Highway Engineering

The passenger car equivalent (pce), or the number of equivalent private cars
that would occupy the same quantity of road space, for each of the above types
of heavy vehicle is primarily dependent on the terrain of the highway under
examination, with steep gradients magnifying the performance constraints of
the heavy vehicles.
The pce’s for trucks (ET), buses (EB), and recreational vehicles (ER), are
defined for three different classes of terrain:

Level terrain: This is categorised as gradients or horizontal/vertical alignments
that allow heavy vehicles to maintain the same speeds as private cars. Upward
and downward gradients of not more than 1–2 % are normally consistent with
this classification.

Rolling terrain: Those gradients or horizontal alignments that result in the
speed of the heavy vehicle in question being lowered to a value substantially
below those of the private car on the same stretch of roadway. The heavy vehicle
is not operating at its maximum speed for a substantial distance.

Mountainous terrain: Those gradients or horizontal alignments that result in the
heavy vehicle operating at its maximum speed for a substantial distance.

Values given by the Transportation Research Board are noted in Table 4.4.

Basic Elements of Highway Traffic Analysis 83

Adjustment factor,fw

Obstruction on one side of Obstruction on both sides of

roadway roadway

Distance of obstruction

Lane width (m) Lane width (m)

from travelled edge (m) 3.65 m 3.36 m 3.05 m 2.75 m 3.65 m 3.36 m 3.05 m 2.75 m

1.83 or greater 1.00 0.97 0.91 0.81 1.00 0.97 0.91 0.81

1.22 0.99 0.96 0.90 0.80 0.98 0.95 0.89 0.79

0.61 0.97 0.94 0.88 0.79 0.94 0.91 0.86 0.76

0 0.90 0.87 0.82 0.73 0.81 0.79 0.74 0.66

Table 4.3Correction factors for non-ideal lane widths and clearances from obstructions (multi-
lane highways) (Source:Highway Capacity Manual(TRB, 1985))

Type of terrain

Correction factor Level Rolling Mountainous

ETfor trucks 1.7 4.0 8.0

EBfor buses 1.5 3.0 5.0

ERfor RVs 1.6 3.0 4.0

Table 4.4Passenger car

equivalents for different

classes of heavy vehicles

(Source:Highway

Capacity Manual(TRB,

1985))