Lec 12 Capacity Analysis (Transportation Engineering Dr.Lina Shbeeb)

1. Transportation Engineering
Lina Shbeeb
Capacity Analysis

2. Traffic flow theory in practice
 Used in facility planning
 The understanding of facility capacity
 Used in design
 The sizing and geometric features of facilities
 Used in performance measure
 The level of service offered by a specific facility
 Delay
 Travel speeds
 Travel times

3. Applications of Traffic Flow Theory
 Determining turning lane lengths
 Average delay at intersections
 Average delay at freeway ramp merging areas
 Change in the levels of freeway performance
 Simulation (mathematical models (algorithms) to study
interrelationship among the different elements of traffic
itcdemo3d.ram

4. Various Types of Traffic Volumes
 DailyVolume
 AverageAnnual DailyTraffic (AADT)
 AverageAnnualWeekdayTraffic (AAWT)
 Average DailyTraffic (ADT)
 AverageWeekdayTraffic (AWT)
 HourlyVolume
 SubhourlyVolume

5. AADT and AAWT
 AverageAnnual DailyTraffic (AADT)
 Average 24-hour traffic volume at a location over a full 365-day
year, which is the total number of vehicles passing the location
divided by 365
 AverageAnnualWeekdayTraffic (AAWT)
 Average 24-hour traffic volume on weekdays over a full year,
which is the total weekday traffic volume divided by 260

6. ADT and AWT
 Average DailyTraffic (ADT)
 Average 24-hour traffic at a location for any period less than a
year (e.g. six months, a season, a month, a week or even two
days)
 AverageWeekdayTraffic (AWT)
 Average 24-hour traffic volume on weekdays for any period less
than a year

7. DDHV/PHV
 Directional Design Hourly volume (DDHV) ~ Peak
HourlyVolume
DDHV (veh/hour) =AADT x K x D
where AADT = Average Annual DailyTraffic (veh/day)
K = Proportion of daily traffic occurring in
peak hour (decimal)
D = Proportion of peak hour traffic traveling
in the peak direction (decimal)
Contd…

8. DDHV/PHV
 For design purposes, K factor is generally chosen as 30 HV and D
factor as the percentage of traffic in predominant direction during
the design hour
 General ranges for K and D factors
Facility Type K Factor D Factor
Rural 0.15 – 0.25 0.65 – 0.80
Suburban 0.12 – 0.15 0.55 – 0.65
Urban 0.07 – 0.12 0.50 – 0.60

9. Sub-Hourly Volume
 Sub-hourlyVolume ~ 15-minVolume
Suppose, the peak 15-min volume observed = 750 veh
So, the HourlyVolume =
15-min volume x 4
= 750 x 4
= 3000 veh/hour

10. Peak Hour Factor
 Relationship between hourly volume and maximum
rate of flow within the hour
For intersection:
PHF = =
where HV = Hourly volume (veh/hour)
V15 = max. 15-min volume within the hour (veh)
Hourly Volume
Max. Rate of Flow
HV
4 x V15

11. Example of PHF Calculation
 Data collected are as follows:
Time Interval Volume (veh)
4:00-4:15 pm 950
4:15-4:30 pm 1100
4:30-4:45 pm 1200
4:45-5:00 pm 1050
Hourly Volume 4300

12. Example of PHF Calculation
We find from the table,
HV = 4300 veh/hour
V15 = 1200 veh
Therefore,
PHF = = = 0.90
HV
4 x V15
4300
4 x 1200

13. Peak Hour Factor
For freeway/expressway:
PHF =
where HV = Hourly volume (veh/hour)
V5 = max. 5-min volume within the hour (veh)
HV
12 x V5

14. Traffic Volume
 Number of vehicles passing a given point or a section of a
roadway during a specified time
 Data collected by
 Manual counting
 Electro-mechanical devices

15. Relationship of Highest Hourly Volume and
ADT on Rural Arterials

16. Travel Time and Delay Surveys
 Want to determine travel time between two points or causes of
delay along a route
 Used for
Performance monitoring
Before and after evaluation of traffic management
schemes or new infrastructure

17. Stream measurements: the moving
observer method
 Moving Observer
 stopwatch, pen and paper
 laptop PC
 instrumented vehicle
=> Chase car versus floating car techniques
 Stationary Observer
 number plate survey
 Field staff or optical character recognition from video
 Vehicle electronic tags
 Mobile phone

18. Travel Time
By travelling several times in a car fromA to B, a
person notes down the time taken in each run and
then comes up with a statistical average ofTravel
Time.
A
B

19. Delay
Delay =Actual travel time - Expected travel time
 Stopped time delay
 Travel time delay

20. Calculation of Delay
 Travel time delay
 Actual travel time data collected by traveling in a car through
the stretch of the road under study.
 Expected travel time calculated from the distance and average
speed.
 Travel time delay calculated from the difference between actual
travel time and expected travel time.
 Stopped time data for the vehicular speed of 0 to 5 mph.

21. Introduction
 Shock wave - a rapid change in traffic conditions (speed,
density, and flow)
 A moving boundary between two different traffic states
 Forward, backward, and stationary shock waves

22. Travel Time
By travelling several times in a car fromA to B, a
person notes down the time taken in each run and
then comes up with a statistical average ofTravel
Time.
A
B

23. Delay
Delay =Actual travel time - Expected travel time
 Stopped time delay
 Travel time delay

24. Calculation of Delay
 Travel time delay
 Actual travel time data collected by traveling in a car through
the stretch of the road under study.
 Expected travel time calculated from the distance and average
speed.
 Travel time delay calculated from the difference between actual
travel time and expected travel time.
 Stopped time data for the vehicular speed of 0 to 5 mph.

25. Level of Service (LOS)
25
 Concept – a qualitative measure describing operational
conditions within a traffic stream and their perception by
drivers and/or passengers
 Levels represent range of operating conditions defined
by measures of effectiveness (MOE)

26. LOS A (Freeway)
 Free flow conditions
 Vehicles are unimpeded in
their ability to maneuver
within the traffic stream
 Incidents and breakdowns
are easily absorbed
26

27. LOS B
 Flow reasonably free
 Ability to maneuver is
slightly restricted
 General level of physical and
psychological comfort
provided to drivers is high
 Effects of incidents and
breakdowns are easily
absorbed
27

28. LOS C
 Flow at or near FFS
 Freedom to maneuver is
noticeably restricted
 Lane changes more difficult
 Minor incidents will be
absorbed, but will cause
deterioration in service
 Queues may form behind
significant blockage
28

29. LOS D
 Speeds begin to decline with
increasing flow
 Freedom to maneuver is
noticeably limited
 Drivers experience physical
and psychological discomfort
 Even minor incidents cause
queuing, traffic stream cannot
absorb disruptions
29

30. LOS E
 Capacity
 Operations are volatile, virtually
no usable gaps
 Vehicles are closely spaced
 Disruptions such as lane changes
can cause a disruption wave that
propagates throughout the
upstream traffic flow
 Cannot dissipate even minor
disruptions, incidents will cause
breakdown
30

31. LOS F
 Breakdown or forced flow
 Occurs when:
 Traffic incidents cause a
temporary reduction in capacity
 At points of recurring congestion,
such as merge or weaving
segments
 In forecast situations, projected
flow (demand) exceeds estimated
capacity
31

32. Design Level of Service
32
This is the desired quality of traffic conditions from a driver’s
perspective (used to determine number of lanes)
 Design LOS is higher for higher functional classes
 Design LOS is higher for rural areas
 LOS is higher for level/rolling than mountainous terrain
 Other factors include: adjacent land use type and development
intensity, environmental factors, and aesthetic and historic values
 Design all elements to same LOS (use HCM to analyze)

33. Design Level of Service (LOS)
33

34. Capacity – Defined
34
 Capacity: Maximum hourly rate of vehicles or
persons that can reasonably be expected to pass a point, or
traverse a uniform section of lane or roadway,
during a specified time period under prevailing conditions
(traffic and roadway)
 Different for different facilities (freeway, multilane, 2-
lane rural, signals)
 Why would it be different?

35. Ideal Capacity
35
 Freeways: Capacity (Free-
Flow Speed)
2,400 pcphpl (70 mph)
2,350 pcphpl (65 mph)
2,300 pcphpl (60 mph)
2,250 pcphpl (55 mph)
 Multilane Suburban/Rural
2,200 pcphpl (60 mph)
2,100 (55 mph)
2,000 (50 mph)
1,900 (45 mph)
 2-lane rural – 2,800 pcph
 Signal – 1,900 pcphgpl

36. Principles for Acceptable Degree of
Congestion:
36
1. Demand <= capacity, even for short time
2. 75-85% of capacity at signals
3. Dissipate from queue @ 1500-1800 vph
4. Afford some choice of speed, related to trip length
5. Freedom from tension, esp long trips, < 42 veh/mi.
6. Practical limits - users expect lower LOS in expensive
situations (urban, mountainous)

37. Multilane Highways
37
 Chapter 21 of the Highway Capacity Manual
 For rural and suburban multilane highways
 Assumptions (Ideal Conditions, all other conditions
reduce capacity):
 Only passenger cars
 No direct access points
 A divided highway
 FFS > 60 mph
 Represents highest level of multilane rural and suburban
highways

38. Multilane Highways
38
 Intended for analysis of uninterrupted-flow highway
segments
 Signal spacing > 2.0 miles
 No on-street parking
 No significant bus stops
 No significant pedestrian activities

39. 39
Source: HCM, 2000

40. 40
Source: HCM, 2000
Step 1: Gather data
Step 2: Calculate capacity
(Supply)

41. 41
Source: HCM, 2000

42. 42
Source: HCM, 2000

43. Lane Width
43
 Base Conditions: 12 foot lanes
Source: HCM, 2000

44. Lane Width (Example)
44
Source: HCM, 2000
How much does use of 10-foot lanes decrease
free flow speed?
Flw = 6.6 mph

45. Lateral Clearance
45
 Distance to fixed objects
 Assumes
 >= 6 feet from right edge of travel lanes to obstruction
 >= 6 feet from left edge of travel lane to object in median
Source: HCM, 2000

46. Lateral Clearance
46
TLC = LCR + LCL
TLC = total lateral clearance in feet
LCR = lateral clearance from right edge of travel lane
LCL= lateral clearance from left edge of travel lane
Source: HCM, 2000

47. 47
Source: HCM, 2000

48. 48
Example: Calculate lateral clearance adjustment for a 4-lane
divided highway with milepost markers located 4 feet to the
right of the travel lane.
TLC = LCR + LCL = 6 + 4 = 10
Flc = 0.4 mph
Source: HCM, 2000

49. 49
fm: Accounts for friction between opposing directions of
traffic in adjacent lanes for undivided
No adjustment for divided, fm = 1
Source: HCM, 2000

50. 50
Fa accounts for interruption due to access points along
the facility
Example: if there are 20 access points per mile, what is
the reduction in free flow speed?
Fa = 5.0 mph

51. Estimate Free flow Speed
51
BFFS = free flow under ideal conditions
FFS = free flow adjusted for actual conditions
From previous examples:
FFS = 60 mph – 6.6 mph - 0.4 mph – 0 – 5.0 mph =
48 mph ( reduction of 12 mph)

52. 52
Source: HCM, 2000
Step 3: Estimate
demand

53. 53
Calculate Flow Rate

54. Heavy Vehicle Adjustment
54
 Heavy vehicles affect traffic
 Slower, larger
 fhv increases number of passenger vehicles to account for presence of heavy trucks

55. f(hv) General Grade Definitions:
55
 Level: combination of alignment (horizontal and vertical) that
allows heavy vehicles to maintain same speed as pass. cars
(includes short grades 2% or less)
 Rolling: combination that causes heavy vehicles to reduce
speed substantially below P.C. (but not crawl speed for any
length)
 Mountainous: Heavy vehicles at crawl speed for significant
length or frequent intervals
 Use specific grade approach if grade less than 3% is more than
½ mile or grade more than 3% is more than ¼ mile)

56. 56
Example: for 10% heavy trucks on rolling
terrain, what is Fhv?
For rolling terrain, ET = 2.5
Fhv = _________1_______ = 0.87
1 + 0.1 (2.5 – 1)

57. Driver Population Factor (fp)
57
 Non-familiar users affect capacity
 fp = 1, familiar users
 1 > fp >=0.85, unfamiliar users

58. 58
Source: HCM, 2000
Step 4: Determine
LOS
Demand Vs.
Supply

59. 59
 Calculate vp
 Example: base volume is 2,500 veh/hour
 PHF = 0.9, N = 2
 fhv from previous, fhv = 0.87
 Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1878 pc/ph/pl
0.9 x 2 x 0.87 x 0.85

60. Calculate Density
60
Example: for previous
D = _____1878 vph____ = 39.1 pc/mi/lane
48 mph

61. 61
LOS = E
Also, D = 39.1 pc/mi/ln, LOS E

62. Design Decision
62
 What can we change in a design to provide an acceptable
LOS?
 Lateral clearance (only 0.4 mph)
 Lane width
 Number of lanes

63. Lane Width (Example)
63
Source: HCM, 2000
How much does use of 10 foot lanes decrease free
flow speed?
Flw = 6.6 mph

64. Recalculate Density
64
Example: for previous (but with wider lanes)
D = _____1878 vph____ = 34.1 pc/mi/lane
55 mph

65. 65
LOS = E
Now D = 34.1 pc/mi/ln, on border of LOS E

66. 66
 Recalculate vp, while adding a lane
 Example: base volume is 2,500 veh/hour
 PHF = 0.9, N = 3
 fhv from previous, fhv = 0.87
 Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1252 pc/ph/pl
0.9 x 3 x 0.87 x 0.85

67. Calculate Density
67
Example: for previous
D = _____1252 vph____ = 26.1 pc/mi/lane
48 mph

68. 68
LOS = D
Now D = 26.1 pc/mi/ln, LOS D (almost C)