1. Chapter 3. Highway Design for Performance
Only section 1 is covered in CE361 (We study more about this topic in CEEn 562 Traffic Engineering): By the end of this chapter the student will be able to (p.3.1):
1. Determine the level of service and capacity for a basic freeway segment under specified traffic characteristics.
Lesson objectives of Section 3.1:
Define and calculate the capacity of a basic freeway segment.
Determine the level of service with various traffic mixes and lane configurations.
Basic freeway segment capacity analysis shows you the essence of capacity analysis of other facilities
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2. 3.1.1 Operations on Freeways
Basic freeway segments (BFS): Segments of the freeway that are outside of the influence area of ramps or weaving areas and have uniform traffic and roadway conditions.
I-15 under construction
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3. Density range (pc/mi/ln)
3.1.1 Continued.
LOS A
LOS B
LOS C or D
Level of service
Density range (pc/mi/ln) A B
> C
>18.0 – 26.0 D
>26. – 35.0 E
>35.0 – 45.0 F
> 45.0
LOS E or F
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4. 3.1.1 Continued.
Performance measures can be: Density, speed, and volume-to-capacity (v/c ratio)
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5. 3.1.2 The Highway Capacity Manual Method for BFS
Base conditions for freeway capacity: Good weather, good visibility, no incidents
Start with an ideal situation and then convert specified non-ideal traffic and roadway conditions into adjustment factors that reduce the ideal values of speed or flow to a value that can be compared against a table of LOS standards.
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6. Four types of analysis DDHV = AADT x K x D Type Input Output
Operational I
vp, FFS
LOS II
vp, LOS, FFS
S (Speed)
III
FFS, LOS
Vp (service volume = Vp * PHF)
Planning/Design IV
vp, LOS
N (No. of lanes)
For planning analysis, you need to estimate the directional design hourly volume (DDHV):
DDHV = AADT x K x D
besides all other potential prevailing conditions. Then, V = DDHV/(no. of lanes)
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7. 3.1.2 The HCM Method for Basic Freeway Segments
You want to find out speed, flow rate, density and LOS. The first example in the section goes through these steps.
Step 1: Input data - Define and segment the freeway sections as appropriate
Step 2: Determine the free-flow speed (FFS) on the freeway segment (for each segment for analysis)
Step 3: Select FFS curve
Step 4: Adjust demand volume (Determine vp)
Step 5: Estimate speed (S, not FFS) given vp, and density, D = vp /S
Step 6: Determine LOS
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8. Operational analysis (Step 1: Input data)
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9. Operational analysis (Step 2: Determine FFS)
You need to determine these two values (fLW and fLC).
Determination of free-flow speed (FFS), mph:
TRD = Total number of on- and off-ramps within ±3 miles of the midpoint of the study segment, divided by 6 miles.
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10. Operational analysis (Step 3: Select FFS curve)
Do not interpolate; Find the closest curve as shown below.
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11. Operational analysis (Step 4: Adjust demand volume)
PP = percent passenger cars
PT = percent trucks & buses
PR = percent recreational vehicles (RVs)
ET = PCE for trucks and buses
ER = PCE for RVs
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12. Heavy-vehicle adjustment factor
Extended segments
Type of Terrain
Level
Rolling
Mountains
ET (trucks & buses)
1.5
2.5
4.5
ER (RVs)
1.2
2.0
4.0
Grade and slope length affects the values of ET and ER.
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13. Operational analysis (Step 5: Estimate speed and density)
If vp is less than or equal to the breakpoint value, then S = FFS.
Density:
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14. Operational analysis (Step 6: Determine LOS)
Level of service
Density range (pc/mi/ln) A B
> C
>18.0 – 26.0 D
>26.0 – 35.0 E
>35.0 – 45.0 F
> 45.0
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15. Planning/Design analysis to determine the number of lanes needed for future demand (See Example 3.1)
You want to find out how many lanes are needed for the targeted level of service to meet the design year traffic.
Step 1: Collect input data (with estimated demand)
Step 2: Determine FFS
Step 3: Select FFS curve
Step 4: Estimate the number of lanes needed, N, for targeted LOS
Step 5: Estimate speed and density (to confirm if N is adequate)
Step 6: Determine LOS (to confirm if N is adequate)
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16. Service flow rates vs. service volumes
What is used for capacity analysis is service flow rate. The actual number of vehicles that can be served during one peak hour is service volume. This reflects the peaking characteristic of traffic flow.
Stable flow
SFE
Unstable flow E F
Flow D C
SFA
SVi = SFi * PHF B A
Density
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