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.

 

Truck and bus population in the U.S.(data taken from FHWA, 2000).
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 below.

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Light (pickup trucks, vans, SUVs)
Figure 2: Light (pickup trucks, vans, SUVs)

Medium (delivery trucks)
Figure 3: Medium (delivery trucks)

Heavy (tractor-trailer combinations)
Figure 4: Heavy (tractor-trailer combinations)

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 5 shows a basic breakdown of the truck and bus population in the U.S.

Table 1. Vehicle Manufacturer Truck Classification

Category Class GVWR2 Representative Vehicles
Light 1 0 – 27 kN
0 – 6,000 lbs.
pickup trucks, ambulances, parcel delivery
2 27 – 45 kN
(6,001 – 10,000 lbs.)
3 45 – 62 kN
(10,001 – 14,000 lbs.)
Medium 4 62 – 71 kN
(14,001 – 16,000 lbs.)
city cargo van, beverage delivery truck, wrecker, school bus
5 71 – 87 kN
(16,001 – 19,500 lbs.)
6 87 – 116 kN
(19,501 – 26,000 lbs.)
7 116 – 147 kN
(26,001 to 33,000 lbs.)
Heavy 8 147 kN and over
(33,000 lbs. and over)
truck tractor, concrete mixer, dump truck, fire truck, city transit bus
Truck and bus population in the U.S. (from FHWA, 2000).
Figure 5. Truck and bus population in the U.S. (from FHWA, 2000).

 

Looking at Figure 5, 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[1]).

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[2]). 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 6 through 11 show some FHWA vehicle class examples.

Table 2. FHWA Vehicle Classification (from FHWA, 2001[2])

Class Type Description Typical ESALs per Vehicle2
1 Motorcycles All two- or three-wheeled motorized vehicles. Typical vehicles in this category have saddle type seats and are steered by handle bars rather than wheels. This category includes motorcycles, motor scooters, mopeds, motor-powered bicycles, and three-wheel motorcycles. This vehicle type may be reported at the option of the State. negligible
2 Passenger Cars All sedans, coupes, and station wagons manufactured primarily for the purpose of carrying passengers and including those passenger cars pulling recreational or other light trailers. negligible
3 Other Two-Axle, All two-axle, four tire, vehicles, other than passenger cars. Included in this classification are pickups, panels, vans, and other vehicles such as campers, motor homes, ambulances, hearses, and carryalls. Other two-axle, four-tire single unit vehicles pulling recreational or other light trailers are included in this classification. Other Two-Axle,
4 Buses All vehicles manufactured as traditional passenger-carrying buses with two axles and six tires or three or more axles. This category includes only traditional buses (including school buses) functioning as passenger-carrying vehicles. All two-axle, four-tire single unit vehicles. Modified buses should be considered to be a truck and be appropriately classified. 0.57
5 Two-Axle, Six-Tire, Single Unit Trucks All vehicles on a single frame including trucks, camping and recreational vehicles, motor homes, etc., having two axles and dual rear wheels. 0.26
6 Three-Axle Single Unit Trucks All vehicles on a single frame including trucks, camping and recreational vehicles, motor homes, etc., having three axles. 0.42
7 Four or More Axle Single Unit Trucks All trucks on a single frame with four or more axles. 0.42
8 Four or Less Axle Single Trailer Trucks All vehicles with four or less axles consisting of two units, one of which is a tractor or straight truck power unit. 0.30
9 Five-Axle Single Trailer Trucks All five-axle vehicles consisting of two units, one of which is a tractor or straight truck power unit. 1.20
10 Six or More Axle Single Trailer Trucks All vehicles with six or more axles consisting of two units, one of which is a tractor or straight truck power unit. 0.93
11 Five or Less Axle Multi-Trailer Trucks All vehicles with five or less axles consisting of three or more units, one of which is a tractor or straight truck power unit. 0.82
12 Six-Axle Multi-Trailer Trucks All six-axle vehicles consisting of three or more units, one of which is a tractor or straight truck power unit. 1.06
13 Seven or More Axle Multi-Trailer Trucks All vehicles with seven or more axles consisting of three or more units, one of which is a tractor or straight truck power unit. 1.39

 

FHWA Class 5.
Figure 6: FHWA Class 5.

FHWA Class 8.
Figure 7: FHWA Class 8.

FHWA Class 11.
Figure 8: FHWA Class 11.

FHWA Class 10.
Figure 9: FHWA Class 10.

FHWA Class 13
Figure 10: FHWA Class 13

FHWA Class 4.
Figure 11: FHWA Class 4.

Truck Flows

 

Truck Flow on the National Highway System (from USDOT, 2000) [Thickness of line is proportional to truck volume
Figure 12: Truck Flow on the National Highway System (from USDOT, 2000) [Thickness of line is proportional to truck volume

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[1]). Figure 12 gives an idea of truck travel on National Highway System (NHS) routes.

In their Comprehensive Truck Size and Weight Study (2000[1]), 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[3])

Bus ESALs/Bus Bus ESALs/Bus
AM General Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
1.14
1.67
2.34
2.85
MAN 60‘
• Empty
• 50% Full
• 100% Full
• 130% Full
0.84
1.42
2.20
2.87
AM General Trolley
• Empty
• 50% Full
• 100% Full
• 130% Full
0.80
1.22
1.78
2.19
Flexible Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
0.57
0.94
1.50
1.92
Flyer
• Empty
• 50% Full
• 100% Full
• 130% Full
0.96
1.45
2.11
2.61
GM Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
0.58
0.95
1.46
1.84
Flyer Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
0.85
1.21
1.67
2.02
Breda 60‘
• Empty
• 50% Full
• 100% Full
• 130% Full
2.53
3.63
5.11
6.17
MAN 40‘
• Empty
• 50% Full
• 100% Full
• 130% Full
1.27
1.80
2.67
3.29

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 3.

Table 4. Seattle Metro Bus Data

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, t = 2.5



Footnotes    (↵ returns to text)
  1. Comprehensive Truck Size and Weight Study.  United States Department of Transportation.  Washington, D.C.  http://www.fhwa.dot.gov/reports/tswstudy/index.htm.
  2. Traffic Monitoring Guide.  Federal Highway Administration.  Washington, D.C.  Accessed online at: http://www.fhwa.dot.gov/ohim/tmguide/index.htm.  Accessed 7 August 2001.
  3. Tabular Bus Data provided by Peter DeBoldt to Joe Mahoney.