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Glossary
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 Capacity and LOS: Theory and Concepts

 
Heavy Vehicle Adjustment Factor

Determining the adjustment factor for the presence of heavy vehicles is a two-step process:

1) Finding passenger-car equivalents for trucks, buses and recreational vehicles

2) Using the equivalent values and the percentage of each type to compute the adjustment factor, using the following equation:

fhv = 1/(1 + Pt*(Et-1)+Pr*(Er-1))

where

fHV = heavy-vehicle adjustment factor
ET = passenger-car equivalents for trucks and/or buses
ER = passenger-car equivalents for recreational vehicles
PT, PR = proportion of trucks or buses and RVs in the traffic stream.

Finding ET and ER

There are two methods for finding values of ET and ER and the choice of methods depends on the freeway grade conditions.

Method 1: If an extended length of freeway contains a number of upgrades, downgrades and level segments, but no one grade is long enough or steep enough to have a significant impact on traffic operations, finding ET and ER is relatively straightforward. ET and ER are shown in the following table for extended general segments where no one grade equal to or greater than 3 percent is longer than 1/4 mile, or longer than 1/2 mile for grades less than 3 percent.

 

TYPE OF TERRAIN

CATEGORY

LEVEL

ROLLING

MOUNTAINOUS

ET (Trucks and Buses Combined)

1.5

3.0

6.0

ER  (RVs)

1.2

2.0

4.0

Method 2: There are three tables you can use to find ET and ER for isolated specific up and down grades:

  1. The first table is used to find the passenger car equivalent for trucks and buses on upgrades that are more than 1/2 mile for grades less than 3 percent or 1/4 mile for grades of 3 percent or more.
  2. The second table is used to find the passenger car equivalent for recreational vehicles on upgrades that are more than 1/2 mile for grades less than 3 percent or 1/4 mile for grades of 3 percent or more.
  3. The third table is used to find the passenger car equivalent for trucks and buses on downgrades that are more than 4 miles for grades of 4 percent or more. 

These three tables are significantly larger than the one above and can be viewed and printed by clicking on the link below.

[Go to Tables]