Geometric Design (I)
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Geometric Design (I). Learning Objectives. To understand the considerations and quantifiable aspects of geometric design. Locational Design. Consider Current land use Geology Future land use Existing infrastructure. Controls and Criteria (1). Design Vehicles (p. 168-173)

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Learning objectives
Learning Objectives

  • To understand the considerations and quantifiable aspects of geometric design


Locational design
Locational Design

  • Consider

    • Current land use

    • Geology

    • Future land use

    • Existing infrastructure


Controls and criteria 1
Controls and Criteria (1)

  • Design Vehicles (p. 168-173)

    • 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


Controls and criteria 2
Controls and Criteria (2)

  • Driver

    • Information handling

    • Reaction time

      • Time to perceive + react to a hazard in vehicle’s path

      • Expected/unexpected

    • Speed

    • Driver errors


Controls and criteria 3
Controls and Criteria (3)

  • Traffic

    • Composition and volume

      • Average daily traffic (ADT) is not adequate

      • Design hourly volume (DHV)

      • 30th-highesthourly volume (30HV) in one year

      • K-factor (% of ADT; 8~12% urban, 12~18% rural)

    • Speed

      • Operating Speed (typically the 85th percentile speed)

      • Free-flow Speed (close to zero density)

      • Running Speed (actual speed)

      • Design Speed (as high as practical)


Controls and criteria 4
Controls and Criteria (4)

  • 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)


Stopping sight distance 1
Stopping Sight Distance (1)

  • 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


Stopping sight distance 2
Stopping Sight Distance (2)

  • Assume

    • Driver eye height of 3.5 feet

    • Height of object between 2.0 and 3.5 feet

  • Reaction distance + braking distance

  • Design standard: tr=2.5, a=11.2


Other sight distances
Other Sight Distances

  • Decision sight distance

    • Allow longer tr for information processing for different maneuver conditions (table 6-5)

  • Passing 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
Horizontal Alignment

  • Basic controlling expression

    e = rate of superelevation

    u = side friction factor (dep. on pavement, speed, …)

    V = vehicle speed

    R = radius of curve


Horizontal alignment1
Horizontal Alignment

  • 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.


Superelevation
Superelevation

  • 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

e

1


Side friction
Side Friction

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