1. Freeway Level of Service
CEE 320 Steve Muench

2. Outline Definitions Level of Service (LOS)
Freeway Segment LOS Determination
Free-flow speed
Flow Rate
Example
Design Traffic Volume

3. I-5 Average Daily Traffic
from the WSDOT 2001 Annual Traffic Report

4. Freeway Defined
A divided highway with full control of access and two or more lanes for the exclusive use of traffic in each direction.
Assumptions
No interaction with adjacent facilities (streets, other freeways)
Free-flow conditions exist on either side of the facility being analyzed
Outside the influence or ramps and weaving areas

5. Basic Freeway Segment
From Highway Capacity Manual, 2000

6. Definitions Freeway Capacity
The maximum sustained 15-min flow rate, expressed in passenger cars per hour per lane, that can be accommodated by a uniform freeway segment under prevailing traffic and roadway conditions in one direction of flow.

7. Definitions – Flow Characteristics
Undersaturated
Traffic flow that is unaffected by upstream or downstream conditions.
Queue discharge
Traffic flow that has just passed through a bottleneck and is accelerating back to the FFS of the freeway.
Oversaturated
Traffic flow that is influenced by the effects of a downstream bottleneck.
From Highway Capacity Manual, 2000

8. Speed vs. Flow Sf Free Flow Speed Uncongested Flow Sm
Speed (mph)
Lots of measurements in the top part, a few in the queue part and a few in the congested part
Segment 1 = top part = uncongested
Segment 2 = straight part = queue discharge
Segment 3 = bottom part = within a queue
Optimal flow, capacity, vm
Congested Flow
Flow (veh/hr)

9. Uncongested Flow
From Highway Capacity Manual, 2000

10. Definitions – Free-Flow Speed
Free-Flow Speed (FFS)
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.
Factors affecting free-flow speed
Lane width
Lateral clearance
Number of lanes
Interchange density
Geometric design

11. Definitions Passenger car equivalents Driver population Capacity
Trucks and RVs behave differently
Baseline is a freeway with all passenger cars
Traffic is expressed in passenger cars per lane per hour (pc/ln/hr or pcplph)
Driver population
Non-commuters suck more at driving
They may affect capacity
Capacity
Corresponds to LOS E and v/c = 1.0

12. Definitions – Level of Service (LOS)
Chief measure of “quality of service”
Describes operational conditions within a traffic stream.
Does not include safety
Different measures for different facilities
Six measures (A through F)
Freeway LOS
Based on traffic density

13. Freeway Segment LOS LOS A LOS B Free-flow operation
Reasonably free flow
Ability to maneuver is only slightly restricted
Effects of minor incidents still easily absorbed
From Highway Capacity Manual, 2000

14. Levels of Service LOS C LOS D Speeds at or near FFS
Freedom to maneuver is noticeably restricted
Queues may form behind any significant blockage.
LOS D
Speeds decline slightly with increasing flows
Density increases more quickly
Freedom to maneuver is more noticeably limited
Minor incidents create queuing
From Highway Capacity Manual, 2000

15. Levels of Service LOS E LOS F Operation near or at capacity
No usable gaps in the traffic stream
Operations extremely volatile
Any disruption causes queuing
LOS F
Breakdown in flow
Queues form behind breakdown points
Demand > capacity
From Highway Capacity Manual, 2000

16. Freeway LOS

17. LOS Calculation Does not consider
Special lanes reserved for a particular type of vehicle (HOV, truck, climbing, etc.)
Extended bridge and tunnel segments
Segments near a toll plaza
Facilities with FFS < 55 mi/h or > 75 mi/h
Demand conditions in excess of capacity
Influence of downstream blockages or queuing
Posted speed limit
Extent of police enforcement
Intelligent transportation system features
Capacity-enhancing effects of ramp metering

18. Input BFFS Adjustment Volume Adjustment
Geometric Data
Measured FFS or BFFS
Volume
BFFS Input
BFFS Adjustment
Lane width
Number of lanes
Interchange density
Lateral clearance
Volume Adjustment
PHF
Number of lanes
Driver population
Heavy vehicles
Measured FFS Input
Compute FFS
Compute flow rate
Define speed-flow curve
Compute density using flow rate and speed
Determine speed using speed-flow curve
Determine LOS

19. LOS Criteria for Basic Freeway Segments
From Highway Capacity Manual, 2000

20. Determining FFS Measure FFS in the field
Low to moderate traffic conditions
Use a baseline and adjust it (BFFS)
FFS =
free-flow speed (mph)
BFFS
base free-flow speed, 70 mph (urban), 75 mph (rural)
fLW
adjustment for lane width (mph)
fLC
adjustment for right-shoulder lateral clearance (mph) fN
adjustment for number of lanes (mph)
fID
adjustment for interchange density (mph)

21. Lane Width Adjustment (fLW)
Base condition (fLW = 0)
Average width of 12 ft. or wider across all lanes
From Highway Capacity Manual, 2000

22. Lateral Clearance Adjustment (fLC)
Base condition (fLC = 0)
6 ft. or greater on right side
2 ft. or greater on the median or left side
Measured from the edge of the paved shoulder to the nearest edge of the traveled lane
No adjustments for left side
From Highway Capacity Manual, 2000

23. Number of Lanes Adjustment (fN)
Base condition (fN = 0)
5 or more lanes in one direction
Do not include HOV lanes
fN = 0 for all rural freeway segments
From Highway Capacity Manual, 2000

24. Interchange Density Adjustment (fIC)
Base condition (fIC = 0)
0.5 interchanges per mile (2-mile spacing)
Interchange defined as having at least one on-ramp
Determined over 6-mile segment
3 miles upstream and 3 miles downstream
From Highway Capacity Manual, 2000

25. Determining Flow Rate Adjust hourly volumes to get pc/ln/hr vp =
15-minute passenger-car equivalent flow rate (pcphpl) V
hourly volume (veh/hr)
PHF
peak hour factor N
number of lanes in one direction
fHV
heavy-vehicle adjustment factor fP
driver population adjustment factor
You measure or are given V

26. Peak Hour Factor (PHF) Typical values 0.80 to 0.95
Lower PHF characteristic or rural or off-peak
Higher PHF typical of urban peak-hour V =
hourly volume (veh/hr) for hour of analysis
V15
maxiumum 15-min. flow rate within hour of analysis 4
Number of 15-min. periods per hour

27. Heavy Vehicle Adjustment (fHV)
Base condition (fHV = 1.0)
No heavy vehicles
Heavy vehicle = trucks, buses, RVs
Two-step process
Determine passenger-car equivalents (ET)
Determine fHV
3 miles upstream and 3 miles downstream

28. Passenger-Car Equivalents (ET)
Extended segments method
Determine the type of terrain and select ET
No one grade of 3% or more is longer than 0.25 miles OR
No one grade of less than 3% is longer than 0.5 miles
Level = any combo of alignments permitting heavy vehicles to maintain approximately the same speed as passenger cars. Generally includes short grades of no more than 2%
Rolling = causes heavy vehicles to reduce speed substantially below passenger cars but does not cause them to operate at their limiting speed for the given terrain for any significant length of time or at frequent intervals
Mountainous = causes heavy vehicles to operate at their limiting speed for significant distances or at frequent intervals
From Highway Capacity Manual, 2000

29. Passenger-Car Equivalents (ET)
Specific grades method
Any grade of 3% or more that is longer than 0.25 miles OR
Any grade of less than 3% that is longer than 0.5 miles
No RVs for downgrades
From Highway Capacity Manual, 2000

30. From Highway Capacity Manual, 2000

31. Determine Average PC Speed (S)
Freeway LOS
Determine Average PC Speed (S)
For 70 < FFS ≤ 75 mph AND (3400 – 30FFS) < vp ≤ 2400
For 55 < FFS ≤ 70 mph AND (3400 – 30FFS) < vp ≤ ( FFS)
For 55 < FFS ≤ 75 mph AND vp < (3400 – 30FFS)

32. Passenger-Car Equivalents (ET)

33. Passenger-Car Equivalents (ET)
Composite grades method
Determines the effect of a series of steep grades in succession
Method OK if…
All subsection grades are less than 4% OR
Total length of composite grade is less than 4000 ft.
Otherwise, use a detailed technique in the Highway Capacity Manual (HCM)
No RVs for downgrades
From Highway Capacity Manual, 2000

34. Determine fHV fHV = Heavy vehicle adjustment factor ET, ER
Passenger-car equivalents for trucks/buses and RVs
PT, PR
Proportion of trucks/buses and RVs in traffic stream

35. Driver Population Adjustment (fP)
Base condition (fP = 1.0)
Most drivers are familiar with the route
Commuter drivers
Typical values between 0.85 and 1.00
Two-step process
Determine passenger-car equivalents (ET)
Determine fHV

36. Define Speed-Flow Curve
Select a Speed-Flow curve based on FFS
Interpolation is OK
75 mph is dashed because is was extrapolated from the 70 mph curve
From Highway Capacity Manual, 2000

37. Determine Average PC Speed (S)
Use vp and FFS curve to find average passenger car speed (S)
Interpolation is OK
75 mph is dashed because is was extrapolated from the 70 mph curve
From Highway Capacity Manual, 2000

38. Determine Density Calculate density using: D = density (pc/mi/ln) vp
flow rate (pc/hr/ln) S
average passenger-car speed (mph)

39. LOS Criteria for Basic Freeway Segments
Determine LOS
LOS Criteria for Basic Freeway Segments
Usually you can easily tell from the graph but it’s a good idea to check here
Density defines LOS!
From Highway Capacity Manual, 2000

40. from WSDOT’s SRWeb http://srview.wsdot.wa.gov/
Example
Determine the typical LOS for SR 520 eastbound near Microsoft (MP – shown in the picture below) at 7 a.m. and 10 p.m.
Geometry
11 ft. lane width
Left lateral clearance = 5 ft.
Right lateral clearance = 4 ft.
Other
7 am PHF = 0.95
10 pm PHF = 0.99
2% trucks
3% buses
from WSDOT’s SRWeb

41. Determine FFS FFS = BFFS – fLW – fLC – fN – fID
Assume BFFS is 70 mph for urban freeway
fLW = 1.9
fLC = 0.8 (do we use 2 or 3 lanes in one direction?) use 3 – this determines speed

42. Determine FFS FFS = BFFS – fLW – fLC – fN – fID
Assume BFFS is 70 mph for urban freeway
fLW = 1.9
fLC = 0.8 (do we use 2 or 3 lanes in one direction?) use 3 – this determines speed
fN = 4.5 (do not include the HOV lane)

43. from Microsoft MapPoint
Determine FFS
from Microsoft MapPoint
In a 6-mile stretch from I-405 to Redmond there are 5 interchanges
IC density = 5/6 = interchanges/mile
Interpolating from the table we get 1.68

44. Determine FFS FFS = BFFS – fLW – fLC – fN – fID
FFS = 70 – 1.9 – 0.8 – 4.5 – 1.7 = 61.1 mph

45. Determine Flow Rate (vp)
½ hour volume is in both directions
Therefore the 1-hour volume in one direction is the same because there are 4 lanes total (2 in each direction)
At 7am the ½ hour volume is about 4000 veh/hr At 10 pm the ½ hour volume is about 1700 veh/hr
Graph from the Puget Sound Regional Council’s Puget Sound Trends, No. T6, July 1997

46. Determine Flow Rate (vp)
Assume there are no RVs
fHV = 1/(1+PT(ET-1) + PR(ER-1)
fHV = 1/(1+0.05(1.5-1) + 0(1.2-1) =
Assume commuters, therefore fP = 1.00
Vp = 4000 vph / (0.95)(2)(0.9756)(1.00) = 2158 pcplph

47. Determine LOS FFS = 61.1 mph Vp = 2158 pcplph S = about 56 mph
Looks like LOS E
Density = 2158/56 = 38.5 pc/mi/ln

48. LOS Criteria for Basic Freeway Segments
Density = 2158/56 = 38.5 pc/mi/ln
LOS E at 7am
At 10 pm
Vp = 1700/(0.99)(2)(0.9756)(1.00) = 880 pc/ln/hr
S = 61.1 mph (still in free-flow area)
D = 880/61.1 = 14.4 pc/mi/ln
LOS B
From Highway Capacity Manual, 2000

49. Design Traffic Volumes

50. Design Traffic Volumes
Need to select the appropriate hourly traffic volume to get the design LOS

51. Definitions Annual average daily traffic (AADT)
Annual traffic averaged on a daily basis
Design hourly volume (DHV)
Traffic volume used for design calculations
Typically between the 10th and 50th highest volume hour of the year (30th highest is most common)
K-factor
Relationship between AADT and DHV

52. Definitions Directional distribution factor (D)
Factor reflecting the proportion of peak-hour traffic traveling in the peak direction
Often there is much more traffic in one direction than the other
Directional design-hour volume (DDHV)

53. Typical Graph
Highest 100 Hourly Volumes Over a One-Year Period for a Typical Roadway
0.15
0.14
0.13
Hourly volume as a proportion of AADT
0.12
0.11
0.10 20 40 60 80
100
Number of hours (annually) with specified or greater volumes

54. WSDOT Graphs

55. Primary References
Mannering, F.L.; Kilareski, W.P. and Washburn, S.S. (2003). Principles of Highway Engineering and Traffic Analysis, Third Edition (Draft). Chapter 6
Transportation Research Board. (2000). Highway Capacity Manual National Research Council, Washington, D.C.