week 2

• Road vehicle performance


  Forms the basis for:

  • Highway design guidelines
  • Traffic analysis
  • Selection and design of traffic control devices
  • Determination of speed limits
  • Timing and control of traffic signal systems
• Tractive effort


  Force available at the road to perform work
• Resistance


  Force impeding vehicle motion
• Major sources of vehicle resistance

  • Aerodynamic resistance
  • Rolling resistance from the road surface-tire interface
  • Grade or gravitational resistance
• Both tractive effort and resistance are measured in newton (N)
• Diagram showing forces acting on a road vehicle (After Mannering et al. 2009)
• Basic equation of vehicle motion

  Equation describing the motion of a vehicle considering tractive effort and resistance
• Aerodynamic resistance


  • Significant impact on vehicle performance
  • Concerns over fuel efficiency lead to efficient aerodynamic designs
  • Turbulent air flow around the vehicle body is the primary source (over 85%)
• Equation for determining aerodynamic resistance

  Specific equation provided to calculate aerodynamic resistance
• Power required to overcome aerodynamic resistance

  Calculation of power required to overcome aerodynamic resistance
• Effect of operational factors on the drag coefficient of an automobile

  Analysis of operational factors affecting the drag coefficient
• Rolling resistance


  • Resistance generated from internal mechanical friction and tire-roadway interaction
  • Tire deformation is a primary source
  • Penetration of the tire into the surface and corresponding surface compression also contribute
• Sources of rolling resistance

  • Tire deformation as it passes over the pavement surface (90% of total)
  • Penetration of the tire into the surface and surface compression (4%)
  • Frictional motion and air circulation around the tire (6%)
• Factors controlling rolling resistance


  • Rigidity of tire and pavement surface
  • Tire conditions including inflation pressure and temperature
  • Vehicle's operating speed affects tire deformation and rolling resistance
• Evaluation of rolling resistance

  • Coefficient of rolling resistance (frl) approximated as: frl = 0.01 + 0.0095 (V)
  • Rolling resistance (Rrl) calculated based on vehicle speed
  • Power required to overcome rolling resistance (Prl) in watts: Prl = frl * W * V
• Grade resistance


  • Component of vehicle weight parallel to the pavement surface: Rg = W sin(θ)
  • For small highway grades: sin(θ) ≈ θ
  • Grade G = vertical rise / horizontal distance
• Available tractive effort - Maximum tractive effort

  • Excess force results in spinning tires without overcoming resistance or accelerating
  • Rear-wheel-drive vehicles: TEmax = μ (Wr + hP/b)
  • Front-wheel-drive vehicles: TEmax = μ (Wf - hP/b)
• Fuel efficiency - Critical determinants


  • Engine design improving air and fuel delivery and reducing friction enhances efficiency
  • Resistance-reducing measures like lowering vehicle weight, improving aerodynamics, and optimizing tire designs
• Principles of braking: Braking forces

  Braking characteristics are crucial for vehicle performance
• Typical values of coefficients of road adhesion
• Braking forces


  • Maximum braking forces develop at the impending slide point
  • Locked wheels result in reduced braking efficiency
  • Antilock systems prevent wheel lock and optimize braking forces
• Principles of braking: Braking force ratio

  • Maximum deceleration achieved by distributing braking forces proportionally between front and rear brakes
  • Braking force ratio determined by the ratio of maximum braking forces on front and rear axles
• Principles of braking: Proportional allocation of braking force

  Proportional allocation of braking force to front (PBFf) and rear (PBFr) axles for optimal braking
• Principles of braking: Braking efficiency

  • Optimal braking force proportioning varies with vehicle and road conditions
  • Front-wheel lockup preferable to rear-wheel lockup to avoid uncontrollable spins
  • Braking efficiency term reflects deviation from optimal braking force proportioning
• Principles of braking: Antilock braking systems


  • Antilock braking systems prevent wheel lock and maintain high braking efficiency
  • Aim to keep the coefficient of road adhesion from dropping to slide values
• Principles of braking: Theoretical stopping distance


  Calculation of minimum stopping distances
• Principles of braking: Practical stopping distance


  • Ensuring adequate driver sight distance for safe stopping
  • Consideration of various driver skill levels, vehicle types, and weather conditions in highway design
• Principles of braking: Equation for practical stopping distance

  AASHTO (2004) recommended deceleration rate of 3.4 m/s²
• Principles of braking: Distance travelled during driver perception/reaction

  • Importance of considering perception/reaction distance in sight distance provision
  • Perception/reaction time influenced by age, physical condition, emotional state, and situation complexity
• Principles of braking: Conservative perception/reaction time for highway design

  • 2.5 seconds
  • Average driver perception/reaction times: 1.0 to 1.5 seconds
• Principles of braking: Total required stopping distance


  Summary of the required stopping distance considerations
• Highway alignment


  The position or the layout of the centre of the highway on the ground
• Types of highway alignment

  • Horizontal alignment: Straight path, horizontal deviations and curves
  • Vertical alignment: Changes in gradient and vertical curves
• Disadvantages of improper alignment

  • Increase in construction cost
  • Increase in maintenance cost
  • Increase in vehicle operation cost
  • Increase in accident rate
• Once the highway is aligned and constructed, it is not easy to change the alignment due to increase in cost of adjoining land and construction of costly structures by the highway side
• Basic requirements of an ideal alignment between two terminal stations

  • Short: a straight alignment is the shortest
  • Easy: for construction and maintenance of roads with minimum problems
  • Safe: for construction and maintenance of roads from the view point of stability of natural hill slopes, embankment and cut slopes and foundation of embankments; also for traffic operation with safe geometric features
  • Economical: the lowest total cost including initial cost, maintenance cost and vehicle operation cost. The alignment would offer maximum utility by serving maximum population and products
• Factors controlling alignment


  • Obligatory points
  • Traffic
  • Geometric design
  • Economics
  • Other considerations
• Obligatory points: Points through which the alignment has to pass


  • Straight alignment AB is deviated along the hill side pass, thus avoiding a tunnel or heavy cutting
  • The bridge should be located only at place where the river has straight and permanent path and where the bridge abutment and pier can be properly founded
• Obligatory points: Points through which the alignment should not pass


  • Costly structures
  • Religious places (churches, temples, mosques, grave or tomb
  • Unsuitable land (marshy, peaty, and water logged areas
  • Lake or pond
• Origin and Destination study

  Carried out in the area and the desired lines be drawn showing the trend of traffic flow