Example 4.4
A 2-lane highway has lane widths of 9 ft (2.75 m), with 6 ft (1.83 m) clear
hard shoulders. There are no-passing zones along 40% of its length. The
directional split is 70/30 in favour of the peak direction.
The percentages for the various heavy vehicle types are:
PT– 10%
PB–4%
PR– 2%The terrain is rolling.
Calculate the service flow of the highway when running at full capacity.The level of service values in Table 4.7 assumes that passing is available over
all sections of the road and that the terrain is level. If this is not the case and
passing is prohibited on portions of the highway and/or the terrain is rolling or
mountainous, the ratios of volume to capacity for the different levels of service
must be adjusted accordingly. Ratios for the different terrain types and for zero,
40% and 100% no-passing zones are given in Table 4.11.88 Highway Engineering
v/cratio
Level terrain Rolling terrain Mountainous terrain
% no-passing % no-passing % no-passing
LOS Avg. 0 40 100 Avg. 0 40 100 Avg. 0 40 100
speed* speed* speed*
A ≥ 58 0.15 0.09 0.04 ≥ 57 0.15 0.07 0.03 ≥ 56 0.14 0.07 0.01
B ≥ 55 0.27 0.21 0.16 ≥ 54 0.26 0.19 0.13 ≥ 54 0.25 0.16 0.10
C ≥ 52 0.43 0.36 0.32 ≥ 51 0.42 0.35 0.28 ≥ 49 0.39 0.28 0.16
D ≥ 50 0.64 0.60 0.57 ≥ 49 0.62 0.52 0.43 ≥ 45 0.58 0.45 0.33
E ≥ 45 1.00 1.00 1.00 ≥ 40 0.97 0.92 0.90 ≥ 35 0.91 0.84 0.78
F < 45 ———< 40 ———< 35 ———Table 4.11Level of service values for 2-lane highways for different percentage no-passing zones
(Source:Highway Capacity Manual(TRB, 1985))*Average speeds in miles per hour, intermediate ratios for 20%, 60% and 80% no-passing zones
are available.Contd