King fahad University of Petroleum and Minerals Civil engineering Department

1
King fahad University of Petroleum and
MineralsCivil engineering Department

• CE-576-Geometric highway Design
• Chapter II
• Instructor Dr. Nedal T. Ratrout

2
Chapter II Design Highway Controls(Highway
Capacity)
3
Objective

• Highway Capacity
• Capacity as a Design Control
• Factors other than Traffic Volume That Affect
  Operating Conditions
• Levels of Service
• Design Service Flow Rate

4
Highway capacity
5
General Characteristics

• The term capacity is used to express the maximum
  hourly rate at which persons or vehicles can
  reasonably be expected to traverse a point during
  a given time period under prevailing roadway
  traffic conditions.

6
ApplicationsHighway capacity analysis serves
three general purposes

• Transportation planning studies Highway
  capacity analysis is used to assess the adequacy
  of existing highways network to serve current
  traffic, and its used to estimate the time when
  traffic growth may overtake the capacity of
  highway or reach a level of congestion below
  capacity which is undesirable.

7

• Highway Design
• Highway capacity is essential to fit a planned
  highway to traffic demands its used to select
  the highway type to determine dimensions such
  as the number of lanes and the minimum lengths of
  weaving sections.

8

• Traffic Operational analysis
• Highway capacity analysis is used in identifying
  bottleneck locations in preparing estimates of
  operational improvements that may be expected to
  result from prospective traffic control measures
  or from spot alternations in highway geometry.

9
Capacity as a Design Control
10
Design Service Flow Rate Vs. Design Volume

• Design Volume is the volume of traffic projected
  to use a particular facility during the design
  year, which is usually 10 to 20 year in the
  future. Design volumes are estimated in the
  planning process and are often expressed as DHV.

11

• Design service flow rate is the maximum hourly
  flow rate of traffic that a highway with
  particular design features would be able to serve
  without the degree of congestion falling below a
  pre-selected level.

12

• A designed facility with dimensions alignment
  should serve the design service flow rate, which
  should be at least as great as the the flow
  rate during the peak 15-minute period of the
  design hour.

13
Measures of Congestion

• The key considerations in geometric design are
  roadway design, the traffic using the roadway and
  the degree of congestion on the roadway. The
  first two considerations can be measured in exact
  units but a scale values for expression the
  degree of congestion is elusive measure.

14

• Numerous measures of degree of congestion have
  been suggested , including safety, freedom to
  maneuver, the ratio of traffic volume to capacity
  (v/c), operating speed, average running speed and
  others, but in case of signalized intersections
  the stopped delay encountered by motorists is
  used as measure of congestion.

15

• For uninterrupted traffic flow. When density
  increases rate of flow increases and speed begins
  to decline until maximum rate of flow reached
  (facility capacity), this will form upstream
  queues breakdown flow. Thus, facilities are
  designed to operate at volume less than their
  capacity.

16

• For interrupted flow average stopped-time delay
  is the principal measure of effectiveness of
  signalized intersections.

17
Relation Between Congestion and Traffic Flow Rate

• Congestion increases with an increase in flow
  rate until the flow rate is almost equal
  facilitys capacity, at this point congestion
  becomes acute.

18
Acceptable Degrees of Congestion

• The degree of congestion that should not be
  exceeded during the design year can be assessed
  by reconciling the demands of the motorists the
  general public with the finances available to
  meet those demands.

19
Principles for Acceptable Degree of Congestion

• The highway should be so designed that when its
  carrying the design volume, the traffic demand
  will not exceed the capacity of the facility even
  during short intervals of time.
• The design volume per lane should not exceed the
  rate at which traffic can dissipate from standing
  queue.

20

• Drivers should be afforded some choice of speed.
  The latitude in choice of speed should be related
  to the length of trip.
• Operating conditions should be such that they
  provide a degree of freedom from driver tension
  that is related to or consistent with the length
  duration of the trips.

21

• There are practical limitations that preclude
  the design of an ideal freeway.
• The attitude of motorists toward adverse
  operating conditions is influenced by their
  awareness of the construction and right-of-way
  costs that might be necessary to provide better
  service.

22
Reconciliation of Principles for Acceptable
Degree f Congestion

• Freeways (Short Trips) The density of traffic
  on urban freeways preferably should not exceed 26
  passenger cars per kilometer per lane.
• Freeways (Long Trips) Travel time is more
  important to the user, so 20 passenger cars per
  kilometer per lane will result acceptable degree
  of freedom.

23

• Rural Freeways Travel speed is dominator
  consideration. A density of 13 passenger cars per
  kilometer per lane will permit desirable
  operations in rural areas.
• Other Multilane Highways Except where traffic is
  controlled by signals, measures of congestion on
  other multilane highways are similar to those of
  freeways.

24
Factors Other Than Traffic Volume That Affect
Operating Conditions
25
Highway Factors

• For highway that is deficient in some of its
  characteristics and where the traffic stream is
  composed of a mixture of vehicles classes,
  compensatory adjustment factors need to be
  applied to the traffic flow rates used as design
  values for ideal highway conditions. These
  adjustments are necessary to determine the volume
  of mixed traffic that can be served under minimum
  accepting conditions

26
Alignment

• For traffic traveling at any given speed, the
  better the roadway alignment, the more traffic it
  can carry. Thus, gentle alignment will be
  identified as the critical feature limiting
  roadway capacity.

27
Weaving Sections

• Some reduction in operating efficiency through
  weaving section can be done if reduction is minor
  and the frequency of occurrence is not high. A
  reduction in operating speed of about 10 km/h
  5mph below that for which the highway as a
  whole operates can be considered a tolerable
  degree of congestion for weaving sections.

28

• Operating conditions within weaving sections are
  affected by both the length and width of the
  sections as well as by the volume of traffic in
  the served area.

29
Ramp Terminals

• When congestion develops at freeway ramp
  junction, some through vehicles avoid the outside
  lane of the freeway adding to the congestion in
  the remaining lanes. Thus, if there are only two
  lenes in one direction, the efficiency per lane
  is not as high on the average as that for three
  or more lanes in one direction.

30

• The degree of congestion for ramp is related to
  the total volume of traffic in the outside lane
  in the vicinity of the ramp junction.

31
Traffic Factors

• Consideration should be given to composition of
  traffic and fluctuations in flow, in deciding
  upon volumes of traffic that will result in
  acceptance degrees of congestion and also upon
  the period of time over which flow should extent.

32

• Passenger-car-equivalency (PCE) factors can be
  obtained by converting of mixed traffic to
  equivalent volumes of passenger cars. These
  factors are differ between facility types.

33
Peak Hour Factor

• The HCM considers operating conditions prevailing
  during the most congested 15-minute period of the
  peak hour to establish service level for the hour
  as a whole. Accordingly, the total hourly volume
  that can be served without exceeding a specified
  degree of congestion is equal to or less than
  four times the maximum 15-minute count.

34

• Peak hour factor (PHF) is used to convert the
  rate of flow during the highest 15-minute period
  to the total hourly volume. It may be described
  as the ratio of the total hourly volume to the
  number of vehicles during the highest 15-minute
  period multiplied by 4. PHF is never greater than
  1.00 and is normally within the range of 0.75 to
  0.95.

35
Level of Service
36

• A level of service is the quality of traffic
  service provided by specific highway facilities
  under specific traffic demands.
• The levels of service range from level-of-service
  A(least congested) to level-of-service F (most
  congested).

37
The table below shows the general definitions of
the levels of service. Specific definition of
level-of-service A through F vary by facility
type.
38
The relationship between highway type location
the level of service appropriate for design is
summarized in the following table. This
relationship is derived from the criteria for
acceptable degree of congestion
39
Design Service Flow Rate
40
General

• Service flow rates are the traffic rates that can
  be served at each level of service. If level of
  service has been identified as applicable for
  design, the corresponding service flow rate
  logically becomes the design service flow rate.

41
Weaving Section

• Weaving section occur when one-way traffic
  streams cross by merging and diverging maneuvers.
  The following figure shows the principal types of
  weaving sections.

42

• Weaving may be considered as simple or multiple.
  Simple weaving section is a single entrance
  followed by single exit. A multipleweaving
  section consists of two or more overlapping
  weaving section, it may also defined as that
  portion of a one-way roadway that has two
  consecutive entrances followed closely by two or
  more exits. The following figure shows the two
  types of weaving sections.

43

• The design level of service of a weaving section
  is dependent on its length, number of lanes,
  acceptable degree of congestion, and relative
  volumes of individual movements.
• Level-of-service criteria for weaving section is
  based on the average running speeds.

44

• There is a definite limit to the amount of
  traffic that can be handled on a given weaving
  section without undue congestion. This limiting
  volume is a function of the distribution of
  traffic between the weaving movements, the length
  of weaving section the number of lanes.

45
Multiple Highways without Access Control

• Multilane highways may be treated as similar to
  freeways if major crossroads are infrequent, or
  if many of the crossroads are grade separated ,
  and if adjacent development is sparse so as to
  generate little interference.

46

• Where there are major crossroads or where
  adjacent development results in more than slight
  interference, the facility should be treated as a
  multi lane highway without access control.

47
Arterial Streets Urban Highways

• The level of service provided by such facilities
  does not remain stable with the passage of time
  and tends to deteriorate in unpredictable manner.

48

• The capacity of an arterial is generally
  dominated by the capacity of its individual
  signalized intersections. The level of service
  for a section of an arterial is defined by the
  average overall travel speed for the section.

49
Intersections

• Design capacities of intersections can be
  estimated by procedures for signalized and
  unsignalized intersections given in HCM.
• The design spacing of signalized intersections
  should also be coordinated with traffic signal
  design and phasing.

50
Pedestrians Bicycles

• The level of service for pedestrian bicycle
  facilities can be evaluated using procedures
  presented in HCM.

51
Chapter III
52
Objective

• Passing Sight Distance for Two-Lane Highway
• Sight Distance for Multilane Highways
• Criteria for Measuring Sight Distance
• Theoretical General Considerations of
  Horizontal Alignment

53
Passing Sight Distance for Two-Lane Highway
54
Criteria for Design

• Passing sight distance is the length needed to
  complete normal passing maneuvers in which the
  passing driver can determine that there are no
  potentially conflicting vehicles ahead before
  beginning the maneuver.

55
Criteria for Design

• The following assumptions are made concerning
  driver behavior in passing maneuvers

1- The overtaken vehicle travels at uniform speed
2- The passing vehicle has reduced speed and
trails the overtaken vehicle as it enters a
passing section. 3- When the passing section is
reached, the passing driver needs a short period
of time to perceive the clear passing section
to react to start his maneuver.
56

• 4- Passing is accomplished under what may be
  termed a delayed start and a hurried return in
  the face of opposing traffic.
• 5- When the passing vehicle returns to its lane,
  there is a suitable clearance length between it
  and an oncoming vehicle in the other lane.

57
Criteria for Design

• The minimum passing sight distance for two-lane
  highways is determined as the sum of the four
  distances shown below.

58

• The following figure shows various distances for
  the components of passing maneuvers, based on
  extensive field observations of driver behavior
  are presented for four passing speed groups.

59

• The minimum passing sight distances presented in
  the previous table are generally conservative for
  modern vehicles.

60

• The following table shows the passing sight
  distances for design of two-lane highway based on
  vehicle passing performance . The ranges of
  speeds in this table are affected by traffic
  volume, if traffic volume is low
  (level-of-service A) there are few vehicles need
  to be passed, but if we have level-of-service D
  or lower there are few passing opportunity.

61
Initial Maneuver Distance (d1)

• The distance d1 traveled during the initial
  maneuver period is computed with the following
  equation

(
)
(
)
62
Distance While Passing Vehicle Occupies Left Lane
(d2)

• Passing vehicles were found in the study to
  occupy the left lane from 9.3 to 10.4 s. The
  distance d2 traveled in the left lane by passing
  vehicle is computed with the following equation

63
Clearance Distance (d3)

• Vary from 30 to 75 m 100 to 250 ft

64
Distance Traversed by an Opposing Vehicle (d4)

• During the first phase of the passing maneuver,
  driver can return to the right lane if an
  opposite vehicle is seen. It is unnecessary to
  include this trailing time interval in computing
  d4 . This time about one-third the time passing
  vehicle occupies the left lane, so d4 is the
  distance traversed by opposite vehicle during
  two-thirds of the time the passing vehicle
  occupies the left lane.

65

• The opposing vehicle is assumed to be traveling
  at the same speed as the passing vehicle, so d4
  2/3 d2 .

66
Design Values

• The speed of passed vehicle has been assumed to
  be the average running speed at a traffic volume
  near capacity, the speed of the passing vehicle
  is assumed to be 15 km/h 10mph greater. The
  assumed speeds of passing vehicles in the table
  of passing sight distances presents the likely
  passing speeds their passing sight distances
  would accommodate a majority of the designed
  passing maneuvers and correspond to the total
  curve shown in the following figure

67
Effects of Grade on Passing Sight Distance

• On downgrade passing is easier because the
  overtaken vehicle can accelerate more rapidly
  than on level which will reduce the time of
  passing. The over taken vehicle can also
  accelerate easier so that situation may result
  racing contest.

68

• The sight distance needed to permit vehicles
  traveling upgrade to pass safely are greater than
  derived design values. Compensating for this that
  the passed vehicle frequently is truck that
  losses some speed on upgrades and that many
  drivers are aware of the greater distances needed
  for passing upgrade compared with level
  conditions.

69
Frequency Length of Passing Sections

• The frequency length of passing section for
  highways depend principally on topography, the
  design speed of highway, and the cost for
  streets, the spacing of intersections is the
  principal consideration.

70

• Where high traffic volumes are expected on a
  highways a high level of service is to be
  maintained, frequent or nearly continuous passing
  sight distances should be provided.

71
Sight Distance for Multilane Highways
72

• It is not necessary to consider passing sight
  distance on highways or streets that have two or
  more traffic lanes in each direction of travel.
• Passing maneuvers that involve crossing the
  centerline of four-lane undivided roadways or
  crossing the median of four-lane roadways should
  be prohibited.
• Multilane roadways should have continuously
  adequate stopping sight distance, with
  greater-than-design sight distances preferred.

73
Criteria for Measuring Sight Distance
74
General

• Sight distance is the distance along a roadway
  throughout which an object of specified height is
  continuously visible to the driver.
• This distance is dependent on the height of the
  drivers eye above the road surface, the
  specified object height above the road surface,
  and the height and lateral position of sight
  obstructions within the drivers line of sight.

75
Height of Drivers Eye

• For passenger vehicles, the height of drivers
  eye is considered to be 1,080mm 3.5 ft above
  the road surface. Its appropriate for measuring
  stopping passing distances.
• For trucks, the drivers eye height ranges from
  1,800 to 2400mm 5.9 to7.9ft. The recommended
  value is 2,330mm 7.6ft.

76
Height of Object

• For stopping sight distance calculations, the
  height of object is considered to be 600mm
  2.0ft above the road surface. For passing sight
  distance calculations, the height of object is
  considered to be 1,088mm 3.5ft above the road
  surface.

77

• Stopping sight distance object
• 1- It is considered that an object 600mm 2.0ft
  high is representative of an object that involves
  risk to drivers and can be recognized by a driver
  in time to stop before reaching it.
• 2- Objects height of less than 600mm 2.0ft
  could substantially increase construction costs
  because additional excavation would be needed to
  provide the longer crest vertical curves.

78

• Passing sight distance object An object height
  of 1,080mm 3.5ft is adopted for passing sight
  distance. Passing sight distance calculated on
  this object height is adequate for night
  condition . The choice of an object height equal
  to the driver eye height makes passing sight
  distance design reciprocal (i.e. drivers of
  passing opposing vehicle can see each other).

79
Sight Obstructions

• On tangent roadway, the obstruction that limit
  the drivers sight distance is the road surface
  at some point on a crest vertical curve.
• On horizontal curves, the obstruction that limits
  the drivers sight distance may be the road
  surface at some point on a crest vertical curve,
  or it may be some physical feature outside of the
  traveled way, such as longitudinal barrier, a
  bridge approach fill slope, a tree, foliage, or
  the backslope of a cut section.

80
Measuring Recording Sight Distance on Plans
81

• Sight distance records for two-lane highways may
  be used effectively to tentatively determine the
  marking of no-passing zones.
• Sight distance records are useful on two-lane
  highways for determining the percentage of length
  of highways on which sight distance is restricted
  to less than passing minimum, which is important
  in evaluating capacity.

82
Horizontal Alignment
83
Theoretical Consideration

• From laws of mechanics, the basic formula that
  governs vehicle operation on a curve is

84
Superelevation

• There are practical upper limits to the rate of
  superelevation on a horizontal curve. These
  limits relate to considerations of climate,
  constructability, adjacent land use, and the
  frequency of slow-moving vehicles.

85

• High rates of superelevation are undesirable on
  high-volume roads, where there are numerous
  occasions when vehicle speeds may be reduced
  because of the volume of traffic or other
  conditions.
• Some vehicles have high centers of gravity and
  some passenger cars are loosely suspended on
  their axels. When these vehicles travel slowly on
  steep cross slopes, a high percentage of their
  weight is carried by the inner tiers. A vehicle
  can roll over if this condition become extreme.

86
Side Friction Factors

• The side friction factor represents the vehicles
  need for side friction, also called side friction
  demand it also represents the lateral
  acceleration af (af fg).

87

• The following is the side friction equation

88

• A key consideration in selecting maximum side
  friction factors for use in design is the level
  of centripetal or lateral acceleration that is
  sufficient to cause drivers to experience a
  feeling of discomfort to avoid higher speed.
  So, increased amount of side friction could be
  used in design of horizontal curves.
• Ball-Bank indicator is used as a uniform measure
  of lateral acceleration to set speeds on curves
  that avoid driver discomfort.

89

• In a series of tests it was concluded that speeds
  on curves that avoid driver discomfort are
  indicated by ball-bank readings of 14 degrees for
  speed 30 km/h 20mph or less, 12 degrees for
  speeds of 40 and 50 km/h 25 30mph, and 10
  degrees for speeds of 55 through 80 km/h 35
  through 50 mph. These ball-bank readings are
  indicative of side friction factors of 0.21, 0.81
  and 0.15 respectively.

90
Side Friction Factor

• From other tests maximum side friction factor of
  0.16 for speeds upto 100 km/h 60mph was
  recommended. For higher speeds, an incremental
  reduction of this factor was recommended. Speed
  studies on the Pennsylvania Turnpike led to
  conclusion that the side friction factor should
  not exceed 0.10 for design speeds of 110 km/h
  70mph and higher.

91

• The following curves summarize the findings of
  side friction factors recommended for curve
  design. Although some variation in the test
  results is noted, all are in agreement that the
  side friction factor should be lower for
  high-speed-design than for low speed design.

92

• The following curves shows the comparison of side
  frictions factors assumed for design of different
  types of highway facilities.

93
Thank You