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Civil Engineering Highway & Transportation Engineering Geometric Design of Highway


Geometric design of highways


Learning Objectives

To understand the considerations and quantifiable aspects of geometric design consider

  1. Locational Design
  2. Current land use
  3. Geology
  4. Future land use
  5. Existing infrastructure

Controls and Criteria

  • Design Vehicles
    • Passenger cars, buses, trucks, RVs
    • Physical characteristics: weight, dimensions
    • Establish intersection radius, pavement markings
  • Vehicle Performance
    • Operating characteristics: accel/decel
    • Impacts air quality, noise, land use


  • Information handling
  • Reaction time Driver Errors Graph Statistics
    • Time to perceive + react to a hazard in vehicle’s path
    • Expected/unexpected
  • Speed
  • Driver errors
  • Traffic
  • Composition and volume
    • Average daily traffic (ADT) is not adequate
    • Design hourly volume (DHV)
    • 30th-highest hourly volume (30HV) in one year
    • K-factor (% of ADT; 8~12% urban, 12~18% rural)
  • Speed of the vehicles
    • Operating Speed (typically the 85th percentile speed)
    • Free-flow Speed (close to zero density)
    • Running Speed (actual speed)
    • Design Speed (as high as practical)
  • Capacity
    • Maximum hourly flow rate (per lane) under prevailing conditions
    • Determines adequacy of existing roadways
    • Helps select roadway type
    • Helps define needs
    • Design level of service (LOS)

Capacity Variance by Free Flow



Stopping and Sight distance

Length of roadway that should be visible ahead of you in order to ensure that you will be able to stop if there is an object in your path.

Calculate the SSD for a vehicle traveling on your roadway at the design speed, and then make sure the actual sight distance that you provide is at least as great as the stopping sight distance

  • Assume
    • Driver eye height of 3.6 feet
    • Height of object between 2.0 and 3.6 feet
  • Reaction distance + braking distance

Braking Distance Reaction

  • Design standard: tr=2.5, a=11.2

Othe sight distances

  • Decision sight distance
    • Allow longer tr for information processing for different maneuver conditions (table 6-5)
  • Passing sight distance Other Sight Distance
    • Ensure safe passing maneuver (figure 6-5)
    • 4 distance components (figure 6-6)
  • At 70 mph
    • SSD = 730 ft
    • DSD = 1445 ft (maneuver E)
    • PSD = 2480 ft


Horizontal Alignment

Basic controlling expression

Horizontal Alignment Formula

e = rate of superelevation
u = side friction factor (dep. on pavement, speed, …)
V = vehicle speed
R = radius of curve


Overall design procedure

  • Determine a reasonable maximum superelevation rate.
  • Decide upon a maximum side-friction factor.
  • Calculate the minimum radius.
  • Iterate and test several different radii until you are satisfied with your design.
  • Make sure that the stopping sight distance is provided.
  • Adjust your design if necessary.
  • Design the transition segments.

Super Elevation of roadsSuper Elevation

  • Tilting the roadway to help offset centripetal forces developed as the vehicle goes around a curve
  • General Practice
    • Highways, no ice/snow
      emax = 0.10
    • Highways, snow/ice
      emax = 0.06
    • Traffic congestion or roadside development, limit speeds
      emax = 0.04 ~ 0.06

Side Friction

Design based on point where centrifugal force creates feeling of discomfort for driver

Speed u max u design
20 0.50 0.17
30 0.35 0.16
40 0.32 0.15
50 0.30 0.14
60 0.29 0.12
70 0.28 0.10

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