Highway Engineering

4.4.4 Maximum service flow rates for 2-lane highways

Where one lane is available for traffic in each direction, a classification of 2-lane

highway applies. In such a situation, if a driver wishes to overtake a slower

moving vehicle, the opposing lane must be utilised. This manoeuvre is therefore

subject to geometric constraints, most noticeably passing sight distances but also

the terrain of the stretch of road in question.

The capacity of such highways is expressed as a two-directional value rather

than the one-directional value used in the previous section for multi-lane high-

ways. Under ideal conditions, the capacity of a 2-lane highway is set at 2800

passenger car units per hour. If one adjusts this value by a ratio of flow to capac-

ity consistent with the desired level of service, the following formula for service

flow is obtained:

(4.21)

Ideal conditions assume the following:


 Passing is permissible throughout 100% of the section of highway in

question


 Lane widths are at least 12 ft (3.65 m)


 Clearance on hard shoulders of at least 6 ft (1.83 m)


 Design speed of a least 60 miles per hour (96 km/h)


 The traffic stream entirely composed of private cars


 The flow in both directions exactly evenly balanced (50/50 split)


 Level terrain


 No obstructions to flow caused by vehicle turning movements, traffic sig-

nalisation, etc.

If ideal conditions obtain, the service flow is obtained using the ratios of flow

to capacity associated with the required level of service, as given in Table 4.7.

When conditions are non-ideal, the capacity of the highway reduces from

2800 pcu/hour based on the following equation:

SFi (4.22)

i

dwHV

v

c

∏¥ 2800 ÊË ˆ ̄ ¥¥¥fff

SFi

i

v

c

∏¥ 2800 ÊË ˆ ̄

86 Highway Engineering

Level of service Average speed v/cratio

A ≥ 58 0.15

B ≥ 55 0.27

C ≥ 52 0.43

D ≥ 50 0.64

E ≥ 45 1.0

F < 45 —

Table 4.7Level of

service values for 2-lane

highways under ideal

conditions (average

travel speeds assume a

design speed of 60 mph

applies) (Source:

Highway Capacity

Manual(TRB, 1985))