Module 5 : Uninterrupted Flow
Lecture 24 : Freeway Operations
1 2 3 4 5 6 7 8 9 10
 

Calculating average passenger car speed ($ S$)

The average passenger car speed depends on the free flow speed (FFS) and flow rate as calculated earlier and can be given as - For, $ 90 \leq FFS \leq 120$ and $ V_p \leq (3100 - 15FFS), $

$\displaystyle S = FFS$ (1)

For, $ 90 \leq FFS \leq 120$ and $ (3100-15FFS) \leq v_P \leq (1800+5FFS)$

$\displaystyle S=FFS-\left[1/28(23FFS-1800\left(\frac{V_p+15FFS-3100}{20FFS-1300}\right)26\right]$ (2)

The average of all passenger-car speeds measured in the field under low- to moderate- volume conditions can be used directly as the FFS of the freeway segment.

Concept of free flow speed (FFS)

Free flow speed can be defined as:
the mean speed of passenger cars that can be accommodated under low to moderate flow rates on a uniform freeway segment under prevailing roadway and traffic conditions.
FFS is the mean speed of passenger cars measured during low to moderate flows (up to 1,300 pc/h/ln). For a specific segment of freeway, speeds are virtually constant in this range of flow rates. Two methods can be used to determine the FFS of a basic freeway segment: field measurement and estimation with guidelines provided in this section. The field-measurement procedure is provided for users who prefer to gather these data directly. If field measurement of FFS is not possible, FFS can be estimated indirectly on the measurement is not possible basis of the physical characteristics of the freeway segment being studied. The physical characteristics include lane width, number of lanes, right-shoulder lateral clearance, and interchange density. Equation given below is used to estimate the free-flow speed of a basic freeway segment:

$\displaystyle FFS = BFFS - f_{LW} - f_{LC} -f{N} - f_{ID}$ (3)

where, $ FFS$ = free flow speed (km/h), $ BFFS$ = base free flow speed (km/h), $ f_{LW}$ = adjustment for lane width (km/h), $ f_{LC}$ = adjustment for right shoulder clearance (km/h),$ f_N$ = adjustment for no. of lanes (km/h), $ f_{ID}$ = adjustment for interchange density (km/h) Estimation of FFS for an existing or future freeway segment is accomplished adjusting a base free-flow speed downward to reflect the influence of four factors: lane width, lateral clearance, number of lanes, and interchange density. Thus, the analyst is required to select an appropriate BFFS as a starting point.

Adjustment for Lane Width

The base condition for lane width is 3.6 m or greater. When the average lane width across all lanes is less than 3.6 m, the base free-flow speed (e.g., 120 km/h) is reduced. Adjustments to reflect the effect of narrower average lane width are given in Table 1.
Table 1: Adjustment for Lane Width (reduction in free-flow speed for various widths of lane
Lane Width (m) $ f_{LW}$(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

Adjustment for Lateral Clearance

Base lateral clearance is 1.8 m or greater on the right side and 0.6 m or greater on the median or left side, measured from the edge of the paved shoulder to the nearest edge of the travelled lane. When the right-shoulder lateral clearance is less than 1.8 m, the BFFS is reduced. Adjustments to reflect the effect of narrower right-shoulder lateral clearance are given in Table 2.
Table 2: Adjustment for Lateral Clearance (reduction in free-flow speed for various values of lateral clearance)
Right Shoulder $ f_{LC}$ (km/h)
Lateral Lanes in One Direction
Clearance (m) 2 3 4 $ \geq$5
$ \geq$1.8 0.0 0.0 0.0 0.0
1.5 1.0 0.7 0.3 0.2
1.2 1.9 1.3 0.7 0.4
0.9 2.9 1.9 1.0 0.6
0.6 3.9 2.6 1.3 0.8
0.3 4.8 3.2 1.6 1.1
0.0 5.8 3.9 1.9 1.3

Adjustment for Number of Lanes

Freeway segments with five or more lanes (in one direction) are considered as having base conditions with respect to number of lanes. When fewer lanes are present, the BFFS is reduced. Table 3 provides adjustments to reflect the effect of number of lanes on BFFS. In determining number of lanes, only mainline lanes, both basic and auxiliary, should be considered.
Table 3: Adjustment for number of lanes (reduction in free-flow speed for number of lanes in one direction)
Number of Lanes $ f_N$ (km/h)
$ \geq$ 5 0.0
4 2.4
3 4.8
2 7.3

Adjustment for Interchange Density

The base interchange density is 0.3 interchanges per kilometer, or 3.3-km interchange spacing. Base free-flow speed is reduced when interchange density becomes greater. Adjustments to reflect the effect of interchange density are provided in Table 4. Interchange density is determined over a 10-km segment of freeway (5 km upstream and 5 km downstream) in which the freeway segment is located. An interchange is defined as having at least one on-ramp. Therefore, interchanges that have only off-ramps would not be considered in determining interchange density. Interchanges considered should include typical interchanges with arterial or highways and major freeway-to-freeway interchanges.
Table 4: Adjustment for Interchange Density (Reduction in Free-Flow Speed for various values of interchange density)
Interchanges per km $ f_{ID}$ (km/h)
$ \leq$ 0.3 0.0
0.4 1.1
0.5 2.1
0.6 3.9
0.7 5.0
0.8 6.0
0.9 8.1
1.0 9.2
1.1 10.2
1.2 12.1

Calculation of Density and determining LOS

Level of service on the basis of density can be calculated using the equation 4

$\displaystyle D= \frac{V_p}{S}$ (4)

Where, $ D$ = density (pc/km/ln), $ V_p$= flow rate (pc/h/ln), $ S$ = average passenger car speed (km/h). The density of the traffic stream can be used to determine the level of service of a freeway segment. Level-of-service thresholds based on density for a basic freeway segment are summarized in the Table 5 shown below.
Table 5: LOS for a freeway segment
LOS Density Range (pc/km/ln)
A 0 - 7
B $ >$7 - 11
C $ >$11 - 16
D $ >$16 - 22
E $ >$22 - 28
F $ >$28