4.4.3 Maximum service flow rates for multi-lane highways
The Highway Capacity Manual generates maximum flow values obtainable on
a multi-lane highway given a certain speed limit and prevailing level of service.
The values assume that ideal conditions exist, i.e. all carriageways are a stan-
dard width (3.65 m), there are no obstructions within 3.65 m of their edge, there
are no heavy goods vehicles, buses or recreational vehicles on the road, the driver
population consists of regular weekday drivers and the road is divided by a
physical barrier and rural-based.
Given the existence of ideal conditions, the maximum service flow, SFMax(i),
can be defined as:
(4.17)
Nis the number of lanes in each direction, and Cjis the capacity of a standard
highway lane for a given design speed j. Its values are shown in Table 4.1:
SFMax i j
i
C
v
c
()=¥ÊË ˆ ̄ ¥N
Basic Elements of Highway Traffic Analysis 81
Design speed (km/h)
70 60 50
Cj(v/h) 2000 2000 1900
Table 4.1Values ofCj
for different design
speeds (Source:
Highway Capacity
Manual(TRB, 1985))
Level of service v/c(C 70 ) v/c(C 60 ) v/c(C 50 )
A 0.36 0.33 —
B 0.54 0.50 0.45
C 0.71 0.65 0.60
D 0.87 0.80 0.76
E 1.0 1.00 1.00
FVariable Variable Variable
Table 4.2Ratios of
flow to capacity for
different levels of
service and design
speeds (Source:
Highway Capacity
Manual(TRB, 1985))
The maximum ratios of flow to capacity for each level of service and design
speed limit are given in Table 4.2.
Example 4.2
A rural divided 4-lane highway has a peak hour volume (V) in one direction
of 1850 vehicles per hour. Ideal conditions apply, therefore there are no heavy
goods vehicles, buses or recreational vehicles in the traffic. The peak hour
factor is 0.8. The design speed limit is 70 mph.
Determine the level of service being provided by the highway.
Contd
