TM 5-822-5/AFM 88-7, Chap. 1
CHAPTER 3
VEHICULAR TRAFFIC
weights and the maximum allowable weights. For
3-1. Effect on Pavement Design.
group 2 and group 3 vehicles, maximum allowable
Pavement thickness must be designed to withstand
weights are based on single-axle and tandem-axle
the anticipated traffic, categorized by type and
loadings not exceeding 18,000 and 32,000 pounds,
weight of vehicles, and measured by average daily
respectively. Since traffic rarely will be composed
volume (ADV) of each type for the design life of
of vehicles from a single group, pneumatic-tired
the pavement. For most pavements, the magnitude
vehicular traffic has been classified into five general
of the axle load is of greater importance than the
categories based on the distribution of vehicles
gross weight of pneumatic-tired vehicles because
from each of the three groups listed above. These
axle spacings are generally so large that there is
traffic categories are defined as follows --
little interaction between the wheel loads of one
axle and the wheel loads of the other axles. Thus,
Category I. Traffic composed primarily of pas-
for the case of pneumatic-tired vehicles having
senger cars, panel and pickup trucks (group 1
equal axle loads, the increased severity of loading
vehicles), but containing not more than 1 percent
imposed by conventional four- or five-axle trucks
two-axle trucks (group 2 vehicles).
as compared with that imposed by two- or three-
Category II. Traffic composed primarily of pas-
axle trucks is largely a fatigue effect resulting from
senger cars, panel and pickup trucks (group 1
an increased number of load repetitions per vehicle
vehicles), but may contain as much as 10 percent
operation. For forklift trucks where the loading is
two-axle trucks (group 2 vehicles). No trucks
concentrated largely on a single axle and for
having three or more axles (group 3 vehicles) are
tracked vehicles where the loading is evenly divided
permitted in this category.
between the two tracks, the severity of the vehicle
Category III. Traffic containing as much as 15
loading is a function of the gross weight of the
percent trucks, but with not more than 1 percent
vehicle and the frequency of loading. Relations
of the total traffic composed of trucks having
between load repetition and required rigid
three or more axles (group 3 vehicles).
pavement thickness developed from accelerated
traffic tests of full-scale pavements have shown
Category IV. Traffic containing as much as 25
that, for any given vehicle, increasing the gross
percent trucks, but with not more than 10 percent
weight by as little as 10 percent can be equivalent
of the total traffic composed of trucks having
to increasing the volume of traffic by as much as
three or more axles group 3 vehicles).
300 to 400 percent. On this basis, the magnitude of
Category IVA. Traffic containing more than 25
the vehicle loading must be considered as a more
percent trucks.
significant factor in the design of pavements than
the number of load repetitions.
b. Tracked vehicles and forklift trucks. Tracked
vehicles having gross weights not exceeding 15,000
3-2. Traffic Evaluation.
pounds and forklift trucks having gross weights not
exceeding 6,000 pounds may be treated as two-
Procedures for the evaluation of traffic and selec-
axle trucks (group 2 vehicles) and substituted for
tion of design index are as follows.
trucks of this type in the traffic categories defined
a. Pneumatic-tired vehicles. To aid in
above on a one-for-one basis. Tracked vehicles
evaluating vehicular traffic for the purpose of
having gross weights exceeding 15,000 pounds but
pavement design, pneumatic-tired vehicles have
not 40,000 pounds and forklift trucks having gross
been divided into the following three groups --
weights exceeding 6,000 pounds but not 10,000
Group 1. Passenger cars, panel trucks, and
pounds may be treated as group 3 vehicles and
pickup trucks
substituted for trucks having three or more axles in
Group 2. Two-axle trucks
the appropriate traffic categories on a on-for-one
Group 3. Three-, four-, and five-axle trucks
basis. Traffic composed of tracked vehicles ex-
The design weights for various pneumatic-tired ve-
ceeding 40,000 pounds gross weight and forklift
hicles have been based on average weights, as de-
trucks exceeding 10,000 pounds gross weight has
termined from Federal Highway Administration
been divided into the following three categories --
traffic surveys made on public highways, plus one-
fourth of the difference between these average
3-1
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