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Planning and Design of Interchanges in Urban and Suburban Areas

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									Urban Interstate Planning and
 Design Issues and Concerns


                  James P McCarthy, PE, PTOE
FHWA - Minnesota Division and Resource Center
        Suite 500 Galtier Plaza, 380 Jackson St.
                              St. Paul, MN 55101
                                   (651) 291-6112
Urban Interstate Operating
Environment

• High Volume
• Limited capacity
• Closely spaced interchanges;
  multiple opportunities for exiting and
  entering
• Low v/c; variable speeds throughout
  typical day
• Mix of trip types and travelers
• Driver/user expectancies re:
  interstates
Challenges

• Limited R/W
• Conflicting demands to serve
  multiple users, trips
• Expense of maintenance and
  reconstruction
• Design for Level of Service D to E is
  inevitable
• Community values and concerns
  may conflict with mobility interests
Key Urban Interstate System
Features

• Maintain Route Continuity
• Maintain Basic Number of
  Lanes; use auxiliary lanes for
  local operational needs
• System Interchanges
• Primacy of System movements
  over local access
Importance of System Features
and Design Characteristics

 System features produce
 desirable operational and safety
 performance:
  – minimal lane changing (greater
    throughput per lane)
  – optimal traffic distribution across
    the lanes
  – minimal conflicts and crashes
  – greater reliability
Maintain Route Continuity

       I-10 and I-17, Phoenix, AZ




      Mainline (60 mph)
      Design Speed for I-10
      (designated as major
      route)
Maintain Basic Number of
Continuous Lanes for Through
Traffic
Changing the Basic Number of Lanes
   • Basic lanes should be
     added or deleted one at
     a time, at major system
     network locations
   • Remove basic lanes
     following significant
     changes in traffic
     volumes                   Major arterial or
                               freeway



           3            3                      2
Auxiliary lanes provide local capacity
and operational efficiency
 Primacy of Movements
• Primary movements (must provide)
  – ‘Through’ Interstate (e.g., I-94)
  – Interstate to Interstate (e.g., I-35W to I-94)
  – Longer distance trips (e.g., Hiawatha to I-
    94 WB)
• Secondary movements (important)
  – Arterial to arterial; long to mid-range trips
• Tertiary movements (discourage or
  minimize)
  – Short trips from one street/interchange to
    the next
Separate local (service) access
from system movements
                         Service
                         interchange




                   Service
                   interchange
Arrangement and spacing of
ramps ‘drives’ operational quality
and ultimate capacity




                     AASHTO values
Use right-hand ramps only; left hand
ramps reduce lane continuity and
increase lane changing and weaving




 System interchange         System interchange
Weaving degrades capacity,
produces poor distribution of
traffic, and increases crash risk
Design
solutions
to eliminate
weaving
problems
associated
with closely
spaced
ramps
Meet user expectations of service
interchanges

• Provide complementary movements
• Avoid partial interchanges
• Interchange directly with crossroad
• Arrange ramps in logical sequence
  (signing)
• Separate service (local) ramps from
  system ramps
Cross section

• Lanes and Lane Width
• Shoulders and Shoulder Width
                           Operational
                           functions of
                           shoulders

                         • Enhance roadside
                           safety
                         • Facilitate routine
• Contribute to            maintenance activities
  capacity               • Provide flexibility for
• Provide space for        maintenance of traffic
                           during resurfacing or
  emergencies,
                           reconstruction
  incidents, accidents
                         • Facilitate snow
• Support law              removal
  enforcement
Consider Innovative Solutions to
Urban Freeway Problems
             • High Occupancy
               Vehicle/High Occupancy
               Toll Lanes (HOV/HOT)
             • ‘Managed Lanes’
Design Philosophy

• Select reasonable design criteria
  within AASHTO Policy
• Understand (design) implications of
  meeting at least minimum criteria
• Design exceptions are probably
  inevitable; but don’t start there
• Decisions re: design exceptions
  should be performance driven
  (operational quality and safety)
Project Design Goals

• FHWA encourages MnDOT to
  follow good design principles in
  development of alternative
  scenarios
• FHWA supports efforts such as
  this to assure corridor
  investments make sense and will
  bring value
Questions

								
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