Traffic Impact Analysis

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Traffic Impact Analysis

• SVC 411 Transportation Planning
• Department of Civil and Biosystems Engineering
• University of Pretoria
• Siegfried Grosskopf, Pr Eng
• Africon Engineering International

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Rational Transportation Planning Process
Vision

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Traditional 4-step Modelling
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Transportation Planning Process

• Levels of Planning
• Regional
• Sub-regional
• Local

• Levels of Assessment

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TIA vs Transport Assessment

• TIA
• Narrow focus
• Predict and Provide
• Status Quo maintain
• Past Experience
• Comparability
• Stress vehicle trips
• Car trips
• Negative impacts
• Increase road capacity
• Add safety features

• Transport Assessment
• Wider focus
• Sustainability
• Change behaviour
• Future perspective
• Changed outcomes
• Stress person trips
• Push other modes
• Negative impacts
• Wider assessment, Travel plans, Financial
  incentives
• Extra transport capacity

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Basic Premises

• Adequate provision for movement to and from
  developments
• Key Issues
• Accessibility
• Capacity
• Adequate on-site circulation

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Traffic Planning

• Traffic Impact Analysis fits into
• Regional planning
• Regional/ corridor development
• Identification of road hierarchy in network
• Structure planning (sub-regional level to local
  level urban centres)
• Road, public transport pedestrian systems
• Parking and traffic management
• Development planning
• Planning approvals
• Support the design
• Access and egress
• Bulk service contributions

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Guidelines for TIA

• International
• Institute of Transportation Engineers, US 1991
• Institute for Highways and Transportation UK,
  1994
• South Africa
• Riglyne vir Verkeersimpakstudies, NDoT 1990
• Manual for traffic impact studies, NDoT 1995
• Individual metropolitan areas

CAUTION New Gauteng policy under development To
be utilised by all metro and local authorities
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Traffic Impact Analyst

• Registered Engineers and Technologists
• Traffic Engineering Training
• Traffic Engineering Experience
• Execute TIA
• Certify TIA

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Objective for TIA

• To determine the Effect that a
• Change in Land-use
• or
• Transportation Infrastructure
• may have on
• Traffic Conditions

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Warrants for TIA
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Extent of Traffic Impact Analysis

• The extent of a Traffic Impact Study should
  include all intersections or other elements of
  the road network at which the performance might
  deteriorate significantly due to the traffic
  generated by the development or by proposed
  revisions to the network in the horizon year

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Study Area

• All elements included in a Traffic Impact
  Statement
• All elements of the road infrastructure where 75
  or more additional trips are added to the sum of
  the critical lane volumes
• 1 1.5km from the site along the network
• Discretion of the road authority

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Analysis Period
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Analysis of Peak Periods

• Analyse the worst situation
• (Operational condtns Queuing capacity, etc)
• Development traffic peak flow
• Background traffic peak flow
• Weekend or other typical off-peak conditions
• Most critical peak hour (AM or PM peak)
• Impact of Special Events
• Non-coinciding peak hours

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Methodology

• Existing background traffic
• (Traffic counts)
• Development traffic
• Manual method
• Trip generation
• Manually assigned to network
• Impact assessed per intersection
• Modelling method
• (Large developments with regional impact)

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Framework for TIA

• Describe Existing Conditions
• Describe Proposed Development
• Background Traffic
• Development Traffic
• Trip Generation
• Trip Distribution
• Modal Split
• Trip Assignment

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Framework for TIA (2)

• Capacity Analysis
• Level of Service
• Access Arrangements
• Parking
• Site Development
• Upgrading of Road Network
• Other issues
• Road Safety
• Piecemeal Developments

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Existing Conditions

• Description of relevant conditions
• Traffic conditions flow rates, etc
• Pedestrians, public transport
• Road safety
• Site development constraints
• Nearby/ Adjacent developments
• Approved developments
• Committed road upgrading

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Proposed Development

• Detail of land use applied for
• Site area
• Size of development
• Permissible area, FAR, coverage, GLA
• Existing use
• Proposed access arrangements
• Hours of operation
• Servicing arrangements
• Development phasing
• Parking standards and provision

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Background Traffic

• Components
• Through traffic
• Traffic generated by other developments
• Growth in through traffic
• Projecting background traffic
• Build-up method
• Various simultaneous projects
• Care with different land uses
• Modelled traffic volumes
• Regional impacts
• Care with different networks/ transportation plan
  forecasts
• Growth method
• Grow non-site traffic
• Care with existing network capacity

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Site TrafficTrip Generation

• Basic principles
• Based on past experience/ comparable
• Externalities
• Mixed land-uses or multi-use developments
• Characteristics of road network accessibility
  LOS
• Public transport
• Socio-economic characteristics of customers
• Estimating Number of Trips
• Analytical
• Trip rate tables
• Modelling

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Site TrafficTrip Generation

• Number of trips
• Primary trips (single purpose / new trips)
• Non-primary trips (multi purpose /
• transferred
  trips)
• Diverted trips
• Pass-by trips (care with two-way traffic)
• Existing trips (replacement of land use)

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Site TrafficTrip Distribution

• Gravity Model

• Methods
• Analogy
• Surrogate
• Gravity Model

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Site TrafficModal Split

• Availability of public transport
• Modes Bus, rail, combi-taxi
• Trip generation in Person trips
• Estimate of vehicle trips
• Occupancy per mode
• Schedules/ routes/ frequency of service
• Bus stops or Rail stations

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Trip Assignment

• Assignment of the vehicle trips
• to the road network
• Shortest time
• Shortest distance
• Models
• Manual
• Logical routes
• All or nothing
• Proportional
• Available capacities
• Bottlenecks
• Critical right turns
• Future network
• Future Land-use
• Diverted Traffic

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Capacity Analysis

• Level of Service
• Qualitative assessment of operational conditions
• LOS A to LOS F LOS E max flow rate
• Based on various Measures of Effectiveness
• Calculated as per HCM

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Level of ServiceSignalised Intersections

• Measure of Effectiveness
• Currently HCM definition
• Average stopped delay
• A dlt5s Good progression, Few stops
• B dlt15s Good progression, More stops
• E dlt60s Poor progression, Long queues
• Historically
• V/C ratio
• A 0 0.1 Free flow
• B 0.1 0.3 Low stable flow
• E 1.0 Unstable flow

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LOS Delay plus Degree of Saturation
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Road Network Analysis

• Recommended Procedure
• Determine functional classification
• Identify the elements to be analysed
• Identify traffic control at the intersections
• Analyse the intersections
• Justify traffic signals

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Access Arrangements

• Factors to Consider
• Location and spacing of accesses
• Uniform spacing for progression on arterials
• Number of accesses and lane configuration
• User characteristics
• Peak traffic flow rate
• Capacity constraints of street system
• Access control
• Strategy and service rate

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Access Arrangements

• Factors to Consider
• Traffic control
• Carefully consider right turn movements and
• interaction with main street traffic
• Geometric Layout
• Storage space
• Capacity of accesses
• Filling stations
• Service access
• Goods vehicle characteristics
• Frequency of deliveries
• Type of goods vehicle

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Parking

• Requirements
• Customers
• Accessible
• User-friendly
• Sufficient
• Convenient and safe
• Service vehicles
• OFTEN ONLY AN AFTERTHOUGHT !

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Site Development Plan

• Elements of the Site Development Plan
• Conflicts between vehicles and pedestrians
• Vehicle/ vehicle conflicts
• Sight distances and geometric standards
• Traffic circulation and channeling
• Pedestrian circulation routes
• Location, shape and access to buildings
• Parking relative to buildings
• Fire and emergency routes
• Service areas and refuse removal
• Surface drainage and landscaping
• ..

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Upgrading of the road network

• Possible policies
• Restore v/c ratio to pre-development
• Provide for 5 years growth after opening of
  development
• Maintain 10 reserve capacity at the design year
• NDoT
• All elements of intersections LOS D or better
• AND v/c lt 0.95 during peak hour
• If baseline LOS E and v/c gt 0.95
• Maintain or improve baseline conditions

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Upgrading of the road network

• Programming
• Scenario analysis with and without
  development for opening and horizon years
• LOS worse than LOS D with development AND
• LOS better than LOS D without development
• Developer responsible for upgrade
• LOS worse than LOS D with development AND
• LOS worse than LOS D without development
• Developer responsible for incremental
  upgrade to LOS and v/c without development

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Upgrading of the road network

• Extent of Upgrading
• Traffic control
• Geometric improvements
• Extra lanes
• Slip lanes
• Channelisation
• Improved turning radii
• Widening
• Revise intersection layout
• New road links

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Cost Apportionment

• Concept
• Road users must pay proportionally for the road
  network used by them
• Road bulk contribution
• To pay for proportion of vehicle trips generated
  by the development in proportion to total number
  of trips using the network for access and the
  capacity of the network taken up by the generated
  trips during daily and peak periods
• Problems
• Equitability
• Redevelopments and cascading developments

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Other Issues

• Road Safety
• Increased conflict between vehicles and
  vulnerable road users
• Piecemeal developments
• Collective impact

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Contact Details Siegfried Grosskopf AFRICON
Tel [phone] PO Box 905 Fax 012 427
2933 0001 PRETORIA Cell 082 564 2896 South
Africa e-mail siegfriedg_at_africon.co.za