Geometric Design of Highways

Geometric Design of Highways

• Geometric highway design involves the dimensions and layout of visible features, vertical alignments, sight distances, and intersections.
• Highway geometrics offer efficient traffic operations, maximum safety, and reasonable costs.

Factors for Road Design

• Geometric designs should align with expected future traffic volumes.
• They should lead to safe driving and convey a sense of safety.
• Designs should integrate and avoid abrupt changes in curves.
• Designs must be extensive, including traffic controls, signs and lighting.
• Designs should be economical regarding construction, maintenance, and operation costs.

Elements of Highway Geometric Design

• Cross-section elements
• Sight distance considerations
• Horizontal alignment details
• Vertical alignment details
• Intersection elements

Factors Affecting Highway Design

• Design speed affects geometric design, which accounts for overall highway needs.
• Topography/terrain: classified by general slope as plain, rolling, mountainous, or steep.
• Traffic: Vehicular and road user characteristics.
• Design Hourly Volume and Capacity: accounts for fluctuating traffic with time.
• Environmental and other factors include aesthetics, landscaping, air/noise pollution, and local conditions.
• Pavement surface depends on the pavement type and is characterized by friction, evenness, and light reflection.

Cross Section Elements

• Pavement surface characteristics: qualities pertaining to the road's surface.
• Width of Pavement or Carriageway: overall span, taking into account several ways vehicles travel.
• Cross Slope or Camber: sideways angle to allow drainage.
• Median or Traffic Separator: division of traffic.
• Kerbs: road boundary.
• Road Margins: edge of road.
• Width of Formation: overall road size.

Cross Section Element Details

• Lane width affects vehicle speed.
• Lane widths typically range from 2.7 to 3.75 m; in residential areas, lane width should not be less than 2.7 m.
• Avoid lane widths less than 3.0 m in urban streets.
• Ideal lane width on a two-lane undivided highway is 3.6 m or more.
• The number of lanes should accommodate design volumes; normally, two lanes are appropriate.
• AASHTO allows 16 lanes, with 8 lanes for free passage for each direction and 4 external lanes for mass transit and commercial vehicles.
• A shoulder is designed to provide structural lateral support for the pavement which provide space for corrective driving and bicycle use.
• Ideal shoulder width: 3.0 m although 1.80 to 2.40 m provides sufficient clearance.
• Typical shoulder slope range: 2-6%.
• Kerbs: indicate the pavement and median boundary and are grouped by function.
  • Low/mountable.
  • Semi-barrier type.
  • Barrier type.
• Medians: provided in divided carriageways to separate opposite traffic; the main function is to prevent head-on collisions.
  • Medians channel traffic, protect pedestrians, and isolate slow traffic.
  • IRC recommends a minimum desirable width of 5.0 m for rural highways and 3.0 m when land is restricted.
  • Width of medians may be reduced to 1.2-1.5 m on long bridges.
    • The allowable width in urban areas: 1.2 m, and the desirable width: 5.0 m.
    • Desirable median width for expressways: 15 m; allowable minimum: 10 m.
    • If median barriers are provided, the median width is reduced to 4.5 m.
• Frontage/service roads: prevent direct contact between highways and nearby activities and property.
  • AASHTO defines frontage road width as one paved lane with a shoulder width of 2.5 meters.
  • These roads should be allocated in one direction, they should not obstruct traffic on intersecting highways.
• Side slopes provide roadway stability and give drivers an opportunity to regain control.
  • Back slopes should be 1V:3H or flatter.
  • Installation of retaining walls should occur in urban areas where slopes are steeper than 1V:2H.
• Guard rails are installed on the edge of the road shoulder to prevent vehicles from running off the embankment when the fill exceeds 3 meters.
• Right-of-way: land acquired for the road's alignment; its width is known as land width.
  • Increase width to avoid removing residential or commercial land near the road.
• Road fences: installed along highways to control access by pedestrians/vehicles.
  • They prevent crossing of a median by reducing headlight glare and preventing animals from entering.
  • Create fences: Generally set at 1.8 m high.
    • 1.2 m-high fences are used in rural areas.
• Cross slope/camber: provides the slope of the road to drain the rain water.
  • This prevents water from penetrating the pavement layers and allows the surface to dry quickly.
  • Camber depends on the Type of pavement surface and amount of rainfall.
• Straight line camber is generally provided for CC pavement.
• Parabolic shape is ideal for roads that undergo frequent lane changes.
• Combined shape has a straight line in the middle that curves on edges.
  • Road design Exercise:
    • Two-lane State Highway with bituminous concrete surface and Major District Road of WBM pavement (3.8 m wide)
    • The height of the crown = 7.0m
    • BC surface slope= 1/50,. rise of crown = (7.0/2.0) X (1/50) = 0.070 m
  • WBM roads with a 1 in 33 provide a slope of 1 in 33
  • rise of crown = (3.8/2.0) X (1/33) = 0.058 m
• Sidewalks: should be there for urban collectors. These are urban areas used for schools, parks and commercial areas.
  • Width should be 1.2m but range up to 2.4m
• Parking lanes: they should allow kerbs for parking within urban roads but should only grant parallel parking to be safer.
  • It should be greater than 3.0m
  • Bus bays are to avoid problems with traffic 75m away from intersection.
• Lay byes provided near public stops with guide maps in 3.0m widths and 30m Lengths.
• Cycle tracks: Provide in urban area traffic
  • 2m is standard but add 1 if there is extra traffic.