Module 5 : Uninterrupted Flow
Lecture 23 : Multilane Highways
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Determination of free-flow speed

When field data are not available, the free-flow speed can be estimated indirectly as follows:

$\displaystyle FFS = BFFS - f_{LW} - f_{LC} - f_{M} - f_{A}$ (1)

where, $ FFS$ is the estimated FFS (km/h), $ BFFS$= base FFS (km/h), $ f_{LW}$= adjustment for lane width, from Table 3 (km/h), $ f_{LC}$= adjustment for lateral clearance, from Table 4 (km/h), $ f_{M}$= adjustment for median type, from Table 5 (km/h), and $ f_{A}$= adjustment for access points, from Table 6 (km/h). FFS on multilane highways under base conditions is approximately 11 km/h higher than the speed limit for 65 and 70 km/h speed limits, and it is 8 km/h higher for 80 and 90 km/h speed limits. BFFS is approximately equal to 62.4 km/h ( i.e decrease in 1.6 km/h) when the 85 th percentile speed is 64 km/h, and it is 91.2 km/h ( i.e decrease in 4.8 km/h) when the 85 th percentile speed is 96 km/h and the in between speed values is found out by interpolation.

Table 1: Adjustment for lane width (Source: HCM, 2000)
Lane Width (m) Reduction in FFS(km/h)          
3.6 0.0          
3.5 1.0          
3.4 2.1          
3.3 3.1          
3.2 5.6          
3.1 8.1          
3.0 10.6          

According to Table 3, the adjustment in km/h increase as the lane width decreases from a base lane width of 3.6 m. No data exist for lane widths less than 3.0m.

The adjustment for lateral clearance ($ T_{LC}$) is given as:

$\displaystyle T_{LC} = L_{CL} + L_{CR}$ (2)

where, $ T_{LC}$ = Total lateral clearance (m), $ L_{CL}$ = Lateral clearance (m), from the right edge of the travel lanes to roadside obstructions (if greater than 1.8 m, use 1.8 m), and $ L_{CR}$= Lateral clearance (m), from the left edge of the travel lanes to obstructions in the roadway median (if the lateral clearance is greater than 1.8 m, use 1.8 m). Once the total lateral clearance is computed, the adjustment factor is obtained from Table 4.

Table 2: Adjustment for lateral clearance(Source: HCM, 2000)
Four-Lane Highways Six-Lane Highways
Total Lateral Reduction in FFS Total Lateral Reduction in FFS
Clearance a (m) (km/h) Clearance a (m) (km/h)
3.6 0.0 3.6 0.0
3.0 0.6 3.0 0.6
2.4 1.5 2.4 1.5
1.8 2.1 1.8 2.1
1.2 3.0 1.2 2.7
0.6 5.8 0.6 4.5
0.0 8.7 0.0 6.3


Table 3: Adjustment to free flow speed for median type(Source: HCM, 2000)
Median Type Reduction in FFS (km/h)
Undivided highways 2.6
Divided highways 0.0

For undivided highways, there is no adjustment for the right-side lateral clearance as this is already accounted for in the median type. Therefore, in order to use Table 5 for undivided highways, the lateral clearance on the left edge is always 1.8 m, as it for roadways with TWRTLs.

Table 4: Adjustment to free flow speed for Access-point density(Source: HCM, 2000)
Access Points/Kilometer Reduction in FFS (km/h)
0 0.0
6 4.0
12 8.0
18 12.0
$ \geq 24$ 16.0

The access-point density, which is use in Table 6, for a divided roadway is found by dividing the total number of access points (intersections and driveways) on the right side of the roadway in the direction of travel being studied by the length of the segment in kilometers. The adjustment factor for access-point density is given in Table 6. Thus the free flow speed can be computed using equation 1 and applying all the adjustment factors.