Pavement Performance Considerations
For Heavy Traffic Loads
Buses; Refuse Trucks; Concrete Trucks; Fire Trucks
By
Richard E. Raymond P.E.
Principal Engineer
City of Spokane
Division of Public Works and Utilities
Capital Programs/GIS Section

August 31, 2004

Scope. The purpose of this paper is to identify and quantify the more significant, heavier
vehicular loads to which the city’s streets are subjected, and provide a means of visualizing and
understanding how the various loads affect the service life of the city’s pavement infrastructure –
particularly the local access streets.
Background. The development of hard surfaces for paths and roads was borne of the necessity to
accommodate and enhance mobility during all climatic conditions. Over the years, practitioners
have experimented with many ways to create all-weather roads. Early methods utilized stones,
branches and logs, whereas modern methods rely primarily on the use of naturally occurring and
processed mineral aggregates, asphalt concrete, and cement concrete – either separately or in
combination – to produce smooth, functional, long-lasting surfaces.
Over the years, the methodology for designing suitable pavement structures has evolved from
trial and error to the use of computers employing sophisticated numerical methods. The goal was
and is to produce a roadway surface that is suitably smooth, and upon which people can travel
with a reasonable expectation of being able to do so safely, under all environmental conditions.
A number of factors must be considered when designing modern pavement structures, three of
which include: (1) the ability of the underlying soils to support loads, (2) the type and
availability of construction materials, and (3) the degree of loading to be accommodated – the
traffic loads.
Traffic loading refers not only to the magnitude of the loads – the weight that is being applied to
the pavement section – but also the nature or arrangement of the applied loads, and the frequency
of the loading, that is, how many times that weight is applied, or the axle load accumulation. As
an example, the design of the frame for a semi-trailer must consider two basic elements: (1) the
frame must be strong enough to support the load that the trailer is intended to carry, and (2) the
frame must be tough enough to resist the repeated stress fluctuations resulting from the bouncing
action as the vehicle travels down the road; that is, the frame must also be fatigue resistant.

-1-

 Likewise, it is intuitive that the useful life of a roadway section will similarly be affected by the
number of applied loadings.
During the late 1950’s, the American Association of State Highway Officials (AASHO) – now
called the American Association of State Highway and Transportation Officials (AASHTO) –
undertook an extensive research effort, called the AASHO Road Test, to “...establish
relationships showing how performance of pavements is influenced by structural design,
represented by component thicknesses of pavement structure, and loading, represented by the
magnitude and frequency of axle loads, for both rigid and flexible pavements of conventional
design.”
The AASHO Road Test showed that the damaging effect of the passage of an axle of any mass –
load – can be represented by a number of 18,000 pound equivalent single axle loads or ESALs.
For example, one application of a 12,000 pound single axle was found to cause damage equal to
approximately 0.23 applications of an 18,000 pound single axle load; or, conversely about four
applications of a 12,000 pound single axle were required to cause the same damage (or reduction
in serviceability) as one application of an 18,000 pound single axle.
Further analysis of the AASHO Road Test resulted in the realization that the amount of damage
inflicted on a pavement structure by the application of varying axle loads is non-linear. That is,
the reduction in pavement serviceability index (PSI) – the “damage” to the roadway – for a load
that is twice as large as an initial load is far greater than two times that of the initial load. In fact,
the damage is exponential; as a rule-of-thumb, roughly the fourth power. So, doubling a load (for
a given wheel and axle configuration) will inflict about sixteen times the amount of “damage”
(reduction in PSI) on a pavement structure. It must be understood that this is an approximation,
but that it is also reflective of the generalized relationship observed in the test data.
Load Equivalency Factors. Subsequent work has resulted in the creation of tabular data that are
utilized by pavement design engineers to rationally transform traffic number forecasts into the
predicted number of ESALs a pavement structure must accommodate over the chosen or
designated analysis period. The predicted ESAL count is then used in conjunction with other
pertinent information to design a suitable pavement section.
To express varying axle loads in terms of a single design parameter, axle load equivalency
factors – LEFs – were developed. It is these numbers that are shown in the various tables. They
relate the potential for reduction in PSI for a given load to the potential for reduction in PSI for
one ESAL. For example, a loading – load “A” – represented by an LEF of .05 imparts only 5%
of the “damage” to a pavement structure as that of a loading – load “B” – represented by an LEF
of 1.00 – one ESAL. Conversely, it takes approximately 20 repetitions (1÷0.05) of load “A” to
equal the amount of damage imparted by one repetition of load “B”.
The total amount of traffic expected over the analysis period is calculated by taking the current
traffic volume, applying an appropriate growth model – often an assumed annual growth rate –
and then summing up all the traffic over the analysis period. Once the total number of vehicles is
known, the mix of traffic – percentage of heavy and light trucks, buses, cars, etc. – is applied,
and the total number of each vehicle type is calculated. Then, knowing the axle weights, number

-2-

 of axles, and axle arrangement (single, dual, triple) for each vehicle type, the ESALs over the
analysis period are calculated by applying and summing the appropriate LEFs from the table for
each vehicle type.
More recent analysis of the AASHO Road Test Data by the Trucking Research Institute (TRI)
suggests that LEFs for both flexible and rigid pavements should be larger for lighter loaded axles
and smaller for heavier loaded axles as compared to AASHTO LEFs. This means that the fourth
power relationship for reduction in PSI may be less – about the 3.5 power, using the TRI
numbers.
There are yet other factors – beyond the scope of this paper – that affect the overall relationship
of load magnitude, arrangement and repetition to pavement damage. Nonetheless, the conclusion
remains unchanged:
For an equal number of applications, heavier loads produce appreciably
more damage to a roadway pavement than do lighter loads.
Or, put another way:
For a given period of time, higher numbers of ESALs produce appreciably
more damage to a roadway pavement than do lower numbers of ESALs.
A corollary to the above would be:
For a given pavement section, an increase in loading applications beyond
the assumed design loading model will hasten the deterioration rate of the
pavement, thus causing the pavement to reach its terminal serviceability
index prematurely.
Vehicle Load Factor. For any vehicle, when the loads on the individual axles or duals/triples are
known, then the sum of all the LEFs for each axle or axle group will yield the total number of
ESALs for that vehicle. This is also sometimes called the Truck Factor in other literature. For the
purposes of this paper, however, the total number of ESALs for any vehicle will be referred to as
the Vehicle Load Factor – VLF.
Sample Vehicle Load Factors. Using the tables from Appendix D of the 1993 AASHTO Guide
for Design of Pavement Structures, and the actual axle weight data for the indicated vehicles, the
following VLFs are calculated for various vehicle configurations found on City of Spokane
streets, for average conditions:
Passenger
Cars
Vehicle
VLF
Equivalent
Passenger car (assumed base line) ...................................................0.0004
Central pre-mix 7yd3 concrete truck ................................................1.84

-3-

1
4,600

 Central pre-mix 10yd3 concrete truck ..............................................2.03
STA Boyertown streetcar:

STA bus, GMC T8H603:

STA bus, FLXIBLE 870:

STA bus, MAN articulated – SG310:

City garbage truck: Front loader
City garbage truck: Rolloff
City garbage truck: traditional rear loader
City garbage truck: residential curbside

City fire truck: older engines
City fire truck: newer engines
City fire truck: downtown ladder
City fire truck: new tillered ladder
City fire truck: L-2 (due 2005)

5,100

empty....................1.35
100% full..............2.76
150% full..............3.80
empty....................1.15
100% full..............2.98
150% full..............3.89
empty....................1.25
100% full..............3.49
150% full..............5.55
empty....................0.81
100% full..............2.45
150% full..............4.59

3,400
6,900
9,500
2,900
7,500
9,700
3,100
8,700
13,900
2,000
6,100
11,500

empty....................n/a
full ........................5.48
empty....................1.91
full ........................5.48
empty....................n/a
full ........................3.37
empty....................2.01
full ........................4.71

n/a
13,700
4,800
13,700
n/a
8,400
5,000
11,800

full ........................0.21
full ........................0.68
full ........................4.37
full ........................3.45
full ........................6.87

500
1,700
10,900
8,600
17,200

Average ................2.74

6,800

In terms of absolute effect (highest VLF) for any single load application, it can be seen that
empty buses rank below the average; full buses and garbage trucks rank above average; and fire
trucks are mixed, some ranking well below average, others a bit above average about like the
buses and garbage trucks; and one (the proposed new fire truck) ranking well above average.
Cumulative Impact. Understanding the one-time impact of these vehicles is only half the story;
the overall impact must consider the number of times these vehicles use the streets during the
pavement analysis period.
In the case of passenger cars, cumulative impact is essentially moot because of the extremely
small VLF associated with passenger cars – pavement deterioration in this case is primarily
associated with environmental effects, or perhaps the application of unforeseen low frequency,
but very massive loads.

-4-

 Consider that during a typical 20-year analysis period, some blocks of
residential streets may see fewer than one million passenger cars – around
100 per day – which would equate to only 400 ESALs during the analysis
period. Other blocks might see more, depending on the geometric layout
of the roadway grid for accessing the arterial network. In contrast, it is
not uncommon to design an average arterial street for millions of
ESALs during its analysis period, and tens of millions for busier
arterials and highways.
As for garbage trucks, for the most part we would consider that they use a local access street
perhaps once a week. For fire trucks the usage might even be less than the garbage trucks. For
buses, the usage is a function of the bus route and schedule. As an example, an inspection of
STA’s various bus schedules indicates that bus trips vary from fewer than twenty to more than
sixty per day (in one direction), depending on the route.
Of course for buses, garbage trucks and fire trucks, the nearer to their main functional nodes, the
more concentrated is their traffic, and thus their effect on the roadway system. As an example,
for buses we would be interested in the bus operations facility on west Broadway Avenue; the
downtown transit plaza; and the various park and ride locations. For garbage trucks we would be
concerned with the waste-to-energy plant; the transfer stations; and the Solid Waste yards near
Perry Street and Madelia Avenue.
According to STA the Monroe Street Bridge and Monroe Street, proper immediately north and
south of the bridge, which feed the downtown bus plaza were accommodating in the
neighborhood of 600 buses per day at the time the bridge was shut down to bus traffic just prior
to the bridge reconstruction project. The data in the above table suggest that this level of bus
traffic would be roughly equivalent to 1.2 MILLION passenger cars EVERY DAY, in terms of
the reduction in serviceability index imparted to the pavement structure!
OBSERVATION: on a trip-for-trip basis, bus loads are less significant
than those for most garbage trucks and fire trucks. However, for those
streets utilized by the transit system, when taking trip frequency into
account, buses account for perhaps THE most significant loading on the
city’s streets (see the example, below) – certainly so for local access
streets.
As stated in the Washington State Department of Transportation Pavement Guide Interactive –
http://hotmix.ce.washington.edu/wsdot_web – ”... 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.” See Attachment 1, herein.
During the City of Spokane’s residential bond resurfacing initiative in the mid-1980s, there were
many local access streets that had been in service for 50 years or more, whose major distress was
the result of environmental conditions – primarily pavement oxidation resulting from exposure to

-5-

 the ultraviolet rays contained in normal sunlight. These areas responded well to minimal
treatment. However, it was not uncommon to find a local access street that had undergone total
structural failure intermingled with other streets that were in reasonably good shape. Invariably,
these areas of structural failure were on bus routes. In fact, in at least one case, only one-half of a
street had failed structurally and as might be expected, that side of the roadway was located on
the return leg of a bus route.
Other Considerations. The above information notwithstanding, the FHWA Vehicle
Classifications would classify a “typical” bus as a (FHWA) Class 4 vehicle with 0.57 ESALs per
vehicle. In their Pavement Management System, the Washington State Department of
Transportation (WSDOT) assigns 0.4 ESALs to their single unit category, which includes the
FHWA Class 4 vehicle. However, based on other data WSDOT assigns 1.6 ESALs to noninterstate urban buses.
Example. Assume a new local access street has just been put in service. The analysis period was
20 years, and the anticipated loading was based on a current service level consisting of the
occasional delivery truck (assume 10 per day; assume 0.5 ESAL per truck), local single
passenger vehicles (assume 200 per day; assume 0.0004 ESAL per vehicle), and 2 garbage trucks
per week (assume 3 ESALs per truck). For the ease of calculation, assume that no growth was
anticipated.
Over the course of the 20 year analysis period, then the total ESAL count assumed for the design
of the pavement structure was about 20*(365*(10*0.5+200*.0004)+52*2*3.0), or only about
43,000 ESALs. This is about 6 ESALs per day.
Now, assume that after, say 2 years the roadway became designated a bus route, with an average
of 30 buses per day. Assuming that each bus equated to about 1.25 ESALs, the same 43,000
ESALs would be reached in only about 2+((43,000-(365*2*6))/(6+30*1.25))/365 = 4.4 years!
To be sure, the minimum pavement thickness specified by many jurisdictions can accommodate
considerably more than 43,000 ESALs during a 20-year analysis period, assuming average
structural and environmental conditions. Typically, then, the minimum pavement thicknesses can
be expected to last longer than the normal 20-year analysis period, assuming the normally
smaller traffic of local access roads. However, it is readily apparent that the addition of
numerous heavy axle loads will significantly reduce the service life of a (local access) roadway.
Conclusions and limitations. It is important that the above information be considered within the
paper’s intended scope. The fact is, the numbers are based primarily on empirical data from the
AASHO Road Test of the late 1950’s, together with subsequent industry observations and
analytical work. The numbers must not be considered “exact”. Rather, they must be viewed as
being generally representative of the observed performance of numerous past and current
pavement systems, and as having been demonstrated suitably appropriate for predicting future
pavement performance.

-6-

 Consideration of the above Vehicle Load Factors and accompanying discussion reveals a number
of interesting, even startling relationships concerning the damage – reduction in serviceability
index – imparted to the street system by various vehicles:









The average EMPTY bus in the above data is about equivalent to nearly 3,000 passenger
cars in terms of “damage” imparted to the pavement infrastructure.
Some empty buses are about equivalent to a loaded 7 cubic yard concrete truck.
Full buses exceed the “damaging” effect of a loaded 10 cubic yard concrete truck.
During the course of an average day, the pavement “damage” along a typical transit
route that is attributable to the bus traffic alone is roughly equivalent to that imparted by
60 thousand passenger cars (assuming 30 buses per day) – nearly 200 thousand ESALs
during a typical 20-year analysis period – and that’s assuming the lightest, EMPTY
bus contained in the above table.
Although some garbage and fire trucks may have a larger ESAL total (VLF) than some
buses, garbage and fire trucks typically impart nowhere near the “damage” imparted by
buses, for those (local access) streets on a transit route. This is due to the reduced
number of garbage and fire truck trips as compared to the bus trips.
On probably all residential bus routes and many – if not most – arterial bus routes, bus
traffic is arguably the single defining loading for which the pavement section should be
designed.

Recommendations. Clearly, heavy traffic – most notably bus traffic – is a major factor in the life
of a street, particularly a local access street. Consequently, attention must be paid to how these
heavy loads will circulate within and through neighborhoods.
While it is possible to anticipate heavy loads and design pavement sections accordingly, it does
not make economic sense to do so if such loads do not subsequently materialize – there is simply
too much demand for current money. Perhaps equally important, any consideration to apply
heavy loads to a street not appropriately designed therefor – e.g. changing a garbage truck route,
or even more seriously changing a bus route – should be made with full knowledge of the
ramifications.
Accordingly, it would not be inappropriate to require any agency, jurisdiction or entity that is
considering actions that would impart significant heavy loading to a pavement structure not
intended for that use – or, for that matter to any pavement structure – to pay into a fund to offset
the cost associated with the inevitable accelerated pavement deterioration and related early
required maintenance and repair. Perhaps the monetary “damages” could/should be related to the
increase in ESALs imparted by the action of the responsible agency or entity. This notion is very
similar to the concept of developer impact fees relating to residential or commercial/industrial
development, and their effects on the transportation network.
It is especially appropriate that STA take into account these pavement service life factors and
associated real – not “soft” – cost implications when considering route changes, particularly if
the changes affect local access streets. It is important for the citizens of Spokane to understand
the full implications of any decisions that have major effects on their – not “the City’s” –
infrastructure. If it is subsequently determined that “hard” payment is not appropriate, then the

-7-

 related costs should be accounted for as social costs or in some other manner so that they appear
in the balance sheet, and do not become hidden and thus forgotten.

-8-

 Attachment 1
From Washington State Department of Transportation, Pavement Guide Interactive
Module 4, Section 3.6.1, Additional Information on Trucks and Buses link

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)

Bus
• AM General
Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
• AM General
Trolley
• Empty
• 50% Full
• 100% Full
• 130% Full
• Flyer
• Empty
• 50% Full
• 100% Full
• 130% Full
• Flyer Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
• MAN 40'
• Empty
• 50% Full
• 100% Full
• 130% Full

ESALs/Bus

Bus
• MAN 60'
• Empty
• 50% Full
• 100% Full
• 130% Full

1.14
1.67
2.34
2.85

• Flexible
Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
• GM Diesel
• Empty
• 50% Full
• 100% Full
• 130% Full
• Breda 60'
• Empty
• 50% Full
• 100% Full
• 130% Full

0.80
1.22
1.78
2.19
0.96
1.45
2.11
2.61
0.85
1.21
1.67
2.02
1.27
1.80
2.67
3.29

Note: 130% Full is all seats filled with standing passengers

-9-

ESALs/Bus
0.84
1.42
2.20
2.87

0.57
0.94
1.50
1.92
0.58
0.95
1.46
1.84
2.53
3.63
5.11
6.17

 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.

- 10 -

 Seattle Metro Bus Data

Table Wan 
733.: 
5H3: HEH 2.. r. HH a. 522$ was:
9. Ian
:1.:5er 
_mm
2+ mam 1.55:;
3 
HH 
t: In 
v.23; 1:22? HEW: nrm.n_ :m m_ .3. 
S. 2U: .3.
.47: Fri: HH ., SH 

34; .52.: HH ., 3er 3.5 
Tam 
v.41. w. Eda _235 34,.
v.74. v.74. Tmyn; 
_En _5n 3 

 

 

 

 

 

 

 

 

 

:5 

 

 

:5 

 

 

 

 

 

 

37x. 

 

 

 

 

 

 

-11-

-12-