lecture 3(a)
DESCRIPTION
civil engineeringTRANSCRIPT
Agenda
• Recap and definitions
• LoS and highway capacity
• Factors affecting transportation:
– Physiological and psychological behaviour of road
users
– Vehicles
Highway capacity and level of service
• Capacity and Level of service are two related terms.
• HOW??
Highway capacity and level of service
• The capacity of highways may be described as its ability to accommodate
traffic.
• Highway capacity itself is limited by the physical features of the highway,
which do not change unless the geometric design of the highway changes.
Highway capacity and level of service
Road type Standard
Maximum
Dual: 2-lane 2400 3200
Dual: 3-lane 3600 4800
Single: 10m
wide
1900 2300
Single: 7.3m
wide
1200 1600
Design flows for all-purpose dual/single carriageway roads
(Peak hourly flow( veh/hour/carriageway ))
Highway capacity and level of service
Factors affecting capacity:
Lane width:
Alignment:
Gradient:
Traffic composition:
Allowed for by converting to passenger car equivalents.
Highway capacity and level of service
• A term closely related to capacity and often confused
with it is service volume.
• When capacity gives a quantitative measure of traffic,
level of service or LOS tries to give a qualitative
measure.
Factors affecting transportation
• The success of transportation engineering depends upon
the co-ordination between the three primary elements,
namely the vehicles, the roadways, and the road users.
• Their characteristics affect the performance of the
transportation system and the transportation engineer should have fairly good understanding about them.
Components of Traffic System • Primary components of the traffic system:
• The general environment has an impact on traffic operations but it is difficult to assess.
• Traffic engineering would be great if these components had uniform characteristics
Road Users
• Very important as all transportation facilities have to be eventually used efficiently and safely by “Humans”.
• The study of human behaviour and the limitations of human performance in the varied circumstances of the road environment.
• Driver (user) response is a major component of planning and design of transportation systems.
• Human beings have a wide range of characteristics that influence the driving task.
Road Users
• Two of the important constituents of transportation
system are drivers and users/passengers.
• Understanding of certain human characteristics like
perception - reaction time and visual acuity and their
variability are to be considered by Traffic Engineer.
• Because of the variability in characteristics, the 85th
percentile values of the human characteristics are used
as standards for design of traffic facilities.
Road Users
• A major task is to find ways to provide drivers with information in a clear, effective, manner that induces safe and proper responses.
• Two important driver characteristics are
Q: Define the driving task and list the factors affecting a road user.
Road Users
• The factors affecting a road user in the driving task can
be divided into 3 main categories:
– Physiological factors
– Psychological factors
– Modifying factors
• Physiological factors include:
– Visual Intake of information of drivers and pedestrians
– visual acuity: data stored to be used afterwards and in
near future
– Stereoscopy
Road Users – Adaptation of the eye to varying illumination and
impaired night vision
– Colour blindness
– Posture and operating conditions leading to fatigue and
impaired performance
– hearing
• Psychological factors include:
– Aggression
– Risk taking
– Exhibitionism
– Lack of concentration
• Modifying factors include:
– Age, injury and disease
– Fatigue
Road Users
– Alcohol and drugs
– Illness
– Weather
– Posture
– Motivation
– Environmental influences
– Education
– Conditioning
Road Users
Visual acuity
• relates to the field of clearest vision. The most acute vision is within
a cone of 3 to 5 degrees, fairly clear vision within 10 to 12 degrees
and the peripheral vision will be within 120 to 180 degrees.
• This is important when traffic signs and signals are placed, but other
factors like dynamic visual acuity, depth perception etc. should also
be considered for accurate design.
Road Users
Perception-reaction time
Four distinct processes:
- Detection: recognition that something requiring response is present
- Identification: driver acquires sufficient information concerning the object or condition to allow the consideration of a proper response
- Decision: analyse the information and make a decision about how to respond
Road Users
Perception-reaction time
- Response: physical response that result from the decision
• Perception-reaction time depends on many factors: age, gender, medical condition, alcohol and drug use, fatigue, sleep deprivation, emotional condition, type of vehicle, experience and knowledge, etc.
Road Users (contd.)
Perception-reaction time
- The American Association of State Highway and Transportation Officials (AASHTO) recommends 2.5 sec for design value.
- For signal timing, the Institute of Transportation Engineers (ITE) recommends 1.0 sec.
- For specific situations, a longer time may be recommended.
- ORN 6- 2.0 sec
Road Users
Perception-reaction time
- Reaction distance: distance the vehicle travels while the driver goes through the process of perception reaction time
Road Users Pedestrians
• A critical safety problem in the street and highway system involves the interactions of vehicles and pedestrians.
• For pedestrians, it has to be considered:
- Walking speed for signalized intersections
- Gap acceptance for unsignalized intersections-
Vehicles • For purpose of geometric design, motor vehicles into
main categories (ORN 40- Table 1):
- Passenger cars
-Motorcycles
- Small/ Large Buses
- Light/ Medium/ heavy goods
- Others: Tractors, road rollers
• Critical vehicles properties that need to be considered:
Vehicles
• The vehicle has less variable characteristics than the
road users, and these factors can be legislatively
controlled within defined limits.
• Legislation can impose limits on the overall performance,
weight, size, as well as specifying the minimum
requirements for individual items of equipment such as
brakes, lighting and indicators.
• Operation and performance of vehicle
Vehicles Turning Characteristics of vehicles
- Low speed turns (≤ 10 mph) (50 mph is equivalent to 80 km/h)
• Limited by the characteristics of the vehicle (minimum radius allowed). AASHTO specifies minimum design radii for each of the design vehicles
- High speed turns (≥ 10 mph)
• Limited by the dynamics of side friction between the roadway and the tyres, and by the superelevation of the roadway.
Acceleration performance of vehicles
• Passenger cars – high acceleration rates
• Heavy vehicles – lower rates of acceleration
Vehicles
Braking performance
• Most critical performance characteristics: braking performance.
• The time and distance required to stop
• Primary consideration in virtually every aspect of traffic system design and operation.
• Factors: vehicles braking system, type & condition of tyres