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Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

CHAPTER 7 Two-Lane Highway Segment Analysis Methodology

The core two-lane highway analysis methodology is contained in HCM7 Chapter 15, Two-Lane Highways. The motorized vehicle analysis methodology process is given in HCM7 Exhibit 15-9 (p. 15-16). The specific equation and exhibit numbers used in the methodology, in the general order in which they are applied, are as follows.

7.1 Equations and Exhibits

The variable definitions for these equations are shown in Section 7.2 Nomenclature.

Step 1: Identify Facility Study Boundaries and Corresponding Segmentation

Exhibit 15-10: Minimum and maximum segment lengths for use in computing segment speeds and percent followers.

Vertical Class	Passing Constrained: Minimum - Maximum (mi)	Passing Zone: Minimum - Maximum (mi)	Passing Lane: Minimum - Maximum (mi)
1	0.25 - 3.0	0.25 - 2.0	0.5 - 3.0
2	0.25 - 3.0	0.25 - 2.0	0.5 - 3.0
3	0.25 - 1.1	0.25 - 1.1	0.5 - 1.1
4	0.5 - 3.0	0.5 - 2.0	0.5 - 3.0
5	0.5 - 3.0	0.5 - 2.0	0.5 - 3.0

Step 2: Determine Demand Flow Rates, Capacity, and d/c Ratio

Equation 15-1: Analysis flow rate estimation; uses results from Equation 4-2.

v_{d} = \frac{V_{d}}{P H F}

(HCM7 Eq. 15-1)

Equation 4-2: Peak hour factor (PHF); in HCM7 Chapter 4, Traffic Operations and Capacity Concepts.

P H F = \frac{V}{V_{15} \times 4}

(HCM7 Eq. 4-2)

Page 82

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Step 3: Determine Vertical Alignment Classification

Exhibit 15-11: Classifications for vertical alignment.

Segment Length (mi)	Segment Grade (%)
Segment Length (mi)	≤ 1	>1 ≤ 2	>2 ≤ 3	>3 ≤ 4	>4 ≤ 5	>5 ≤ 6	>6 ≤ 7	>7 ≤ 8	>8 ≤ 9	>9
≤ 0.1	1 (1)	1 (1)	1 (1)	1 (1)	1 (1)	1 (1)	1 (1)	2 (1)	2 (2)	2 (2)
>0.1 ≤ 0.2	1 (1)	1 (1)	1 (1)	1 (1)	2 (1)	2 (2)	2 (2)	3 (2)	3 (3)	3 (3)
>0.2 ≤ 0.3	1 (1)	1 (1)	1 (1)	2 (1)	2 (2)	3 (2)	3 (3)	4 (3)	4 (4)	5 (5)
>0.3 ≤ 0.4	1 (1)	1 (1)	2 (1)	2 (2)	3 (2)	3 (3)	4 (4)	5 (4)	5 (5)	5 (5)
>0.4 ≤ 0.5	1 (1)	1 (1)	2 (1)	2 (2)	3 (3)	4 (3)	5 (4)	5 (5)	5 (5)	5 (5)
>0.5 ≤ 0.6	1 (1)	1 (1)	2 (1)	3 (2)	3 (3)	4 (4)	5 (5)	5 (5)	5 (5)	5 (5)
>0.6 ≤ 0.7	1 (1)	1 (1)	2 (1)	3 (2)	4 (3)	4 (4)	5 (5)	5 (5)	5 (5)	5 (5)
>0.7 ≤ 0.8	1 (1)	1 (1)	2 (1)	3 (3)	4 (4)	5 (4)	5 (5)	5 (5)	5 (5)	5 (5)
>0.8 ≤ 0.9	1 (1)	1 (1)	2 (1)	3 (3)	4 (4)	5 (5)	5 (5)	5 (5)	5 (5)	5 (5)
>0.9 ≤ 1.0	1 (1)	1 (1)	2 (2)	3 (3)	4 (4)	5 (5)	5 (5)	5 (5)	5 (5)	5 (5)
>1.0 ≤ 1.1	1 (1)	1 (1)	2 (2)	3 (3)	4 (4)	5 (5)	5 (5)	5 (5)	5 (5)	5 (5)
>1.1	1 (1)	1 (1)	2 (2)	4 (4)	4 (4)	5 (5)	5 (5)	5 (5)	5 (5)	5 (5)

Step 4: Determine the FFS

Equation 15-3: FFS estimation; uses results from Equations 15-2, 15-4, 15-5, and 15-6 and Exhibit 15-12.
FFS = BFFS − a(HV%) − f_ls − f_A (HCM7 Eq. 15-3)
Equation 15-2: BFFS estimation.
BFFS = 1.14 × S_p (HCM7 Eq. 15-2)

Equation 15-4: Coefficient estimation for heavy vehicle percentage (HV%) parameter in Equation 15-3.

a = \max [0.0333, a_{0} + a_{1} \times B F F S + a_{2} \times L + \max (0, a_{3} + a_{4} \times B F F S + a_{5} \times L) \times \frac{v_{0}}{1, 000}]

(HCM7 Eq. 15-4)

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Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Exhibit 15-12: Coefficient values for Equation 15-4.

Vertical Class	a₀	a₁	a₂	a₃	a₄	a₅
1	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000
2	−0.45036	0.00814	0.01543	0.01358	0.00000	0.00000
3	−0.29591	0.00743	0.00000	0.01246	0.00000	0.00000
4	−0.40902	0.00975	0.00767	−0.18363	0.00423	0.00000
5	−0.38360	0.01074	0.01945	−0.69848	0.01069	0.12700

Equation 15-5: Adjustment for lane and shoulder width.
f_LS = 0.6 × (12 − LW) + 0.7 × (6 − SW) (HCM7 Eq. 15-5)

Equation 15-6: Adjustment for access points.

f_{A} = \min (\frac{A P D}{4}, 10)

(HCM7 Eq. 15-6)

Step 5: Estimate the Average Speed

Equation 15-7: Average speed estimation; uses results from Equations 15-8 to 15-16 and Exhibits 15-13 to 15-22.
(HCM7 Eq. 15-7)

Equation 15-8: Slope coefficient (m) for Equation 15-7.

m = \max [b_{5}, b_{0} + b_{1} \times F F S + b_{2} \times \sqrt{\frac{v_{0}}{1, 000}} + \max (0, b_{3}) \times \sqrt{L} + \max (0, b_{4}) \times \sqrt{H V %}]

(HCM7 Eq. 15-8)

Exhibit 15-13: Coefficient values for Equation 15-8 for passing zone and passing constrained segments.

Vertical Class	b₀	b₁	b₂	b₃	b₄	b₅
1	0.0558	0.0542	0.3278	0.1029	0	0
2	5.728	-0.0809	0.7404	Equation 15-9	Equation 15-10	3.1155
3	9.3079	-0.1706	1.1292	Equation 15-9	Equation 15-10	3.1155
4	9.0115	-0.1994	1.8252	Equation 15-9	Equation 15-10	3.2685
5	23.9144	-0.6925	1.9473	Equation 15-9	Equation 15-10	3.5115

Page 84

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Exhibit 15-14: Coefficient values for Equation 15-8 for passing lane segments.

Vertical Class	b₀	b₁	b₃	b₄	b₅
1	-1.1379	0.0941	Equation 15-9	Equation 15-10	0
2	-2.0688	0.1053	Equation 15-9	Equation 15-10	0
3	-0.5074	0.0935	0	Equation 15-10	0
4	8.0354	-0.0860	Equation 15-9	Equation 15-10	4.19
5	7.2991	-0.3535	Equation 15-9	Equation 15-10	4.87

Equation 15-9: Segment length coefficient (b₃) for Equation 15-8.

b_{3} = c_{0} + c_{1} \times \sqrt{L} + c_{2} \times F F S + c_{3} \times (F F S \times \sqrt{L})

(HCM7 Eq. 15-9)

Exhibit 15-15: Coefficient values for Equation 15-9 for passing zone and passing constrained segments.

Vertical Class	c₀	c₂
1	0.1029	0
2	-13.8036	0.2446
3	-11.9703	0.2542
4	-12.5113	0.2656
5	-14.8961	0.437

Exhibit 15-16: Coefficient values for Equation 15-9 for passing lane segments.

Vertical Class	c₀	c₁	c₂	c₃
1	0	0.2667	0	0
2	0	0.4479	0	0
3	0	0	0	0
4	-27.1244	11.5196	0.4681	-0.1873
5	-45.3391	17.3749	1.0587	-0.3729

Equation 15-10: Heavy vehicle percentage coefficient (b₄) for Equation 15-8.

b_{4} = d_{0} + d_{1} \times \sqrt{H V %} + d_{2} \times F F S + d_{3} \times (F F S \times \sqrt{H V %})

(HCM7 Eq. 15-10)

Exhibit 15-17: Coefficient values for Equation 15-10 for passing zone and passing constrained segments.

Vertical Class	d₀	d₁	d₂	d₃
1	0	0	0	0
2	-1.7765	0	0.0392	0
3	-3.5550	0	0.0826	0
4	-5.7775	0	0.1373	0
5	-18.2910	2.3875	0.4494	-0.0520

Page 85

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Exhibit 15-18: Coefficient values for Equation 15-10 for passing lane segments.

Vertical Class	d₀	d₁	d₃
1	0	0.1252	0
2	0	0.1631	0
3	0	-0.2201	0.0072
4	0	-0.7506	0.0193
5	3.8457	-0.9112	0.017

Equation 15-11: Power coefficient (p) for Equation 15-7.

p = \max [\begin{array}{l} f_{8}, f_{0} + f_{1} \times F F S + f_{2} \times L + f_{3} \times \frac{v_{0}}{1, 000} + f_{4} \times \sqrt{\frac{v_{0}}{1, 000}} \\ + f_{5} \times H V + f_{6} \times \sqrt{H V} + f_{7} \times (L \times H V) \end{array}]

(HCM7 Eq. 15-11)

Exhibit 15-19: Coefficient values for Equation 15-11 for passing zone and passing constrained segments.

Vertical Class	f₀	f₁	f₂	f₃	f₄	f₅	f₆	f₈
1	0.67576	0	0	0.1206	-0.35919	0	0	0
2	0.34524	0.00591	0.02031	0.14911	-0.43784	-0.00296	0.02956	0.41622
3	0.17291	0.00917	0.05698	0.27734	-0.61893	-0.00918	0.09184	0.41622
4	0.67689	0.00534	-0.13037	0.25699	-0.68465	-0.00709	0.07087	0.3395
5	1.13262	0	-0.26367	0.18811	-0.64304	-0.00867	0.08675	0.3059

Exhibit 15-20: Coefficient values for Equation 15-11 for passing lane segments.

Vertical Class	f₀	f₁	f₂	f₅	f₇
1	0.91793	-0.00557	0.36862	0.00611	-0.00419
2	0.65105	0	0.34931	0.00722	-0.00391
3	0.40117	0	0.68633	0.0235	-0.02088
4	1.13282	-0.00798	0.35425	0.01521	-0.00987
5	1.12077	-0.00550	0.25431	0.01269	-0.01053

The remainder of the calculations in Step 5 apply only if the segment contains horizontal curvature. Exhibit 15-21 provides a flowchart of the calculation procedure to adjust segment average speed for horizontal curvature. Instead of presenting the flowchart here, the steps to the flowchart are described in the text as follows.

For all horizontal curves within the segment:

Determine the alignment classification using Exhibit 15-22.

Page 86

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Exhibit 15-22: Horizontal alignment classifications.

Radius (ft)	Superelevation (%)
Radius (ft)	<1	≥1 <2	≥2 <3	≥3 <4	≥4 <5	≥5 <6	≥6 <7	≥7 <8	≥8 <9	≥9 <10	≥10
<300	5	5	5	5	5	5	5	5	5	5	5
300–449	4	4	4	4	4	4	4	4	4	4	4
450–599	4	3	3	3	3	3	3	3	3	3	3
600–749	3	3	3	3	3	3	2	2	2	2	2
750–899	2	2	2	2	2	2	2	2	2	2	2
900–1,049	2	2	2	2	2	2	2	2	1	1	1
1,050–1,199	2	2	2	2	1	1	1	1	1	1	1
1,200–1,349	2	2	1	1	1	1	1	1	1	1	1
1,350–1,499	1	1	1	1	1	1	1	1	1	1	—
1,500–1,649	1	1	1	1	1	1	1	1	—	—	—
1,650–1,799	1	1	1	1	1	1	—	—	—	—	—
1,800–1,949	1	1	1	1	1	—	—	—	—	—	—
1,950–2,099	1	1	1	1	—	—	—	—	—	—	—
2,100–2,249	1	1	1	—	—	—	—	—	—	—	—
2,250–2,399	1	1	—	—	—	—	—	—	—	—	—
2,400–2,549	1	—	—	—	—	—	—	—	—	—	—
≥2,550	—	—	—	—	—	—	—	—	—	—	—

Calculate the horizontal curve BFFS using Equation 15-14; uses result from Exhibit 15-22.
BFFS_HCi = min(BFFS_T, 44.32 + 0.3728 × BFFS_T − 6.868 × HorizClass_i) (HCM7 Eq. 15-14)
Calculate the horizontal curve FFS using Equation 15-13; uses result from Equation 15-14.
FFS_HCi = BFFS_HCi − 0.0255 × HV% (HCM7 Eq. 15-13)

Calculate the m coefficient for Equation 15-12, using Equation 15-15.

m = \max (\begin{array}{l} 0.277, - 25.8993 - 0.7756 \times F F S_{H C i} + 10.6294 \times \sqrt{F F S_{H C i}} \\ + 2.4766 \times H o r i z C l a s s_{i} - 9.8238 \times \sqrt{H o r i z C l a s s_{i}} \end{array})

(HCM7 Eq. 15-15)

Calculate the average speed on horizontal curve subsegment using Equation 15-12; uses results from Equations 15-13 and 15-15.

Page 87

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

S_{H C i} = \min (S, F F S_{H C i} - m \times \sqrt{\frac{v_{d}}{1, 000}} - 0.1)

(HCM7 Eq. 15-12)

Set the horizontal curve average speed to the minimum value between the horizontal curve average speed and a comparable tangent segment.

Once the above calculations have been performed for all horizontal curve subsegments within the segment, calculate the adjusted average speed for the segment using Equation 15-16.

S = \frac{\sum_{i} (S u b s e g S p e e d_{i} \times S u b s e g L e n g t h_{i})}{L}

(HCM7 Eq. 15-16)

Step 6: Estimate the Percent Followers

Equation 15-17: Percent followers estimation; uses results from Equations 15-22 and 15-23.

P F = 100 \times [1 - e (m \times {\frac{v_{d}}{1, 000}}^{p})]

(HCM7 Eq. 15-17)

Equation 15-18: Percent followers at capacity flow rate for passing zone and passing constrained segments; uses results from Exhibit 15-24.

\begin{array}{l} P F_{c a p} = b_{0} + b_{1} (L) + b_{2} (\sqrt{L}) + b_{3} (F F S) + b_{4} (\sqrt{F F S}) + b_{5} (H V %) \\ + b_{6} (F F S \times \frac{v_{0}}{1, 000}) + b_{7} (\sqrt{\frac{v_{0}}{1, 000}}) \end{array}

(HCM7 Eq. 15-18)

Exhibit 15-24: Coefficient values for Equation 15-18.

Vertical Class	b₀	b₁	b₂	b₃	b₄	b₅	b₆	b₇
1	37.6808	3.05089	-7.90866	-0.94321	13.64266	-0.00050	-0.05500	7.13758
2	58.21104	5.73387	-13.66293	-0.66126	9.08575	-0.00950	-0.03602	7.14619
3	113.20439	10.01778	-18.90000	0.46542	-6.75338	-0.03000	-0.05800	10.03239
4	58.29978	-0.53611	7.35076	-0.27046	4.4985	-0.01100	-0.02968	8.89680
5	3.32968	-0.84377	7.08952	-1.32089	19.98477	-0.01250	-0.02960	9.99453

Equation 15-19: Percent followers at capacity flow rate for passing lane segments; uses results from Exhibit 15-25.

\begin{array}{l} P F_{c a p} = b_{0} + b_{1} (L) + b_{2} (\sqrt{L}) + b_{3} (F F S) + b_{4} (\sqrt{F F S}) + b_{5} (H V %) \\ + b_{6} (\sqrt{H V %}) + b_{7} (F F S \times H V %) \end{array}

(HCM7 Eq. 15-19)

Exhibit 15-25: Coefficient values for Equation 15-19.

Vertical Class	b₀	b₁	b₂	b₃	b₄	b₅	b₆	b₇
1	61.73075	6.73922	–23.68853	–0.84126	11.44533	–1.05124	1.5039	0.00491
2	12.30096	9.57465	– 30.79427	–1.79448	25.76436	–0.66350	1.26039	–0.00323
3	206.07369	–4.29885	0	1.96483	– 30.32556	–0.75812	1.06453	–0.00839
4	263.13428	5.38749	– 19.04859	2.73018	– 42.76919	–1.31277	–0.32242	0.01412
5	126.95629	5.95754	– 19.22229	0.43238	–7.35636	–1.03017	–2.66026	0.01389

Page 88

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Equation 15-20: Percent followers at 25% of capacity flow rate for passing zone and passing constrained segments; uses results from Exhibit 15-26.

\begin{array}{l} P F_{25 c a p} = c_{0} + c_{1} (L) + c_{2} (\sqrt{L}) + c_{3} (F F S) + c_{4} (\sqrt{F F S}) + c_{5} (H V %) \\ + c_{6} (F F S \times \frac{v_{0}}{1, 000}) + c_{7} (\sqrt{\frac{v_{0}}{1, 000}}) \end{array}

(HCM7 Eq. 15-20)

Exhibit 15-26: Coefficient values for Equation 15-20.

Vertical Class	c₀	c₁	c₂	c₃	c₄	c₅	c₆	c₇
1	18.01780	10.00000	-21.60000	-0.97853	12.05214	-0.00750	-0.06700	11.60405
2	47.83887	12.80000	-28.20000	-0.61758	5.8	-0.04550	-0.03344	11.35573
3	125.40000	19.50000	-34.90000	0.90672	-16.10000	-0.11000	-0.06200	14.71136
4	103.13534	14.68459	-23.72704	0.664436	-11.95763	-0.10000	0.00172	14.70067
5	89	19.02642	-34.54240	0.29792	-6.62528	-0.16000	0.00480	17.56611

Equation 15-21: Percent followers at 25% of capacity flow rate for passing lane segments; uses results from Exhibit 15-27.

\begin{array}{l} P F_{25 c a p} = c_{0} + c_{1} (L) + c_{2} (\sqrt{L}) + c_{3} (F F S) + c_{4} (\sqrt{F F S}) + c_{5} (H V %) \\ + c_{6} (\sqrt{H V %}) + c_{7} (F F S \times H V %) \end{array}

(HCM7 Eq. 15-21)

Exhibit 15-27: Coefficient values for Equation 15-21.

Vertical Class	c₀	c₁	c₂	c₃	c₄	c₅	c₆	c₇
1	80.37105	14.44997	– 46.41831	–0.23367	0.84914	–0.56747	0.89427	0.00119
2	18.37886	14.71856	- 47.78892	–1.43373	18.3204	–0.13226	0.77217	–0.00778
3	239.9893	15.90683	– 46.87525	2.73582	– 42.88130	–0.53746	0.76271	–0.00428
4	223.68435	10.26908	– 35.60830	2.31877	– 38.30034	–0.60275	–0.67758	0.00117
5	137.37633	11.00106	– 38.89043	0.78501	– 14.88672	–0.72576	–2.49546	0.00872

Equation 15-22: Slope coefficient (m) for Equation 15-17; uses results from Exhibit 15-28.

m = d_{1} (\frac{0 - l n [1 - \frac{P F_{25 c a p}}{100}]}{0.25 [\frac{c a p}{1, 000}]}) + d_{2} (\frac{0 - l n [1 - \frac{P F_{c a p}}{100}]}{[\frac{c a p}{1, 000}]})

(HCM7 Eq. 15-22)

Exhibit 15-28: Coefficient values for Equation 15-22.

Segment Type	d₁	d₂
Passing Zone and Passing Constrained	–0.29764	–0.71917
Passing Lane	–0.15808	–0.83732

Page 89

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Equation 15-23: Power coefficient (p) for Equation 15-17; uses results from Exhibit 15-29.

p = e_{0} + e_{1} (\frac{0 - l n [1 - \frac{P F_{25 c a p}}{100}]}{0.25 [\frac{c a p}{1, 000}]}) + e_{2} (\frac{0 - l n [1 - \frac{P F_{c a p}}{100}]}{[\frac{c a p}{1, 000}]}) + e_{3} (\frac{0 - l n [1 - \frac{P F_{25 c a p}}{100}]}{0.25 [\frac{c a p}{1, 000}]}) + e_{4} (\frac{0 - l n [1 - \frac{P F_{c a p}}{100}]}{[\frac{c a p}{1, 000}]})

(HCM7 Eq. 15-23)

Exhibit 15-29: Coefficient values for Equation 15-23.

Segment Type	e₀	e₁	e₂	e₃	e₄
Passing Zone and Passing Constrained	0.81165	0.3792	–0.49524	–2.11289	2.41146
Passing Lane	–1.63246	1.6496	–4.45823	–4.89119	10.33057

Step 7: Calculate Additional Performance Measure Values for a Passing Lane Segment

Equation 15-24: Estimate number of heavy vehicles entering the passing lane segment.

N u m H V = v_{d} \times \frac{H V %}{100}

(HCM7 Eq. 15-24)

Equation 15-25: Estimate proportion of the demand flow in the faster lane (i.e., the lane used by passing vehicles); uses result from Equation 15-24.
PropFlowRate_FL = 0.92183 − 0.05022 × ln(v_d) − 0.00030 × NumHV (HCM7 Eq. 15-25)
Equation 15-26: Estimate demand flow rate in the faster lane; uses result from Equation 15-25.
FlowRate_FL = v_d × PropFlowRate_FL (HCM7 Eq. 15-26)
Equation 15-27: Estimate demand flow rate in the slower lane (i.e., the lane used by non-passing vehicles); uses result from Equation 15-25.
FlowRate_SL = v_d × (1 − PropFlowRate_FL) (HCM7 Eq. 15-27)
Equation 15-28: Estimate percentage of heavy vehicles in the faster lane.
HV%_FL = HV% × HVPropMultiplier_FL (HCM7 Eq. 15-28)

Page 90

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Equation 15-29: Estimate number of heavy vehicles in the slower lane; uses result from Equation 15-28.

N u m H V_{S L} = N u m H V - (F l o w R a t e_{F L} \times \frac{H V %_{F L}}{100})

(HCM7 Eq. 15-29)

Equation 15-30: Estimate percentage of heavy vehicles in the slower lane; uses result from Equation 15-29.

H V %_{S L} = \frac{N u m H V_{S L}}{F l o w R a t e_{S L}} \times 100

(HCM7 Eq. 15-30)

Equation 15-31: Estimate average speed lane differential adjustment.

A v g S p e e d D i f f A d j = 2.750 + 0.00056 \times v_{d} + 3.8521 \times \frac{H V %}{100}

(HCM7 Eq. 15-31)

Equation 15-32: Estimate average speed in the faster lane at the midpoint of the passing lane segment; uses result from Equation 15-31.

S_{P L m i d - F L} = S_{i n i t_F L} + \frac{A v g S p e e d D i f f A d j}{2}

(HCM7 Eq. 15-32)

Equation 15-33: Estimate average speed in the slower lane at the midpoint of the passing lane segment; uses result from Equation 15-31.

S_{P L m i d - F L} = S_{i n i t_F L} - \frac{A v g S p e e d D i f f A d j}{2}

(HCM7 Eq. 15-33)

Step 8: Calculate Follower Density

Equation 15-34: Follower density estimation for passing lane segment midpoint.

F D_{P L m i d} = \frac{(\frac{P F_{P L m i d_F L}}{100} \times \frac{F l o w R a t e_{F L}}{S_{P L m i d_F L}}) + (\frac{P F_{P L m i d_S L}}{100} \times \frac{F l o w R a t e_{S L}}{S_{P L m i d_S L}})}{2}

(HCM7 Eq. 15-34)

Equation 15-35: Follower density estimation for passing zone and passing constrained segments.

F D = \frac{P F}{100} \times \frac{v_{d}}{S}

(HCM7 Eq. 15-35)

Step 9: Determine Potential Adjustment to Follower Density

Equation 15-36: Estimate percentage improvement to percent followers on a segment downstream of a passing lane segment.

I m p r o v e_{P F} = \max (\begin{array}{l} 0, 27 - 8.75 \times l n [\max (0.1, D o w n s t r e a m D i s t a n c e)] \\ + 0.1 \times \max [0, P F - 30] + 3.5 \times l n [\max (0.3, P a s s L a n e L e n g t h)] \\ - 0.01 \times F l o w R a t e \end{array})

(HCM7 Eq. 15-36)

Page 91

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

Equation 15-37: Estimate percentage improvement to the average speed on a segment downstream of a passing lane segment.

I m p r o v e_{S} = \max (\begin{array}{l} 0, 3 - 0.8 \times D o w n s t r e a m D i s \tan c e + 0.1 \times \max [0, P F - 30] \\ + 0.75 \times P a s s L a n e L e n g t h - 0.005 \times F l o w R a t e \end{array})

(HCM7 Eq. 15-37)

Equation 15-38: Adjusted follower density on a segment downstream of a passing lane segment; uses results from Equations 15-36 and 15-37.

F D_{a d j} = \frac{P F}{100} \times (1 - \frac{% I m p r o v e_{P F}}{100}) \times \frac{F l o w R a t e}{S \times (1 + \frac{% I m p r o v e_{s}}{100})}

(HCM7 Eq. 15-38)

Step 10: Determine LOS

Exhibit 15-6: Motorized vehicle LOS criteria for two-lane highways.

Follower Density (followers/mi/ln)

LOS	Posted Speed Limit ≥ 50 mi/h	Posted Speed Limit < 50 mi/h
A	≤ 2.0	≤ 2.5
B	> 2.0–4.0	> 2.5–5.0
C	> 4.0–8.0	> 5.0–10.0
D	> 8.0–12.0	> 10.0–15.0
E	> 12.0	> 15.0

Step 11: Facility Analysis

This step is not included in the rural highway facility analysis.

7.2 Nomenclature

a₀ − a₅ =	coefficient values from Exhibit 15-12.
APD =	access-point density (access points/mi).
b₀ − b₅ =	coefficients for speed–flow slope model, from Exhibit 15-13 for passing zone and passing constrained segments, and from Exhibit 15-14 for passing lane segments.
b₀ − b₇ =	coefficient values for Equation 15-18, from Exhibit 15-24.
b₀ − b₇ =	coefficient values for Equation 15-19, from Exhibit 15-25.
b₃ =	segment length coefficient for speed–flow slope model (decimal).
b₄ =	heavy vehicle percentage coefficient for speed–flow slope model (decimal).

Page 92

Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

BFFS =	base free-flow speed (mi/h).
BFFS_HCi =	base free-flow speed on horizontal curve subsegment i in the analysis direction (mi/h), from Equation 15-14.
BFFS_T =	base free-flow speed on preceding tangent subsegment in the analysis direction (mi/h).
c₀ − c₃ =	coefficients for the b₃ segment length coefficient model, from Exhibit 15-15 for passing zone and passing constrained segments, and from Exhibit 15-16 for passing lane segments.
c₀ − c₇ =	coefficient values for Equation 15-20, from Exhibit 15-26.
c₀ − c₇ =	coefficient values for Equation 15-21, from Exhibit 15-27.
d₀ − d₃ =	coefficients for the b₄ heavy vehicle percentage coefficient model, from Exhibit 15-17 for passing zone and passing constrained segments, and from Exhibit 15-18 for passing lane segments.
d₁ − d₂ =	coefficient values for Equation 15-22, from Exhibit 15-28
DownstreamDistance =	distance downstream from the start of the passing lane segment (mi).
e₀ − e₄ =	coefficient values for Equation 15-23, from Exhibit 15-29.
f₀ − f₈ =	coefficients for the power coefficient model, from Exhibit 15-19 for passing zone and passing constrained segments, and from Exhibit 15-20 for passing lane segments.
f_A =	adjustment for access point density, from Equation 15-6 (mi/h).
f_LS =	adjustment for lane and shoulder width (mi/h), from Equation 15-5.
FD_adj =	adjusted follower density on a segment downstream of a passing lane segment (followers/mi).
FD_F =	average follower density for the facility in the analysis direction (followers/mi).
FD_i =	follower density, or adjusted follower density, for segment i in the analysis direction (followers/mi).
FFS =	free-flow speed in the analysis direction (mi/h).
FFS_HCi =	free-flow speed on horizontal curve subsegment i in the analysis direction (mi/h), from Equation 15-13.
FlowRate =	demand flow rate (veh/h).
FlowRate_FL =	demand flow rate in the faster lane (veh/h).
FlowRate_SL =	demand flow rate in the slower lane (i.e., the lane used by non-passing vehicles) (veh/h).
HorizClass_i =	horizontal classification for subsegment i.
HV% =	percentage of heavy vehicles in the analysis direction (%).
HV%_FL =	percentage of heavy vehicles in the faster lane (%).
HV%_SL =	percentage of heavy vehicles in the faster lane (%).
HVPropMultiplier_FL =	0.4.
%Improve_PF =	% improvement to percent followers on a segment downstream of a passing lane segment.
%Improve_S =	% improvement to the average speed on a segment downstream of a passing lane segment.
L =	segment length (mi), subject to minima and maxima given in Step 1.
L_i =	actual segment length for segment i (mi).
LW =	lane width (ft), constrained to minimum and maximum values of 9 ft and 12 ft, respectively.
m =	slope coefficient (decimal).
NumHV =	number of heavy vehicles entering the passing lane segment (veh).
NumHV_SL =	number of heavy vehicles in the slower lane (veh).
p =	power coefficient (decimal).

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Suggested Citation: "7 Two-Lane Highway Segment Analysis Methodology." National Academies of Sciences, Engineering, and Medicine. 2024. Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27895.

PassLaneLength =	length of passing lane segment (mi).
PF =	percent followers in the analysis direction (%).
PF_cap =	percent followers at capacity flow rate in the analysis direction (%).
PF_{PLmid_FL} =	percent followers in the faster lane at the midpoint of the passing lane segment (%).
PF_{PLmid_SL} =	percent followers in the slower lane at the midpoint of the passing lane segment (%).
PF_25cap =	percent followers of 25% of the capacity flow rate in the analysis direction (%).
PHF =	peak hour factor (decimal).
PropFlowRate_FL =	proportion of the demand flow in the faster lane (i.e., the lane used by passing vehicles) (decimal).
S =	average speed in the analysis direction (mi/h), with consideration of horizontal curvature.
S_HCi =	average speed on horizontal curve subsegment i in the analysis direction (mi/h).
S_{init_FL} =	initial average speed in the faster lane (mi/h).
S_{init_SL} =	initial average speed in the slower lane (mi/h).
S_pl =	posted speed limit (mi/h).
S_PLmid-FL =	average speed in the faster lane at the midpoint of the passing lane segment (mi/h).
S_PLmid-SL =	average speed in the slower lane at the midpoint of the passing lane segment (mi/h).
SubsegLength_i =	length of subsegment (horizontal curve or tangent) i (mi/h).
SubsegSpeed_i =	speed of subsegment (horizontal curve or tangent) i (mi/h).
SW =	shoulder width (ft), constrained to minimum and maximum values of 0 ft and 6 ft, respectively.
V₁₅ =	volume during the peak 15 min of the analysis hour (veh/15 min).
v_d =	flow rate in the analysis direction (veh/h).
v_o =	demand flow rate in opposing direction (veh/h); v_o = 1,500 in passing constrained segments and v_o = 0 in passing lane segments.