CE562 Lecture 24 Freeways

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CE562 Lecture 24Freeways
Text A Policy on Geometric Design, Freeways, pp.
503-554.
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Freeways

• A freeway is the highest type of arterial highway
  with full control of access.
• Freeway elements medians, grade separations,
  ramp connections, and frontage roads.
• Design speed ranges between 80 and 100 (110) km/h
  for urban freeways and 110 (80-100) km/h for
  rural freeways.
• Level of service Level-of-service C is
  acceptable in urban and developed areas, and
  level B in rural areas.

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Design Traffic Volumes and LOS

• Prediction 20 years ahead
• Directional Design Hourly Volume (DDHV) for
• LOS C (D) for urban areas
• LOS B (C) for rural areas

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Lanes and Shoulders

• Lane width is 3.6 m.
• Side slope between 1.5 and 2, acceptable is 2.5
  , unidirectional or two-way.
• Right shoulders - 3.0 m at least, 3.6 m where
  more than 250 trucks per hour (DDHV)
• Left (median) shoulders - 1.2-2.4 m on four-lane
  freeways, 3.0-3.6 m on wider freeways.
• Cross slope of shoulders between 2 and 6 , at
  least 1 steeper than the adjacent through lane
  (on tangents).
• Superelevation max. 8-12 where snow and ice are
  not expected. Flatter superelevation rates are
  recommended. On viaducts should be smaller than
  6-8 percent.

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Maximum Grades
120, 130
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Vertical Clearance

• The vertical clearance should typically be 4.9 m.
  
• Smaller clearance of 4.3 m is allowable if larger
  clearance would be unreasonably costly and there
  is an alternative route with the 4.9-m clearance.
  
• The vertical clearance to sign trusses and
  pedestrian overpasses should be at least 5.1 m.
• An allowance should be added for future
  resurfacing.

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Horizontal Clearance to Obstructions

• The clear zone should be consistent with the
  operating speed.
• Fixed objects should be relocated beyond the
  clear zone, or shielded, or made breakaway.
• Piers and walls should be located beyond the
  shoulder or better 0.6 m from the edge of the
  shoulder.

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Rural Freeways
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Rural Freeways

• Rural freeways have alignment and cross-sectional
• elements liberal in design.
• The design speed is 110 km/h and desirable
  level-of-service is B
• Foreslopes are 16 or flatter
• Backslopes are 13 or flatter
• Frontage roads are relatively short, normally
  with two-way traffic
• Intersections of frontage and cross roads are
  generally located as far as practical from grade
  seperations

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Rural Freeways
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Rural Freeways
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Urban Freeway
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Medians on Urban Freeways

• Medians should be as wide and flat as feasible.
• The minimum median width on four-lane freeway is
  3.0 m (1.20.61.2).
• The minimum median width on freeway wider than
  four lanes is 6.6 m (3.00.63.0).
• If lanes are more than four and truck DDHV is
  larger than 250, then the minimum median width is
  7.8 m (3.60.63.6).
• For the minimum widths, median barriers are
  required.

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Depressed Urban Freeways

• Advantage reduced is the impact on the adjacent
  areas.
• Disadvantage increased is the cost for drainage
  (pumping stations).

Exhibit 8-4
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Depressed Urban Freeways
Exhibit 8-5
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Depressed Urban Freeways
Exhibit 8-6
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Freeways on Viaducts

• The freeway may be elevated where the
  right-of-way is restricted, water table is high,
  or underground utilities are extensive, etc.
• Advantages
• (1) all cross streets can be left open (frontage
  roads are not required),
• (2) existing utilities are hardly affected, and
• (3) detours during construction are limited,
• (4) space under the structure can be used for
  surface streets, parking, transit lines, or other
  purposes
• Disadvantages
• (1) high cost of maintaining the structure,
• (2) closed drainage system,
• (3) icing,
• (4) poor appearance.

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Freeways on Viaducts
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Freeways on Viaducts
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Freeways on Viaducts
Exhibit 8-9
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Freeways on Viaducts
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Freeways on Embankments
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Ground-level Freeways
Restricted Cross Sections

• Ground-level freeways are used
• in flat terrain,
• along railroads,
• along water courses,
• in suburban areas.
• They have many rural characteristics except
• cross sections can be restricted,
• continuous one-way frontage roads are usually
  desirable.

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Ground-level Freeways
Desirable Cross Sections
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Combination-type Freeways
Exhibit 8-16
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Combination-type Freeways
Exhibit 8-17
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Combination-type Freeways
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Combination-type Freeways
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Combination-type Freeways
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Urban Freeways
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Public Transit on Freeways
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Public Transit on Freeways
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Public Transit on Freeways
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Public Transit on Freeways
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Public Transit on Freeways
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HOV Lanes on Freeways
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Public Transit on Freeways
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Suburban Freeway