c. Base and Subbase Materials. Suitable materials for base and subbase are
frequently available on Government-owned land. Determine the cost of process-
ing available materials against the cost of commercial sources.
Section 3. TRAFFIC ANALYSIS
1. TRAFFIC SURVEYS. Traffic surveys should be conducted to determine the
number and weight of the vehicles anticipated to use the pavement in question.
The prediction of traffic intensity should include the present volumes modi-
fied by an appropriate factor for growth. Rely on local practice or the prac-
tice of the state highway department for specific procedures for conducting
surveys. Guidance on traffic studies for military installations is contained
in Military Traffic Management Command Pamphlet No. 55-8, "Traffic Engineering
Study Reference." When designing new roads and streets, consider traffic
requirements for a 20-year design period.
a. Average Daily Traffic. Determine the average daily traffic (ADT) from
available records or vehicle counts for the road under consideration or for an
existing roadway of similar traffic conditions. Where data is not available, a
detailed traffic study may be required.
b. Traffic Weight. The percentage of trucks (T) in a traffic stream should
be determined by traffic counts or current records. A tabulation of the number
of axles observed within certain load groups should be made using the Federal
Highway Administration loadometer tables.
c. Equivalent 18 Kip Repetitions. The basic method of measuring the effects
of mixed traffic on highway pavements is with the use of equivalent 18 kip
single-axle loads (EAL). The concept of the equivalent axle load was developed
from the AASHTO Road Test. A detailed discussion on the development of the
load equivalency factors is contained in AASHO Interim Guide for Design of
Pavement Structures--1972.[1] Load equivalency factors for single and tandem
axle loads are given by Figure 4. For an example of their use see Table 4.
d. Traffic Distribution. Assign 50 percent of the total two-way traffic to
each direction. For two-lane roads 100 percent of the unidirectional traffic
is assigned to the design lane. For four-lane roads 90 percent of the unidir-
ectional traffic should be assigned to the outside lane.
2. DESIGN INDEX. A design index (DI) is used in the flexible pavement thick-
ness design procedures to account for the effects of traffic intensity and
weight.
a. Vehicle Groups. Where detailed traffic survey and axle load data are not
available, spot counts or estimates can be made to ascertain the general
characteristics and volume of traffic. As an aid to determining a DI, vehicles
should be grouped according to the following categories:
(1) Group 1: Passenger cars and panel and pickup trucks.
(2) Group 2: Two-axle trucks.
(3) Group 3: Trucks having three or more axles.
b. Selection of Design Index. From detailed traffic surveys or general
vehicle groups given above, select the DI from Table 5.
c. Design Index for Tracked Vehicles and Forklift Trucks. The DIs given in
Table 5 do not include the effects of tracked vehicles or forklift trucks.
Where significant volumes of tracked vehicles and forklifts are indicated, a
higher DI may be required for thickness design. Procedures for considering
these exceptional vehicles are contained in Flexible Pavements for Roads,
Streets, Walks, and Open Storage Areas, DA TM5-822-5.
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[1] The American Association of State Highway Officials, AASHO, was renamed
American Association of State Highway and Transportation Officials in 1973.
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