2/24/2024

Geometric Design of Highway

Lecture # 1: Functional Highway Classification

Lecturer: Dr. Hardy Kamal Karim
Ph.D. in Highway Engineering
University of Sulaimani

Hierarchies of Movements and Components

• Motor vehicle travel involves a series of

distinct travel movements.

• The six recognizable stages in most trips

include:
1) main movement,
2) transition,
3) distribution,
4) collection,
5) access, and
6) termination.

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Access Control and Mobility Needs

• The two major considerations in classifying highway and street networks
functionally are access and mobility.

• Freeways are provided almost exclusively to enhance mobility for through

traffic.

• Access to freeways is provided only at specific grade-separated interchanges,

with no direct access to the freeway from adjacent land except by way of
those interchanges.

• Access management techniques can be used to reduce the conflicts between

mobility for through traffic and access to adjacent development. However,
such conflicts are unavoidable, especially in urban areas, because access to
adjacent development is essential to thriving communities.

• Collector roads and streets connect arterials to local roads and streets as well
as providing access to adjacent development.

• Mobility for through traffic is less important on collectors than on arterials

because motor vehicles often travel only moderate distances on a collector
before reaching an arterial or local road.

• Local roads and streets exist primarily to serve adjacent development.

Mobility for through traffic is of little importance because the distance from
the origin or destination of a motor vehicle trip to the nearest collector is
usually short.

• Figure 1-3 illustrates the general balance between mobility and access for
each functional class. The figure is conceptual in nature because the extent
and context of development along each road and street varies widely, and
some arterials may have more extensive access needs than many collector or
local roads.

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Context Classification for Geometric Design

• Rural areas
1) Rural context
2) Rural town context

• Urban areas
3) Suburban context
4) Urban context
5) Urban core context

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• The rural context category ranges from no development (natural environment)

to some light development (structures), with sparse residential and other
structures mostly associated with farms 7

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• The rural town category is characterized by low density (low-rise-one or two

story-structures) but a concentrated development of diverse uses- residential
and commercial.
• Rural towns are generally incorporated but have limited government services.
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• Locations classified as suburban include a diverse range of commercial and

residential uses that have a medium density. 11

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• Urban locations are high density, consisting principally of multistory and low-
to medium-rise structures for residential and commercial use. 13

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• Urban cores house the highest level of density with its mixed residential and
commercial uses accommodated in high-rise structures.
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Federal Functional Classification Decision Tree

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FHWA

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Key characteristics of roadway type based on FCS:

1) Interstates/Freeways/Expressways - Corridors of national importance

providing long distance travel
• Limited access.
• Through traffic movements.
• Primary freight routes.
• Possible transit network support.
• No pedestrian or bicycle traffic.
• Guided by FHWA design standards.

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2) Principal Arterial- Corridors of regional importance connecting large centers of

activity
• Through traffic movements.
• Long-distance traffic movements.
• Long-haul public transit buses.
• Primary freight routes.

3) Minor Arterial - Corridors of local importance connecting centers of activity

• Connections between local areas and network principal arterials.
• Connections for through traffic between arterial roads.
• Access to public transit and through movements.
• Pedestrian and bicycle movements.

4) Collector - Roadways providing connections between arterials and local roads

• Traffic with trips ending in a specific area.
• Access to commercial and residential centers.
• Access to public transportation.
• Pedestrian and bicycle movements.

5) Local - All other roads

• Direct property access—residential and commercial.
• Pedestrian and bicycle movements.

• Note: Interstates/Freeways/Expressways are not addressed in the Expanded FCS, and they are
not included in the Expanded FCS matrix because FHWA design standards govern their design.

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Performance - Based Design

Aspects of performance that may be considered in geometric design include any

of the issues that affect the project development process
• Traffic operational efficiency
• Existing and expected future crash frequency and severity
• Construction cost
• Future maintenance cost
• Context classification
• Service and ease of use for each transportation mode:
- automobile
- bicycle
- pedestrian
- transit
- truck

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• Accessibility for persons with disabilities

• Available right-of-way
• Existing and potential future development
• Operational flexibility during future incidents and maintenance activities
• Stakeholder input
• Community impacts and quality of life
• Historical structures
• Impacts on the natural environment:
- air quality
- noise
- wetlands preservation
- wildlife/endangered species
• Preservation of archeological artifacts

Driver Accommodation
• The metrics used to define the context–roadway interaction for drivers are
the target operating speed and the balance between mobility and access.

Target Operating Speed

• Target operating speed is grouped into three categories:
1) low (< 30 mph),
2) medium (30 – 45 mph), and
3) high ( > 45 mph).

• Speed, in general, decreases along the context continuum (from rural to urban
core) as well as along the roadway type (from principal arterials to locals).

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Access and Mobility

• Access is defined as the frequency of driveways or intersections and is grouped
in three categories based on distance between access points:
1) low ( > 0.75 mile),
2) Medium (0.75 – 0.25 mile); and
3) high ( < 0.25 mile).

• Mobility is defined – qualitatively - as a function congestion level:

1) low (congested conditions),
2) medium (some congestion), and
3) high (no congestion; free flow). Volumes referred to here are during the peak
period.

Expanded FCS Matrix Approach

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Design Considerations

Bicyclist Accommodation

Bicycle facilities can be generally categorized based on the amount of

separation they provide from motorized traffic. For the purposes of the
Expanded FCS, bicycle facilities are categorized as follows:
• High separation - provides physical separation from traffic in the form of
physical barrier or lateral buffer.
• Medium separation - provides a dedicated space adjacent to motorized
traffic.
• Low/No separation - provides joint-use facilities for motorized and non
motorized traffic.

The amount of separation necessary for a facility is dependent mostly on the

following:
• The amount of bicycle traffic on the facility.
• The speed of motorized traffic on the adjacent roadway.
• The amount of motorized traffic on the adjacent roadway.

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Pedestrian Accommodation
Pedestrian facilities can be generally categorized by their width.

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Single-Context Application
Example/ Rural Principal Arterial

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Example/ Suburban Minor Arterial

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