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Transportation System Components. Transportation System. A transportation system may be defined as a planned network of elements or physical components that play different roles in the transportation of goods and persons from one place to another.
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Transportation System • A transportation system may be defined as a planned network of elements or physical components that play different roles in the transportation of goods and persons from one place to another. • The elements or physical components of a transport system are referred to as the facilities. • A transport system can therefore be considered as consisting of fixed facilities, the flow entities, and control system that permit people and goods to overcome the friction of geographical space efficiently in order to participate in a timely manner in some desired activity.
Fixed Facilities • The fixed facilities are the physical components of the system that are fixed in space and constitutes the network of links and nodes of the transportation system • For example, the links could be roadway segment and railway track and the nodes could be intersections, interchanges, transit terminals, harbors, and airports • The design includes soil and foundation engineering design, structural engineering design, the design of drainage systems, and geometric design, which is concerned with the physical proportioning of the elements of fixed facilities
Flow Entities • Flow entities are the units that traverse the fixed facilities. These include people, vehicles, container units, railroad cars, and so on… • In the case of a road system, the fixed facilities are expected to accommodate a wide variety of vehicle types, ranging from bicycles to large tractor-trailer combinations
Control System • The control system consists of vehicular control and flow control. Vehicular control refers to the technological way in which individual vehicles are guided on fixed facilities. Such controls can be manual or automated. • The proper geometric design of fixed facilities must consider the characteristics of the vehicle and the characteristics of the vehicular control system.
Mobility & Accessibility • Mobility refers to the ability to move between different activity sites • If a facility could move people and goods very fast then that facility provides very high mobility • Accessibility refers to the number of activity sites connected by the facility • If a facility provides connection to large number of residences, commercial places and industrial places then it provides very high accessibility • Mobility and accessibility are inversely related
A freeway is a highway which operates under the purest form of uninterrupted flow. Access is totally controlled and limited to grade separated interchange ramps. Multilane and movements are separated by medians No grade crossings, no traffic signals, no stop signs Freeway Grade separated interchange ramps
Freeway A freeway is composed of three subcomponents: • Basic freeway segment • Weaving areas • Ramp junctions
Expressway • Expressway is a divided highway facility having two or more lanes in each direction for the exclusive use of traffic, with full control of access and egress • They are also access controlled roads but there may be an occasional at-grade (same level) signalized intersection.
Multilane Highways • A highway with at least two lanes for the exclusive use of traffic in each direction, with no control or partial control of access, but that may have periodic interruptions to flow at signalized intersections no closer than 3.0 km.
Highway Classification Schemes • By Geometric Design • By Route Numbering • Administrative Classification • Functional Classification
Functional Classification ofHighway Systems • Understanding the 6 recognizable stages in most trips (See fig 1.1): - Primary movement - Transition - Distribution - Collection - Access - Termination
Functional Classification ofHighway Systems • Primary System • Freeways • Expressways • National Highways (Multilane highways) • Secondary System • State Highways • Major District Roads • Tertiary System • Other district Roads • Village Roads
Mobility & Accessibility Freeways and Expressways Multilane Highways State Highways Mobility Major District Roads Other District Roads Village Roads Access
Classification of Urban Streets • Primary Arterial streets • Secondary Arterial streets • Collector streets • Local Residential streets
Arterials • Primary ArterialA major surface street with relatively long trips between major points, and with through-trips entering, leaving, and passing through the urban area. • e.g. Freeways and interstate highways
Arterials • Secondary ArterialA signalized street that primarily serves through-traffic and that secondarily provides access to abutting properties, with signal spacing of 3.0 km or less.
Collector Streets • A surface street providing land access and traffic circulation within residential, commercial, and industrial areas. The function of collector street is to collect traffic from local streets and feed it to the primary and secondary arterial or vice-versa
Local Residential Street • These streets provide access to the abutting properties. Unrestricted parking and pedestrian movement is allowed on these streets
Mobility & Accessibility Primary Arterials Secondary Arterials Mobility Collectors Locals Access
Traffic Stream Components • Drivers • Vehicles • Roads and highways • The general environment • Control devices
Driver Characteristics A driver’s decisions and actions dependent principally on information recieved through the senses. In the order of importance, the senses most used by drivers are: • Visual (Sight)- 90% of the information • Kinesthetic (Movement) • Vestibular (Equilibrium) • Auditory (Hearing)
Driver Characteristics • Perception-Reaction Time (PRT): Includes the detection, identification, and decision, and response elements. • Perception-identification-emotion-volition time recommended by Uniform Traffic Control Devices (UTCD) = 3-10 sec • AASHTO PRT recommendation is 2.5 sec
Perception-Reaction Time • Perception-reaction time increases with several factors such as age, fatigue, and the presence of alcohol or drugs… • Then how would you design for a city with large number of citizens????
Driver Characteristics 2. Visual Acuity: Plays important role in the placement of signs and other traffic displays. Drivers should not have to divert their eyes to “read” a sign or device to the extent that the roadway in front of them is no longer in their clear vision field. Thus, signs and devices are placed in locations where they would normally fall w/in the clear vision range of a driver focusing on the roadway ahead.
Driver Information Needs In order to make sound decisions, a driver must receive reliable and understandable information to reduce uncertainity. A driver gathers information from a variety of sources, uses it as a basis for decision making, and then translates those decisions into actions to control the vehicle.
Driver Information Needs 5 basic principles for the systematic presentation of information required by drivers: • First thing first- primacy • Do not overload- processing channel limitations • Do it before they get on the road- a priori knowledge • Keep them busy- Spreading • Do not surprise them- Expectancy
Vehicle Characteristics • Design Vehicles: For what type of vehicle should a particular facility be designed? See Table 3-1.. • Allow minimum turning radii to be established.. Operationally achieved @ 10 mph, controlled by the turning capability of the vehicle.. See Table 3.2.. • At higher speeds, controlled by the dynamics of the situation.. See horizontal alignment
Vehicle Characteristics 2. Acceleration: Vehicles differ in their weight-to-power ratios and thereby in their acceleration performance F- Ra-Rrl-Rg= ma Ra= Aerodynamic Resistance, Turbulent air flow around vehicle body Rrl =Rolling Resistance, Resistance from tire deformation Rg = Grade Resistance, Gravitational force acting on the vehicle
Aerodynamic Resistance Ra Composed of: • Turbulent air flow around vehicle body(85%) • Friction of air over vehicle body (12%) • Vehicle component resistance, from radiators and air vents (3%)
Rolling Resistance Rrl Composed primarily of • Resistance from tire deformation (90%) • Tire penetration and surface compression ( 4%) • Tire slippage and air circulation around wheel ( 6%) • Simplifying approximation:
Grade Resistance Rg Composed of • Gravitational force acting on the vehicle θg For small angles, Rg θg W
Power Requirements Power is the rate at which work is done. The power P required to overcome the various resistances and to propel a vehicle is computed: P= R*v (watts) R= sum of various resistance forces (Newtons) v= vehicle velocity (m/sec)
Vehicle Characteristics 3. Breaking Performance: Breaking distance (SI Units) db= (Si2- Sf2)/ 254 (F±G) Db = breaking distance, meters Si = initial vehicle speed , kph Sf = final vehicle speed, kph F = coefficient of forward friction between tires and roadway G = grade, expressed as decimal, + for upgrades and - for downgrades
Vehicle Characteristics 3. Breaking Performance: Breaking distance (English Units) db= (Si2- Sf2)/ 30 (F±G) Db = breaking distance, ft Si = initial vehicle speed , mph Sf = final vehicle speed, mph F = coefficient of forward friction between tires and roadway G = grade, expressed as decimal, + for upgrades and - for downgrades
Vehicle Characteristics 4. Stopping Sight Distance: How far a vehicle will travel from the point at which a situation is perceived to the time the deceleration is complete • Worst-case conditions • Poor driver skills • Low braking efficiency • Wet pavement • Perception-reaction time = 2.5 seconds
Stopping Sight Distance • Stopping sight distance (d)= the reaction distance (dr) +braking distance (db) SI Units d=0.278 Sit + (Si2- Sf2)/ 254 (F±G) English Units d= 1.468 Sit + (Si2- Sf2)/ 30 (F±G)
Vehicle Characteristics 5. Decision Sight Distance: It is the distance required for a driver to detect an unexpected or otherwise difficult to perceive an information or hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required safety maneuver safely and efficiently. See Table 3.5… ** It is not based upon stopping but rather upon maneuvering.
Geometric Characteristics of Roadways • Horizontal Alignment: Roads are not perfectly straight and do have curves in the horizontal plane. These are referred to as changes in the horizontal alignment.
Superelevation R ≈ Fc α Fcn Fcp α e W 1 ft Wn Ff Wp Ff α
Geometric Characteristics of Roadways • Vertical Alignment: Viewed from a distance, one can see the changes in the vertical alignment, that is the changes in the elevation of highways.
Environment • A set of conditions that cause the driver, vehicle, or road characteristics to vary systematically. e.g. Wet pavements result in different frictions and therefore different braking distances.
Traffic Control Devices • Control devices such as markings, signs, and signals should communicate with the driver, and must do so in a uniform and comprehensible way.
