4.4 . Classified Traffic Counts.
Classified traffic counts were performed at two x c t m of rood lor b d o M *
l l * and 18 March 2012 at the overhead bridft at chamaf* 1200m md next to
Mbagathi Mosque at chainage 2350m. Traffic counts were limited to commercial traffic
only.
The average daily traffic was computed from the data which consisted o fhvt d«y 12
count and two day 24-hr counts on 14th March 2012 and 17* March 2012 The 12 hf
counts were converted to 24-hr counts. The results are shown m Table 4 16 and 4 1 /
Table 4.16: Average daily commercial Traffic for Mbagathi Road Bridge
Vehicle Mon Tue Wed Thur Frl Sat Sun Totals AOT
Type
Bus 217 289 364 240 368 184 US 1.777 254
MGV 768 857 744 415 727 246 173 3,930 561
HGV 180 188 170 91 201 80 21 931 133
A-HGV 67 60 106 51 110 31 10 43S 62
Total 1,232 1,394 1,384 797 1,406 S41 319 7,073 1,010
Table 4.17: Average daily commercial Traffic at the mosque
Thur Fr. Sat Sun Totals AOT
Vehicle Mon Tue Wed
Type
691 253 145 3,357 480
Bus 623 629 609 407
i - - -4
604 245 188 2,993 428
MGV 474 472 620 390
66 24 764 109
HGV 138 109 165
119 143 ■ ■■ i _- —•f
24 26 442 63
A-HGV 88 66 127
58 53
588 383 7,556 1,080
972 1,587
Total 1,274 1,297 1,455 - L _ ------- 1
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The average number of commercial vehicles per day was recorded to be less than 2,000
commercial vehicles per day hence the total commercial traffic in one direction was used
as recommended by Kenya Road Design Manual Part III.
4.4.1. Vehicle Equivalence Factors
The average vehicle equivalence factors have been derived from the maximum legal
limits for vehicular loadings. The legal limits for equivalence factors are shown in table
4.18.
The vehicle classes considered were:
i. Buses (B); passenger vehicles with more than 18 passengers;
ii. Medium Goods Vehicles (MGV); with 2 axles and un-laden weight above 1.5 tons;
iii. Heavy Goods Vehicles (HGV); vehicles having three or four rigid axles; and,
iv. Articulated Heavy Goods Vehicles (AHGV); vehicles with 3 or more articulated
axles.
Table 4.18: Legal Limits of Vehicle Equivalence Factors in Kenya.
Vehicle Type Maximum E.F based on legal limit
Bus 3.4
Heavy Goods Vehicle 3.4
Medium Goods Vehicle 3.0
Articulated Heavy Goods Vehicle 6.8
4.4.2. Design daily Equivalent Standard Axles
The design daily equivalent standard axles (DESA) were calculated by summing up
product of average vehicle equivalence factors (VEF) and the average daily traffic (ADT)
for both directions for each vehicle type. DESA for the most heavily loaded road section
(at the mosque) was adopted for design. The Design daily Equivalent Standard Axles are
shown in Table 4.19. Assuming that slow traffic lanes will carry 80% of commercial
vehicles, the design traffic loading is 2,972 daily equivalent standards axles.
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Table 4.19: Design daily Equivalent Standard Axles
Vehicle Type V.E.F Design ADT DESA
Bus 3.4 480 1632
Heavy Goods Vehicle 3.4 109 370.6
Medium Goods Vehicle 3.0 428 1284
Articulated Heavy Goods Vehicle 6.8 63 428.4
Total 1,080 3,715
4.4.3. Annual Traffic Growth Rate
Annual trends in traffic growth can be estimated from the growth rate of Gross Domestic
Product, GDP (Kenya Road Design Manual, Part III). The annual growth rate of GDP
between the years 1961 to 2011 is given in Table 4.18. The average growth in GDP =
4.51% rounded off to 5.0%. the annual GDP growth rate in Kenya from 1961 to 2010 is
summarized in Table 4.20.
Table 4.20: Annual GDP Growth Rate from 1961 to 2010.
-----------------------------— —
GDP Growth GDP Growth rate GDP Growth
Year Year Year
rate (%) (%> rate (%)
1961 -7.8 1978 6.9 1995 4.4
1962 9.5 1979 7.6 1996 4.1
^ 1963 8.8 1980 5.6 1997 0.5
' 1964 5.0 1981 3.8 1998 3.3
1965 2.0 1982 1.5 1999 2.5
1966 14.7 1983 1.3 2000 0.6
1967 3.4 1984 1.8 2001 3.8
1968 8.0 1985 4.3 2002 0.5
1969 8.0 1986 7.2 2003 2.9
1970 -4.7 1987 5.9 2004 5.1
1971 22.2 1988 6.2 2005 5.9
1972 17.1 1989 4.7 2006 6.3
1973 5.9 1990 4.2 2007 7.0
1974 4.1 1991 1.4 2008 1.6
1992 -0.8 2009 2.6
1975 0.9
1993 0.4 2010 5.3
1976 2.2
1994 2.6 MEAN 4.5%
1977 9.5
Source: World Bank data bank, 2012.
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4.4.4. Cumulative Equivalent Standard Axles, CESA
The cumulative Equivalent Standard Axles, CESA over a chosen design period, n is
obtained by Equation 12.
((1 + r ) B- 1)
C E S A = 365 x T x Equation 12
r
Where:
T - The design daily Equivalent Standard Axles (DESA) for base year (2012);
r —Annual traffic growth rate (5%) expressed as a decimal fraction, and,
n - Design period in years; taken as 15 years to take into account the fact that this study
is being undertaken 5 years after the concrete road was open to traffic.
Seasonal correction factor = 1.0.
CESA = 23.42 x 106 ESA. Traffic class T2.
4.5. Design of Mbagathi road by Thickness Design Method.
Based on the design of Mbagathi Road based on 15 year remaining life, the following
results are obtained.
Assuming subgrade CBR > 30; (white-topping)
Subbase thickness = 150mm and no capping layer required.
Concrete slab thickness = 200mm for JUCP.
Therefore provided 205mm thick slab is adequate for the remaining life of the pavement.
Summary:
Thickness design method indicates that the road will survive its remaining life without
major deterioration. However, special attention must be paid to drainage condi ions
which indicated failure of the pavement.
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