1. Freeway Capacity Analysis
CE331 Transportation Engineering

2. Objectives Understand the concept of level of service (LOS)
Define peak hour factor (PHF)
Determine the LOS for a given freeway
Determine the number of lanes to achieve a given LOS

3. Some Terms
Capacity
Max hourly flow rate for a roadway segment under existing conditions
Free Flow Speed (FFS)
Speed of traffic when no vehicles are present
In practice, measured at flow < 1,300 pcphpl
Level of Service (LOS)
Qualitative measure of traffic conditions
Level of congestion
Mostly determined by density

4. Level of Service (LOS) LOS Density (pc/mi/ln) A 0-11 B 12-18 C 19-26 D
27-35 E
36-45 F
> 45

5. Level of Service A

6. Level of Service F

7. Freeways
Limited access roads with high design standards, divided, and at least two lanes per direction

8. Freeway Components Basic freeway sections Weaving areas
Ramps and ramp junctions
Weaving area Basic freeway segment Ramp junction

9. “Ideal” Conditions 12-foot lanes 6-foot right shoulder
2-foot left shoulder
Only passenger cars
Leveled terrain
Interchange density 0.5/mi (or 2-mile spacing)
Everyday commuters
Five or more lanes per direction (urban only)

10. Procedure for Determining LOS
Estimate free flow speed (FFS)
Calculate equivalent flow rate (vp)
Compute density (D=vp/S) and compare D to values in Exhibit 23-3 to determine LOS

11. Step 1: Free Flow Speed (FFS)
Adjust for existing conditions
FFS = BFFS - fLW - fLC - fN – fID
BFFS – base FFS
Rural: 75mph
Urban: 70mph
fLW – lane width adj. factor, Exhibit 23-4
fLC – lateral clearance adj. factor, Exhibit 23-5
fN – number of lanes adj. factor, Exhibit 23-6
fID – interchange density adj. factor, Exhibit 23-7

12. Step 2: Equivalent Flow Rate (vp)
Adjust for vehicle mix and users
V: directional hourly volume (vph)
PHF: peak hour factor
N: number of lanes in one direction
fHV: heavy vehicle adj. factor
fp: driver population adj. factor, mostly =1

13. Peak Hour Factor (PHF)
Describes concentration of traffic within one hour
Definition:
V – hourly volume
V15 – maximum 15-min flow rate within the hour

14. PHF Example Max Flow Volume PHF = 2150/2300 = 0.94 Time Count 500 550
9:00-9:15
500
9:15-9:30
550
9:30-9:45
525
9:45-10:00
575
Hourly Flow
2000
2200
2100
2300
Max Flow
Total
2150
Volume
PHF = 2150/2300 = 0.94

15. Heavy Vehicle Adj. Factor (fHV)
PT – truck and bus percentage
PR – recreational vehicle percentage
ET – passenger car equivalent for trucks and buses, Exhibits 23-8, 23-9, 23-11
ER – passenger car equivalent for recreational vehicles, Exhibit 23-8, 23-10

16. Step 3: Calculate Density and Compare
Density (D) = vp / S
Compare D with threshold values in Exhibit 23-3 and determine LOS

17. Types of Analysis Operational Determine LOS Design & Planning
Determine number of
lanes

18. Example 1 FFS = BFFS - fLW - fLC - fN - fID
4-Lane rural freeway; V=2,100 vph; 6% trucks;
11-foot lanes; 4-foot right and left shoulders;
3% grade; 0.55 miles; 0.25 interchanges/mile;
PHF 0.9; everyday commuters. LOS? 75
1.9
0.0
1.2
0.0
FFS = BFFS - fLW - fLC - fN - fID
= 71.9 mph

19. Example 1 (cont’d) vp = V PHF N fhv fp 2100 = 1202 pcphpl 1.00 0.90 2
0.06
1.5
0.0
fhv = 1/[1+PT(ET-1)+PR(ER-1)]
= 0.97

20. Example 1 (cont’d)
D = vP/S = 1202/71.9 = vpm

21. Example 2
An urban freeway segment of 1.5 miles is expected to be designed under ideal conditions and it will be located on a 3% upgrade. The directional design hourly volume is expected to be 3,200vph and it will consist of 5% trucks, 5% recreational vehicles, and 90% passenger cars. Most drivers will be everyday commuters. If similar freeways showed a PHF=0.90 and no interchanges are planned within this section, determine the number of lanes required to accommodate the traffic at minimum LOS C.

22. Example 2 (2/7) FFS = BFFS - fLW - fLC - fN - fID 70 (urban) 0.0 0.0
4.5
0.0
FFS = BFFS - fLW - fLC - fN - fID
= 65.5 mph

23. Example 2 (3/7) V vp = PHF N fhv fp 3200 = 1956 pcphpl 1.00 0.90 2
0.05
2.5
1.5
fhv = 1/[1+PT(ET-1)+PR(ER-1)]
= 0.91
0.05

24. Example (4/7)
D = vP/FFS = 1956/65.5 = 31.2 vpm Y LOS D

25. Example 2 (5/7) V vp = PHF N fhv fp FFS = BFFS - fLC - fLW - fN - fID70
0.0
0.0
3.0
0.0
FFS = BFFS - fLC - fLW - fN - fID
= 67.0 mph
3200
vp = V
PHF N fhv fp
= 1304 pcphpl
0.90
1.00 3
0.91

26. Example (6/7)
D = vP/FFS = 1304/67 = 19.5 vpm Y LOS C

27. Example 2 (7/7) pcphpl mph Use Graph or LOS Table:
D = vP/FFS = 1304/67 = 19.5 vpm Y LOS C
Use Graph or LOS Table:
Source: HCM 2000, Ch. 23, p.23-3