms of their configuration rather than weight.  This type of classification system is more conducive to traffic applications but can be adapted for pavement loading applications.  It can also be easily confused with the vehicle manufacturers truck classification system shown in Table 1.  The FHWA Traffic Monitoring Guide (TMG) recommends classifying vehicles into 13 different categories.  All States currently use this classification scheme or some variation of it for classifying vehicles, although few use it exclusively (FHWA, 2001).  States typically aggregate the 13 FHWA categories listed in Table 2 into a small number of categories (about three to five) for ESAL forecasting and estimating.  Figures 4 through 9 show some FHWA vehicle class examples.

Table 2: FHWA Vehicle Classification (from FHWA, 2001)¹

Figure 4: FHWA Class 5	Figure 5: FHWA Class 8
Figure 6: FHWA Class 11	Figure 7: FHWA Class 10
Figure 8: FHWA Class 13	Figure 9: FHWA Class 4

Truck Flows

In addition to the number and weight of trucks, pavement design is also concerned with where these trucks travel.  For instance, many residential streets only experience one or two heavy trucks per week (e.g., the garbage truck) while some of the busiest interstate truck routes can experience volumes of up to 500 trucks/hour (USDOT, 2000).  Figure 10 gives an idea of truck travel on National Highway System (NHS) routes.

Figure 10: Truck Flow on the National Highway System (from USDOT, 2000)
[Thickness of line is proportional to truck volume]

In their Comprehensive Truck Size and Weight Study (2000), the U.S. Department of Transportation made the following general observations on truck flow within the U.S.:

• Truck traffic on the NHS varies widely throughout the country, ranging from an annual average of one or two trucks per hour in each direction to more than 500 trucks per hour.

• Truck volume on most of the NHS in the Western Region is relatively low. Exceptions include major North-South routes in the Interstate Route 5 Coastal Corridor, and major East-West corridors associated with Interstate Route 80, Route 40, Route 10, and Route 20.

• Truck volumes east of the Mississippi on much of the NHS range from modest in the New England States to very high in the mid-Atlantic region.

• Many of the highways in the North-South, mid-continent I-35 Corridor have low to modest truck volumes. The lowest truck volumes in this corridor are at the northern and southern ends, and in the middle of the corridor through Kansas. Dominant trucking activity in the corridor includes East-West trips and travel between most corridor States and the North-Central region of the United States.

Notes on Buses

Although buses are sometimes ignored in truck counts, they can significantly contribute to overall pavement loading - especially in urban areas.  Many times, school buses provide the only major loading for residential pavements.  Furthermore, buses often inflict more pavement damage than much heavier trucks due to their axle configurations and wheel loads.   As shown in Table 3, a heavily loaded, dual powered bus (both diesel and electric power systems) can impart over 6 ESALs per bus.  Table 3 tabulates various bus LEFs for King County (WA) Metro. 

Table 3: Representative Bus ESALs (Metro, 1987; DeBoldt, 1993)

If no other information is known about a bus route other than the volume of buses, use an ESAL/bus corresponding to 50 percent full.  This results in an average ESAL/bus @ 1.60.

Table 4 shows the detailed King County Metro numbers used to calculate the values in Table 4.

Table 4: Seattle Metro Bus Data

1. Reference: Metro, 1987

2. Reference: DeBoldt, 1993

3. Tire inflation pressures range from 95 to 115 psi

4. Tire sizes may vary

Note: LEFs based on single axle, SN = 5, p_t = 2.5