1. Chapter 3. Highway Design for Performance
Only section 2 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:
Determine the level of service on a basic freeway segment under specified traffic characteristics
Lesson objectives of Section 3.2:
Define and calculate the capacity of a multilane interstate highway.
Determine 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
Chapter 3

2. 3.2.1 Freeway definitions
Basic freeway segments: 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
Chapter 3

3. 3.2.2 The “perfect freeway” for maximum flow
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.
Min. lane widths of 12 feet
Min. right-shoulder lateral clearance of 6 feet (median  2 ft)
Traffic stream consisting of passenger cars only
Ten or more lanes (in urban areas only)
Interchanges spaced every 2 miles or more
Level terrain, with grades no greater than 2%, length affects
Driver population dominated by regular and familiar users
Chapter 3

4. Density range (pc/mi/ln)
3.2.3 Freeway performance measures
LOS B
LOS C or D
LOS A
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
LOS E or F
Chapter 3

5. 3.2.3 Freeway performance measures (cont.)
Performance measures can be: Density, speed, and volume-to-capacity (v/c ratio)
Interpolate if necessary.
Chapter 3

6. 3.2.3 Freeway performance measures (cont.)
Density criteria are independent of Free Flow Speed level, Table 3-13.
Chapter 3

7. 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
Chapter 3

8. 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 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)
Chapter 3

9. 3.2.4 Applications (Operational analysis), Operation (LOS)
You want to find out LOS, Speed, and Density.
Step 1: Define and segment the freeway sections as appropriate
Step 2: Based on the measured or estimated free-flow speed (eq. 3.11, See the next slide) on the freeway segment, construct an appropriate speed-flow curve of the same shape as the typical curves shown in Figure 3.9.
Step 3: Using the flow rate, vp, read up to the free-flow speed curve identified in step 2 and determine the average passenger car speed (This is not FFS!) and LOS corresponding to that points
Step 4: Determine the density of flow as D = vp /S
Step 5: Determine the LOS using the density ranges in Table 3.12.
Chapter 3

10. Operational analysis (continued)
You need to determine these two values.
Determination of free-flow speed (FFS), mph:
For freeways, BFFSi (i.e., ideal FFS) is 70 (for urban) or 75 mph (for rural). If field study results exist, use FFS from those studies. Field FFS is estimated by travel time studies. FFS is approximately average speeds taken when flow rate is not more than 1300 pcphpl.
fN adjustment is for urban freeways only. For rural freeways, fN = 0.
fID is determined using a plus/minus 3-mile stretch that includes the study segment (i.e. 6 miles). Must have at least one on-ramps. (# of on-ramps/6 miles). If there are only off-ramps this adjustment is not applied.
Determination of 15-min. passenger-car equivalent flow rate (vp) pcphpl:
Chapter 3

11. Heavy-vehicle adjustment factor
PP = percent passenger cars
PT = percent trucks & buses
PR = percent recreational vehicles (RVs)
ET = PCE for trucks and buses
ER = PCE for RVs
Grade and slope length affects the values of ET and ER.
Chapter 3

12. Once you have FFS and vp…
You can determine density which is the primary measure of effectiveness of basic freeway segments.
For the LOS range from A to D, S (speed) is basically FFS. Toward the upper end of LOS D to E, S does decrease from FFS as shown in the speed-flow rate diagram.
Chapter 3

13. How doe we deal with long, sustained grades…
There are 3 ways to deal with long, sustained grades: extended general freeway segments, specific upgrades, and specific downgrades.
(1) Extended segments: where no one grade of 3% or greater is longer than ¼ mi or where no one grade of less than 3% is longer than ½ mi. And for planning analysis. (we only deal with extended segment cases in this class. The rest is covered by CE562)
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
Chapter 3

14. How we deal with long, sustained grades…(cont
How we deal with long, sustained grades…(cont.) (This will be covered in CE562 in detail.)
(2) Specific upgrades: Any freeway grade of more than ½ mi for grades less than 3% or ¼ mi for grades of 3% or more. (For a composite grade, see the next slide.) Use the tables for ET and ER for specific grades.
(3) Specific downgrades:
If the downgrade is not severe enough to cause trucks to shift into low gear, treat it as a level terrain segment.
Otherwise, use the table for downgrade ET
For RVs, downgrades may be treated as level terrain.
(PC equivalents for these two cases are not covered by the text. Just remember these adjustments may need to be done for long sustained grades.)
Chapter 3

15. 3.2.4 Applications (Planning analysis), Design (N)
You want to find out how many lanes are needed for the targeted level of service to meet the design year traffic.
Step 1: Find fHV using Table 3.14 for ET and ER.
Step 2: Try 2 lanes in each direction, unless it is obvious that more lanes will be needed.
Step 3: Convert volume (vph) to flow rate (pcphpl), vp, for the current number of lanes in each direction, using Equation 3.10.
Step 4: If vp exceeds capacity, add one lane in each direction and return to Step 2.
Step 5: Compute Free Flow Speed using Equation 3.11.
Step 6: Use Table 3.13 to determine the LOS for the freeway with the current number of lanes being considered. If the LOS is not good enough, add another lane and return to Step 3.
Chapter 3