Freeway

1
Freeway Highway Level of Service
CEE 320Anne Goodchild
2
Outline

• Definitions
• Level of Service (LOS)
• Freeway Segment LOS Determination
• Free-flow speed
• Flow Rate
• Multilane Highway LOS
• 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.

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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 influencedby the effects of
  a downstream bottleneck.

From Highway Capacity Manual, 2000
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Speed vs. Flow
SfFree Flow Speed
Uncongested Flow
Sm
Speed (mph)
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

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Definitions

• Passenger car equivalents
• 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)
• Used for performance analysis, investment
  decisions
• Freeway LOS
• Based on traffic density

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Levels of Service

• LOS A
• Free-flow operation
• LOS B
• Reasonably free flow
• Ability to maneuver is only slightly restricted
• Effects of minor incidents still easily absorbed

From Highway Capacity Manual, 2000
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Levels of Service

• LOS C
• 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
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Levels of Service

• LOS E
• 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 gt capacity

From Highway Capacity Manual, 2000
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Freeway LOS
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LOS Calculation
Freeway LOS

• 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 lt 55 mi/h or gt 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

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Freeway LOS
Input Geometric Data Measured FFS or BFFS Volume
(highest)
Base Conditions
BFFS Input
BFFS Adjustment Lane width Number of
lanes Interchange density Lateral clearance
Volume Adjustment PHF Number of lanes Driver
population Heavy vehicles
Adjust for temporal variation
If FFS not known
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
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Define Speed-Flow Curve
Freeway LOS

• Select a Speed-Flow curve based on FFS

From Highway Capacity Manual, 2000
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Base conditions for basic freeway segment

• 12-ft lane widths
• 6-ft right shoulder
• 2-ft median lateral clearance
• Only passenger cars
• 5 or more lanes in each travel direction
• 2-mi or greater interchange spacing
• Level terrain
• Mostly familiar roadway users

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Determining FFS
Freeway LOS

• 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)
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Lane Width Adjustment (fLW)
Freeway LOS

• Base condition (fLW 0)
• Average width of 12 ft. or wider across all lanes

From Highway Capacity Manual, 2000
23
Lateral Clearance Adjustment (fLC)
Freeway LOS

• Base condition (fLC 0)
• 6 ft. or greater on right side
• 2 ft. or greater on the median or left side

From Highway Capacity Manual, 2000
24
Number of Lanes Adjustment (fN)
Freeway LOS

• 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
25
Interchange Density Adjustment (fIC)
Freeway LOS

• 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

From Highway Capacity Manual, 2000
26
Determining FFS
Freeway LOS

• 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)
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Determining Analysis Flow Rate
Freeway LOS

• Adjust hourly volumes to get pc/ln/hr

vp 15-minute passenger-car equivalent flow rate (pcphpl)
V hourly volume (veh/hr) highest, total one direction
PHF peak hour factor
N number of lanes in one direction
fHV heavy-vehicle adjustment factor
fP driver population adjustment factor
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Peak Hour Factor (PHF)
Freeway LOS

• 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 maximum 15-min. flow rate within hour of analysis
4 Number of 15-min. periods per hour
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Heavy Vehicle Adjustment (fHV)
Freeway LOS

• Base condition (fHV 1.0)
• No heavy vehicles
• Heavy vehicle trucks, buses, RVs
• Two-step process
• Determine passenger-car equivalents (ET, ER) for
  this traffic stream
• Determine fHV

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Passenger-Car Equivalents (ET,ER)
Freeway LOS

• Extended segments method
• Determine the type of terrain and select ET,ER
• Level, rolling, mountainous
• 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

From Highway Capacity Manual, 2000
31
Passenger-Car Equivalents (ET)
Freeway LOS

• 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

From Highway Capacity Manual, 2000
32
Freeway LOS
From Highway Capacity Manual, 2000
33
Passenger-Car Equivalents (ER)
Freeway LOS
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Passenger-Car Equivalents (ET,ER)
Freeway LOS

• Composite grades method
• Determines the effect of a series of steep grades
  in succession
• Distance weighted average
• 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)

From Highway Capacity Manual, 2000
35
Determine fHV
Freeway LOS
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
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Determining Analysis Flow Rate
Freeway LOS

• Adjust hourly volumes to get pc/ln/hr

vp 15-minute passenger-car equivalent flow rate (pcphpl)
V hourly volume (veh/hr) highest, total one direction
PHF peak hour factor
N number of lanes in one direction
fHV heavy-vehicle adjustment factor
fP driver population adjustment factor
37
Driver Population Adjustment (fP)
Freeway LOS

• Base condition (fP 1.0)
• Most drivers are familiar with the route
• Commuter drivers
• Typical values between 0.85 and 1.00
• Analyst selects the value using judgement
• Dependent on local conditions (scenic views,
  etc.)

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Define Speed-Flow Curve
Freeway LOS

• Select a Speed-Flow curve based on FFS

From Highway Capacity Manual, 2000
39
Determine Average PC Speed (S)
Freeway LOS

• Use vp and FFS curve to find average passenger
  car speed (S)

From Highway Capacity Manual, 2000
40
Determine Average PC Speed (S)
Freeway LOS

• For 70 lt FFS 75 mph AND (3400 30FFS) lt vp
  2400
• For 55 lt FFS 70 mph AND (3400 30FFS) lt vp
  (1700 10FFS)
• For 55 lt FFS 75 mph AND vp lt (3400 30FFS)

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Determine Density
Freeway LOS

• Calculate density using

D density (pc/mi/ln)
vp flow rate (pc/hr/ln)
S average passenger-car speed (mph)
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Freeway LOS
DetermineLOS
LOS Criteria for Basic Freeway Segments
From Highway Capacity Manual, 2000
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Example
Freeway LOS
Determine the typical LOS the a 6 mile stretch of
roadway with 5 interchanges.

• Geometry
• 11 ft. lane width
• 3 lanes
• Left lateral clearance 5 ft.
• Right lateral clearance 4 ft.
• Other
• 7 am PHF 0.95
• 2 trucks
• 3 buses

Determine free flow speed, analysis flow rate
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Determine FFS
Freeway LOS

• FFS BFFS fLW fLC fN fID
• BFFS is 70 mph for urban freeway
• fLW 1.9

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Determine FFS
Freeway LOS
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Determine FFS
Freeway LOS

• FFS BFFS fLW fLC fN fID
• fLc 0.8
• fN 3.0

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Determine FFS
Freeway LOS
In a 6-mile stretch there are 5 interchanges
5/6.833
Linear interpolation 5(.833-.75)1.3 1.7
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Determine FFS

• FFS BFFS fLW fLC fN fID
• FFS 70 1.9 0.8 3.0 1.7 62.6 mph

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Determine Flow Rate (vp)
Freeway LOS
At 7am the ½ hour volume is about 4000
veh/hrAssume half in each direction, 4000 veh/hr
max
Graph from the Puget Sound Regional Councils
Puget Sound Trends, No. T6, July 1997
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Determine Flow Rate (vp)
Freeway LOS

• Assume there are no RVs
• ET 1.5
• fHV 1/(1PT(ET-1) PR(ER-1)
• fHV 1/(10.05(1.5-1) 0(1.2-1) 0.9756
• Assume commuters, therefore fP 1.00
• Vp 4000 vph / (0.95)(2)(0.9756)(1.00) 2158
  pcplph

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Determine LOS
Freeway LOS
FFS 62.6 mph, Vp 2158 pcplph, S about 57
mph, Looks like LOS E, Density 2158/57 38
pc/mi/ln
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Freeway LOS
LOS Criteria for Basic Freeway Segments
From Highway Capacity Manual, 2000
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Multilane Highway LOS
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Multilane Highway LOS
Multilane Highway LOS

• Similar to Freeway LOS
• A few minor differences

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Base Conditions for Multilane Highway
Multilane Highway LOS

• Level terrain, with grades no greater than 2
  percent
• Minimum lane width 12 ft
• Objects no closer than 6 ft from the edge of the
  traveled pavement (at the roadside or median)
• No direct access points along the roadway
• Divided highway
• Traffic stream composed entirely of passenger
  cars
• Free flow speed of 60 mph or more
• Driver population composed principally of regular
  users

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Free Flow Speed (FFS)
Multilane Highway LOS

• 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, 60 mph is typically used
fLW adjustment for lane width (mph)
fLC adjustment for right-shoulder lateral clearance (mph)
fM adjustment for median type (mph)
fA adjustment for access points (mph)
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Lane Width Adjustment (fLW)
Multilane Highway LOS

• Base condition (fLW 0)
• Average width of 12 ft. or wider across all lanes

Same as Freeway LOS
From Highway Capacity Manual, 2000
58
Lateral Clearance Adjustment (fLC)
Multilane Highway LOS

• Base condition (fLC 0)
• 12 ft or greater TLC
• LCL 6 ft for undivided highways
• (accounted for in median type adjustment)
• LCL 6 ft for two-way left-turn lanes

From Highway Capacity Manual, 2000
59
Median Adjustment (fM)
Multilane Highway LOS

• Base condition (fM 0)
• Divided highway

From Highway Capacity Manual, 2000
60
Access-Point Density Adjustment (fA)
Multilane Highway LOS

• For each access point/mi FFS decreases by 0.25
  mph
• Base condition (fA 0)
• 0 access points per mile
• For NAPM 40 fA 0.25 NAPM
• For NAPM gt 40 fA 10

From Highway Capacity Manual, 2000
61
Determining Flow Rate
Multilane Highway LOS

• 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
Same as Freeway LOS
62
Heavy Vehicle Adjustment (fHV)
Multilane Highway LOS

• Base condition (fHV 1.0)
• No heavy vehicles
• Heavy vehicle trucks, buses, RVs
• Two-step process
• Determine passenger-car equivalents (ET)
• Determine fHV

Same as Freeway LOS
63
Passenger-Car Equivalents (ET)
Multilane Highway LOS

• Extended segments method
• Determine the type of terrain and select ET
• No one grade of 3 or more is longer than 0.5
  miles OR
• No one grade of less than 3 is longer than 1 mile

From Highway Capacity Manual, 2000
64
Passenger-Car Equivalents (ET)
Multilane Highway LOS

• Specific grades method
• Any grade of 3 or more that is longer than 0.5
  miles OR
• Any grade of less than 3 that is longer than 1
  mile

From Highway Capacity Manual, 2000
65
Multilane Highway LOS
Same as Freeway LOS
From Highway Capacity Manual, 2000
66
Passenger-Car Equivalents (ET)
Multilane Highway LOS
Same as Freeway LOS
67
Passenger-Car Equivalents (ET)
Multilane Highway LOS

• 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)

Same as Freeway LOS
From Highway Capacity Manual, 2000
68
Determine fHV
Multilane Highway LOS
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
Same as Freeway LOS
69
Driver Population Adjustment (fP)
Multilane Highway LOS

• Base condition (fP 1.0)
• Most drivers are familiar with the route
• Commuter drivers
• Typical values between 0.85 and 1.00

Same as Freeway LOS
70
Determine Average PC Speed (S)
Multilane Highway LOS

• Use vp and FFS curve to find average passenger
  car speed (S)

From Highway Capacity Manual, 2000
71
Determine Average PC Speed (S)
Multilane Highway LOS

• For 55 lt FFS 60 mph AND vp gt 1400
• For 50 lt FFS 55 mph AND vp gt 1400
• For 55 lt FFS 75 mph AND vp lt (3400 30FFS)
• For vp lt 1400

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Determine LOS
Multilane Highway LOS

• Use vp and passenger car speed (S)

From Highway Capacity Manual, 2000
73
Determine Density
Multilane Highway LOS

• Calculate density using

D density (pc/mi/ln)
vp flow rate (pc/hr/ln)
S average passenger-car speed (mph)
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LOS Criteria for Multilane Highways
Multilane Highway LOS
From Highway Capacity Manual, 2000
75
Design Traffic Volumes
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Design Traffic Volumes

• Need to select the appropriate hourly traffic
  volume to get the design LOS

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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

78
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)

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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
100
60
80
0
Number of hours (annually) withspecified or
greater volumes
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WSDOT Graphs
81
Primary References

• Mannering, F.L. Kilareski, W.P. and Washburn,
  S.S. (2005). Principles of Highway Engineering
  and Traffic Analysis, Third Edition. Chapter 6
• Transportation Research Board. (2000). Highway
  Capacity Manual 2000. National Research Council,
  Washington, D.C.