AASHTO Rural Arterials

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AASHTO Rural Arterials

• CHAPTER VII

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Chapter VIIRural Arterials

• Includes two lane, multilane hwy, freeway
  (chapter 8).
• Designed based on traffic volume needs.
• Geometry are controlled by speed, volume and
  terrain

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Rural Arterials

• Design speed ranges from 60-120 kph. (40-60 _at_
  mount.) (80-100 _at_ rolling), (100-120 _at_ level).
• Design traffic volume ADT for 20 yrs for low
  volume road. 30HV is used for high volume road.
• LOS LOS B norm. but LOS C _at_ mount area.
• Sight distance Stopping sight dist on all the
  road. Passing and decision dist as frequent and
  practical ex 7-1
• Alignment and grades Horz vert algn should
  flat and gradual. Grades (ex 7-2)
• Number of lanes based on Volume, LOS capacity.
• Superelevation not more than 12
• Cross slopes 1.5 to 2 on two lane. Multilane
  either crowned in the middle or sloped in one
  direction.

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EX 7-1
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EX 7-2
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Local Rural Roads

• Vert clearance 4.9 m could be 4.3 if there is
  alt. road.
• Width based on volume (ex 7-3). On high vol
  should should be paved or at least 0.6 m should
  be for safety.
• Horz clearance AASHTO DG. Highly recommended to
  be free from obstacles.
• Passing Hwy is designed on stop concept.
  Passing should be frequent. If not us climbing
  or passing lane.

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Ex 7-3
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Developing a 4 lane Hwy

• Usually two lane is adequate.
• When the anticipated DHV gt service volume of the
  two lane Hwy for certain LOS.an upgrade to 4
  lanes is expected.
• consult HCM to check the whether the two lane can
  maintain the LOS or not.
• If an upgrade is a potential, therefore
  appropriate acquisition of ROW and stage
  construction is recommended. (ex 7-5)

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Ex 7-5
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Multilane Undivided Arterials

• Intended for high traffic in one direction of
  travel. Passing is accomplished w/o crossing the
  yellow line.
• Crash rates are higher than two lane ( higher
  vol, more intersections, access points etc)
• Can live w/o a median in urban suburban areas.
  In rural areas median is recommended.
• Eventually all multilane arterials on new
  locations should be divided.

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Divided Arterials

• When two lanes are provided in each direction
  with a median 1.2 m wide or more.
• Median wide enough for at least protected left
  turn. (could be also barrier)
• Adv increase safety, reduce head-on collision,
  refuge for ped.

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Divided Arterials

• Lane Width 3.6 m due to high vol speed.
• Cross slope to the outer edge usually 1.5 2
• Shoulder full shoulder are justified . Right
  shoulder is has more significance than median
  shoulder.
• Median Barrier consult AASHTO RDG for horz
  clearance and type of barrier.

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Divided Arterials

• Medians 1.2 1.8 m on highways w/o at-grade
  intersection. Wider is better.
• Median with 3.6 - 9 m is the best for left turns
  and as a separator.
• Median width also is a f(design vehicle) eg. 7.5
  m for pass vech, 15 m for bus
• Very wide median is undesirable at hwys with
  intersections. (causes confusion and reduce
  signal efficiency)
• Studies shows that as median width increase on
  rural arterial, undesirable driving behavior in
  crease which leads to more crashes.

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Divided Arterials

• Alignment Profile each direction could have
  different elevation (precautions Rd side design,
  headlight glare, intersections)
• Stopping sight distance govern the design.
• No passing sight distance nevertheless avoid
  roller coaster.
• Generally horz vert allgn should be
  smooth-flowing with gradual change.

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Divided Arterials

• Climbing lane Not strongly justified as in two
  lane hwy since there is the right lane always for
  heavy trucks on upgrade.
• Superelevation Curves should be superelevated
  to ensure safety, pleasant view and economy. We
  have three cases to superelevate with the
  presence of the median mentioned in chapter 3 (ex
  3-37) and ex 7-6
• Ex 7-7 shows median types (note C,D E for
  urban)

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Ex 7-6
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Divided Arterials

• ROW CS Ex 7-8 shows the ROW
• Case 1 (open area) ROW follows the most
  favorable CS
• Case 2 (constrained area) CS follows the
  available ROW.
• When separation of local traffic from through
  traffic is needed, frontage road is introduced as
  in ex 7-9

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Ex 7-8
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Ex 7-9
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Divided Arterials

• Sections with widely separated roadway sometimes
  happen when the expansion is required but terrain
  restrict. Therefore two roadways will be widely
  separated.
• Intersections Undesirable and unexpected on low
  rural Hwy. Curbs are considered obstacles and
  should be avoided.
• Accl Deccl lanes is min for minor
  intersections. Interchanges is recommended on
  principle arterials.

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Divided Arterials

• Access Management Mobility significance
  nature of the arterials dictates reasonable
  access control (full or partial). The control
  factor is the amount of the crossroads and access
  points.
• Access points should be well distanced and
  designed in a manners to provide the shortest
  distance for crossing vehicle.

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Divided Arterials

• Ped Bicycles Undesirable yet if needed safe
  sidewalk and off road bicycle path should be
  studied.
• Bus Turnouts wide and well marked shoulder or
  turnarounds should be provided where concentrated
  population is expected.
• Railroad-hwy crossing should be grade
  separated. Could also be at-grade if deemed
  necessary. Both cases, proper signs, signals,
  gates etc must be provided.
• Rest Areas Desirable on principle arterials.
  Release drivers fatigue. Should be well spaced
  and economical studied.

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• Thank You