3/10/2009
Instractor:
Eyassu Tesfamariam (PhD)
Assist. Instractor:
Sophonias Asrat(MSc)
Design Controls and Criteria
y Functional classification
y Projected traffic volumes and composition
y Design speed and design vehicle
y Topography
y Available Funding
y Driver performance factors
y Safety
y P li i
Politics
y Social and environmental impacts
y Right‐of‐Way (ROW)
y Costs
1
3/10/2009
Road functional classification
Mobility & Accessibility
Mobility: The ability to move goods and
passengers to their destination. (in a
reasonable time)
Accessibility: the ability to reach
desired destination,
Classification of Movements
and Highways
• Hierarchy of Movements
Highway function
Hierarchial system
y based on ppurpose
p and level of importance
p
y Principal arterials
ERA manual
y Minor arterials Trunk roads (Class I)
Mobility Link roads (Class II)
y Major collectors Accessibility Main access roads (Class III)
y Minor collectors Collector roads (Class IV)
y Local roads and streets Feeder roads (Class V)
y Each functional classification can be termed either urban or
rural depending on the location of the planned highway.
2
3/10/2009
Highway functional classification
y Classification of Highways
Arterials
Higher degrees of Mobility
Low degree of access
Collectors
Balance between Arterials and Collectors
Locals
Lower degrees of mobility
High degree of access
Highway function (Cont…)
y Highway Design Manuals
♦ Principal arterial - main movement
(high mobility, limited access)
♦ Minor arterial - interconnects principal arterials
(moderate mobility, limited access)
♦ Collectors - connects local roads to arterials
(
(moderate
d t mobility,
bilit moderate
d t access))
♦ Local roads and streets - permits access to
abutting land (high access, limited mobility)
3
3/10/2009
ERA Design manual 2002
y Highway function
Highway function (Cont…)
y Role of Functional Classification in Design Process
Design Speed Horizontal/Vertical
Alignment
Functional
Classification
Cross Section
The use of functional classification as a design type should
appropriately integrate the highway planning and design process.
4
3/10/2009
Highway functional classification (Cont…)
The first step in the design process is to define the function that the
facility is to serve.
The level of service required to fulfill this function for the anticipated
volume and composition of traffic provides a rational and cost-effective
basis for the selection of design speed and geometric criteria within the
range of values available to the designer (for the specified functional
classification).
Design Speed
Definition: A desgn speed is a selected speed used to determine the various
geometric design features of the roadway.
Design speeds are selected to achieve a desired level of operation and safety on
a highway.
It is important to design facilities with all elements in balance, consistent with
an appropriate design speed.
Design elements such as sight distance, vertical and horizontal alignment, lane
and shoulder widths, roadside clearances, superelevation, etc., are influenced
by design speed.
5
3/10/2009
Design Speed
y Control factors of Design Speed
y Topography
y Flat
y Rolling
y Mountainous
y Escarpment
y The adjacent land use
y The functional classification of highways
y Anticipated Operating Speed
y Operating Speed: Observed free‐flow speed
Design Vehicle
6
3/10/2009
Design Vehicle
y Design vehicles are selected to represent all vehicles on
the highway.
y The vehicle type selected is typically the largest vehicle
likely to use the highway with considerable frequency.
y The weight,
g physical
p y dimensions, and operating
p g
characteristics of the design vehicle will be used to
establish the geometric features of the highway.
Design Vehicle
y The physical characteristics of vehicles directly affect the
required characteristics of the roadways on which they are
driven.
y Jurisdictions evaluate their vehicle fleet populations and
establish a grouping of vehicles on which roadway design can be
based.
y Roadway y features such as intersection corner radii, overpass
p
clearances, ramp widths, and lane widths are all directly affected
by the size and shapes of the vehicles using a facility.
y Design vehicles have representative dimensions and operating
characteristics and are used to establish highway design controls.
7
3/10/2009
Design Vehicle Classes
y Passenger Vehicles
y passenger vehicle
hi l class
l includes
i l d compact, t small,
ll medium,
di and
d large
l passenger
vehicles, including pickup trucks, sport utility vehicles, and vans
y Buses
y include all single unit buses, all articulated buses and intercity busses
y Trucks
y include single‐unit trucks, tractor‐trailers, and semi‐tractor trailer combinations
y Recreational Vehicles
y vehicles with long wheel bases that have tracking characteristics are similar to
single‐unit buses
In the design of any highway facility, the designer should consider the largest
design vehicle likely to use that facility with considerable frequency.
Design Vehicle Classes
8
3/10/2009
Examples of selecting a design vehicle
Design Vehicle
y Vehicle dimension and
turn radius
e.g. Passenger car
9
3/10/2009
Design Vehicle
y Vehicle dimension and
turn radius
e.g. Truck
Turning path data definitions
10
3/10/2009
Turning path data definitions
11
