Insight on Geometric Design on road and highways and the Intersection Design Principles.
1. Geometric Design on road and highways
Basic design controls serve as the foundation for establishing the physical form, safety, and functionality of the transportation facility. Some design controls are inherent characteristics of the facility (e.g., its physical context and the existing transportation demands placed upon it). Other basic design controls are selected or determined by the designer, working with communities and users to address a project’s purpose and need. Selecting appropriate values or characteristics for these basic design controls is essential to achieve a safe, effective, and context sensitive design. Road having following element and their influence on the physical characteristics of a roadway or other transportation facility are:
1. Roadway Context
The context of a roadway is a critical factor to consider in developing a project’s purpose and need, making fundamental design decisions such as cross-section determination, and selecting detailed design elements such as street light fixtures or other construction materials. Development of a roadway design that is sensitive to, and respectful of the surrounding context is important for project success. Historically, the highway design process has focused on a project’s transportation element, particularly those associated with motor vehicle travel. A context-sensitive design should begin with analysis of the contextual elements, such as environmental and community resources, of the area through which a roadway passes. The concept of area types has been developed to help the designer understand the users, constraints, and opportunities that may be encountered in different settings. Once the designer has an understanding of the area surrounding the road and the road’s users, the designer should consider the transportation elements of the roadway, its function within the regional transportation system, and the appropriate level of access control. Thus, three main elements of context considered in design are :
i. Area Type – The surrounding built and natural environment
ii. Roadway Type – The role the roadway plays in terms of providing regional connectivity and local access
iii. Access Control – The degree of connection or separation between the roadway and the surrounding land use.
i. Area Types
The context of a roadway begins with its environmental context,
which includes nearby natural resources, terrain, and the manmade
environment (development patterns, historic, cultural, and
recreational assets). The environmental context can be a
determinant of the desired type of accommodation for different
users. This context often establishes the physical constraints of
the roadway alignment and cross-section, and influences the
selection of motor vehicle design speed. A roadway frequently
traverses a variety of changing environs. Additionally, the volume
and character of pedestrian, bicycle, public transit, and motor
vehicle activity can change considerably along its route. Land use
is the fundamental determinant in the function of a road; as land
use changes along a road, the road's functions also change. Roadway
must be designed in a manner that serves the existing land use
while supporting the community's future land use goals.
ii. Roadway
Types
The transportation network is composed of several types of roadways
that provide different functions, traditionally referred to as an
its functional class. The primary of some roads is to facilitate
movement of vehicles (bicycles, cars, trucks, buses and light rail)
between cities and towns. The primary purpose of other roads is to
provide access to the adjoining land. Most roads provide a
combination of these purposes. The roadway type should be selected
to reflect the actual role that the roadway plays in the
transportation system, as defined through the project development
process. A typical trip will often entail travelling along a
variety of roadway types, each of which provides a different degree
of local access and a different degree of regional
connectivity.
iii. Access Control
Access control is a term used to define how access to adjacent
properties is regulated and designed along a roadway. Access
control is among the most useful tools available to maintain safe
and efficient roadway operations for all users. Judicious use of
medium treatments, driveway permits, and safe driveway geometry can
improve roadway safety and enhance the operation of the road
without undue burden on accessing boarding property. The degree of
access control is influenced by the roadway type and area type. For
example, access controls are usually more stringent on arterials
than on collectors and local roads, reflecting the mobility and
land access functions of these roadways. Likewise, access controls
are often given more consideration in developing areas where there
is flexibility for future land use to conform to an access
management plan than in developed areas where the pattern of land
use has been established. However, the designer should consider
existing access points along a roadway and the possibility for
changes that are consistent with the project’s purpose and need.
For example, it may be possible to relocate, redesign, or
consolidate driveway along an existing roadway. A thorough
understanding of access control will help the designer select an
appropriate design speed, planning parameters, and desired
level-of-service for the facility’s users. Access control is
exercised by statute, zoning, right-of-way purchases, driveway
controls, turning and parking regulations, geometric design (e.g.,
raised medians, grade separations, and frontage roads), and
right-of-way, permitting frequently administered by PWD.
2. Roadway Users
A fundamental expectation in roadway design is that all users will be accommodated safely. Virtually all roadways serve a variety of users including pedestrians, cyclists, motor vehicle drivers and passengers. In a few cases, such as expressways, roadways serve almost extensively motor vehicle traffic. Early in the process, the designer needs to determine the composition of users anticipated for the facility. Appropriately accounting for all user characteristics is essential to obtain a safe and efficient roadway. Experience demonstrates that when human and vehicular factors are properly accommodated, the safety and effectiveness of the highway or road system is greatly enhanced. Consideration of roadway users’ characteristic and selection of appropriate accommodation can also influence the roadway’s effectiveness for businesses and residential users, the economic health of the region, the physical health of the population, and the quality of the built and natural environment. The characteristics of these varied roadway users are important controls that influences the physical design of a roadway, as described in the following sections.
The Cyclist
Safe, convenient and well-designed facilities are essential to
encourage use of bicycle. Roads designed to accommodate cyclists
with moderate skills will meet the needs of most riders. Young
children are primarily the cyclists who may require special
consideration, particularly on neighborhood streets, in
recreational areas, and close to schools. When bicycles are used on
public streets and roads, cyclists are subject to the same traffic
rules as motor vehicle operation.
3. Transportation
Demand
Transportation demands – volume, composition, and patterns – are
important design controls. The greater the demand for a facility,
the more important are its operation and safety characteristics.
The designer must have a good understanding of existing and
anticipated demands by pedestrians, cyclists, and drivers.
Community planning goals, the selected design year, and performance
measures for a project are key determinants of how the design
achieves the project’s purpose and need.
Design Year
Projects are designed to accommodate travel demands likely to occur
within the life of the facility under reasonable maintenance. This
involves projecting future conditions for a selected planning
horizon year. Projections of future demand for major transportation
investments are usually made for the 20 to 30 year range. For large
projects, the designer should usually select 20 years from the
expected facility completion date as the design year. This is a
reasonable compromise between a facility's useful life, the
uncertainties of long-range projections, and the consequences of
inaccurate projections. For smaller, less capital intensive
projects, a 5 to 10 year planning horizon is generally used.
Forecasts of future activity levels should reflect community and
regional plans, community setting, and the project’s purpose and
need. Based on these considerations, a future conditions forecast
represents a technical analysis and policy consensus on the type
and developed intensity of land use, future regional economic
activity, presence of transit service, the needs of pedestrian and
cyclists, and many other factor. Forecasts of future activity
levels should include estimates of pedestrian and bicycles
activities. Particular care must be takien when forecasting
pedestrian and bicycles volumes. Most of the times, there is latent
demand above observed pedestrian and bicycle volumes because
pedestrian and bicycle facilities do not yet exist in the project
area, are substandard, or do not provide complete connectivity to
attractions. It is important to evaluate future land development,
including any potential attractors such as transit stops, schools,
parks and retail uses that may be located near moderate and
high-density residential development. Planners and designers need
to determine the appropriate estimates of activity level design.
For the typical project undertaken within a community, such as an
intersection improvement or a corridor access management project,
the forecast is based on existing conditions. First, traffic counts
(including pedestrian and bicycle trips) are conducted to determine
when the peak hour(s) of traffic occurs. Second, seasonal
adjustment is made, if necessary, to ensure the count data are
representative of at least average annual conditions. Lastly,
future conditions are estimated by adding or subtracting from the
existing traffic volumes to account for known development and
transportation projects, and an annualized factor is generally
applied to account for potential area wide growth or decline.
Regional travel demand models are often used in planning larger
transportation projects. Although the typical process for
forecasting traffic volumes assumes that traffic will increase over
time, there are situations where traffic volumes may decline or
remain relatively constant over time. It is important that traffic
forecasts for a roadway design project reflect likely conditions
over the project's life and are not selected arbitrarily.
Volume and Composition of Demand
The composition of transportation demand is an important element in
the design of roadways. The designer should develop a realistic
design scenario including the volume and mix of activity for all
modes as described below.
a. Pedestrian Demands
Pedestrian counts should be completed to determine pedestrian flows
and patterns. The pedestrian counts should include sidewalk
demands, crossing demands, and storage demands at corners, traffic
islands, and median (total number of pedestrians waiting to cross
the street). In addition to relying on counts of pedestrians, the
designer should also evaluate the project area to determine if
there is latent demand for pedestrian accommodation due to an
uncomfortable existing walking environment, missing links in the
pedestrian network, or expected changes in development patterns.
The likelihood of latent demand can be assessed by looking at
surrounding land uses and their propensity to generate pedestrian
activity. One can also look for conditions like pathways worn along
the roadside to determine if pedestrian connectivity is
underserved. It may be important to complete pedestrian counts for
other times of the day (beyond the typical morning and evening peak
hours) and/or on weekends, depending on the project area. For
example, if a project area is heavily influenced by a school, it is
important to observe pedestrian flows during morning and
mid-afternoon periods. Public assembly facilities and transit stops
or stations also merit special consideration because they can
produce high volumes of pedestrians over short durations. To
determine the appropriate locations for pedestrian counts
(including project area intersections), it is important to review
current pedestrian routes between activity centers. Informal paths
or crossing locations may warrant supplemental pedestrian
observations during project planning.
b. Bicycle Demands
Bicycle demands should be counted during peak hour's concurrent
with vehicle turning movement counts. As the pedestrian activity,
the designer should also evaluate the project area to determine if
there is potential latent demand for bicycles accommodation.
Additional consideration of bicycle demands during other periods of
the day and/or on weekdays may warrant supplemental counts.
c. Motor Vehicle Traffic Volumes
Daily, peak hour, and patterns of motor vehicle traffic are needed
as input to the planning and design of roadway facilities. Some key
definitions of traffic volume measures are listed here:
Design Volumes and Traffic
Composition
The design hourly (DHV), or daily peak hours, will affect many
design elements including the desired number of travel lanes, lane
and shoulder width, and intersection layout. The design volume may
also influence the level of service provided and the accommodation
appropriate for pedestrians and cyclists. Daily traffic estimates
are also useful in making design decisions related to the total
user benefit of a proposed improvement. For example, the benefit of
highway safety roadside improvements is directly related to the
crash exposure (expressed in ADT) on the road. Sometimes selection
of the design hour entails judgement regarding the conversion of
daily traffic to peak hour traffic volumes. Other times, when data
from continuous traffic count stations are used, the design hourly
volume is based on the peaking characteristics of the facility over
an entire year. For rural areas, the DHV is typically based on the
30th or 50th highest hour. In urban areas, the DHV typically
represents the 100th highest hour. In some circumstances, a lesser
design hour is appropriate. These design hour volumes are usually
selected since they capture operating conditions expected to occur
on a regular basis and have been shown to have dependable
statistical relationship to measured ADT on a roadway. The choice
of the design hour volume has a significant impact on the
characteristics of a project. Designers should ensure that the
design volume is selected such as the facility is well-matched to
the traffic volumes it will carry on a regular basis and is not
“over-designed”. For example, accommodating a high volume expected
to occur infrequently will result in a project that is costly and
has significant adverse impacts. Likewise, accommodating a lower
design volume that is frequently exceeded may result in significant
congestion and not meet the level-of-service expectations for
various users. Large or heavy vehicles, such as trucks and buses,
have different operating characteristics from passenger cars and
bicycles and can affect traffic operations. Therefore, the number
of trucks and buses expected to use a facility needs to be
estimated for both the daily and peak hour conditions, in planning
and design. For highway capacity purposes, “heavy vehicle’ are
typically defined as all buses, single-unit trucks, and truck
combinations other than light delivery trucks. (Light delivery
trucks two axles with four tires). In addition, the impact of
transit operations (such as buses making stops along a roadway)
must be considered in operational analysis of the roadway.
4. Measures of Effectiveness
Through the project development
process and with public input, the designer should evaluate the
project (and its alternatives, if applicable) using several
measures of effectiveness. Suggested measures of effectiveness and
analysis techniques for consideration during project planning and
design are described below. Many of these measures of effectiveness
are included in the transportation evaluation criteria used by
transportation agencies for project evaluation and prioritization.
The following sections discuss transportation or contextual of
effectiveness.
Condition of Facilities
National or state transportation policy places an emphasis on
improving the condition of existing facilities. Projects on
existing facilities should return a facility to a state of good
repair by addressing existing structural, pavement surface, or
other deficiencies. Techniques such as pavement testing and bridge
inspections can be used to identify existing deficiencies.
Safety
The safety of transportation facilities is a primary concern in
planning and design. Some projects are specifically proposed to
address known safety problems; however, all projects should result
in a facility that safely accommodates its users. Corridor safety
audits and analysis of crash records can be useful for identifying
existing safety hazards. Project design elements should be selected
based on their historic safety performance and expected operating
characteristics.
Mode Choice
Many projects result in improved accommodation for particular
modes. The effectiveness of these projects can be measured by the
degree to which they allow users to choose the mode best-suited to
their trip purpose and personal values within the broader framework
of the community, the region, and the environment.
5.Speed
Speed is an important factor considered by travellers in selecting a transportation mode or route. Speed can also influence the physical characteristics of the transportation infrastructure. Many design elements such as horizontal and vertical curvature and super elevation are directly related to speed. Other features, such as lane and shoulder width, and the width of the roadside recovery clear zones for errant vehicles, can vary with, but are not a direct function of the design speed. The objective in the planning and design of a roadway is to determine a speed that is appropriate for the context results in a safe facility for all users, is consistent with the community’s goals and objectives for the facility, and meets user’s expectations. Once an appropriate speed is selected, the designer needs to tailor design elements to that speed. Speed is defined as the distance travelled by an object in a certain period of time. Speed is commonly expressed in km/h in the context of transportation planning and design. Several measures and characteristics of speed are important to understand when designing a roadway, as described in the following sections. These measures are most often used to describe motor vehicle operations, although they are also applicable to pedestrian and bicycle movement.
Motor Vehicle Operating
Speed
Operating speed is the measured speed at which drivers are observed
operating their vehicles in fair weather during off-peak hours.
Operating speed is measured at discrete points along a roadway.
Operating speeds are usually reported using percentile speeds with
the 50th percentile (average) and 85th percentile (the speed at
which 85 percent of vehicles are travelling at or below) speeds are
often used to characterize the operating speed on a roadway. The
roadway’s features such as curves and topography, width, access to
adjacent properties, presence of pedestrians and cyclists, parking,
traffic control devices, lighting, etc., affect the operating
speed. During peak periods, when traffic congestion or intersection
operations are controlling movement along a corridor, observed
operating speeds may be substantially lower than the operating
speed measured during off-peak conditions when the roadway’s design
and context are controlling speed. Numerous studies have indicated
that drivers will not significantly alter what they consider to be
a safe operating speed, regardless of the posted speed limit unless
there is constant heavy enforcement.
Target Speed for Motor Vehicles
The target speed is the desired operating speed along a roadway.
The appropriate target speed is determined early in the project
development process, and should consider:
The designer should balance the
benefits of high speed for long distance, regional motor vehicle
travel with environmental, impact community, right of way, and cost
constraints. When high speeds are selected, the designer should
also include design elements to maintain the safety of pedestrians
and cyclists.
Selecting Motor Vehicles Design Speed
Design speed is the selected speed used to determine various
geometric features of the roadway. The design speed should be a
logical one with respect to the target speed and existing operating
speed. When selecting a design speed, understanding the existing
operating speed and target speed addresses: (1) the need to meet
the expectations of drivers based on the roadway environment, and
(2) the ways in which the setting influences the desired speed. It
is important to understand the inter-relationship between speed and
roadway geometry. Selection of a design speed influences the
physical geometrics of the roadway. Similarly, the physical
geometrics of the roadway are important determinates of the
operating speed that will result on the facility. The relatively
wide range of design speed recognizes the range of roadway types,
context, and topography. The provision of a range in design speeds
combined with general guidance on selection of a design speed
represents perhaps the greatest flexibility afforded by the
designer. Designers should exercise judgement in the selection of
an appropriate design speed for particular circumstances and
conditions. In general, an appropriate design speed should be
within approximately 5km/h of travel speed. When determining the
appropriate design speed the designer should also consider the
volumes and composition of the expected non-vehicular and vehicular
traffic, the anticipated driver characteristics, and driver
familiarity with the route. The designer should also consider
expected operations throughout the day, including both peak and
non-peak hours. Indeed, no-peak traffic flow will generally control
the selection of a reasonable design speed. The design speed may
vary from any given route as it traverses rural, suburban, and
urban areas. Once these factors have been evaluated and an
appropriate design speed is determined, the geometric elements
should be designed consistently to the level. The designer should
document the factors leading to the selection of an appropriate
design speed. This documentation is important for selected design
speed below the existing posted speed limit, below the “reasonable
and proper” speed for the type of roadway and area or below the
measured operating speed. Where it is not possible to meet the
selected design speed for one location or design element along a
corridor, a design exception and appropriate warning signage may be
justified. Higher design speeds impose greater challenges and
constraints on designers. Designers faced with difficult or
constrained conditions may consider selecting a lower design speed
for an element or portion of the highway. This practice can cause
problems such as a large number of drivers may not “behave” as the
designer desires or intends them to. Designs based on artificially
low speed can result in inappropriate geometric features that
violate driver expectations and degrade the safety of the highway.
The emphasis should be on the consistency of design so as not to
surprise the motorist with unexpected features. Therefore, the
design speed should only be based on the speed limit if the speed
limit is consistent with existing operating speed or physical
constraints of the built environment. Designers should not propose
an alternative design speed for a highway or segment of a project
as design exception. A serious fundamental problem with accepting
or allowing a design exception for design speed is based on its
important relative to all features of the highway. A reduction in
the design may be unlikely to affect overall operating speed. It
will potentially result in the unnecessary reduction of all the
speed-related design criteria rather than just the one or two
features that led to the need for the exception. The acceptable
alternative approach to a design speed exception is to evaluate
each geometric feature individually, addressing exceptions for each
feature within the context of the appropriate design speed.
Occasionally, projects retain geometric elements, such as tight
curves, super elevation, or restricted sight distances that are
designed for a speed lower than the design speed for the corridor.
This may be due to adjacent land use, or to environmental or
historic constraints. In these cases, the designer should recommend
a posted speed consistent. In these cases, the designer should
recommend a posted speed consistent with the geometric features.
Where it is desirable to maintain a higher consistent speed
throughout a corridor, the designer should install appropriate
cautionary signing at locations with design elements that do not
meet the criteria for the posted speed.
Design Speed and Traffic Calming
The term traffic-calming refers to a variety of physical measures
to reduce vehicular speed primarily in residential neighborhoods.
The lowering of operating speed is often the appropriate solution
to addressing safety problems. Such problems typically involve
vehicle conflicts with pedestrians, cyclists, and school children.
Research has shown that measurable reductions in operating speed
are possible through traffic-calming. A local road or street, and
in some instances other roadways that function as a local road or
street, may have an existing operating speed far in excess of the
speed limit or the target speed. In these cases it may be
acceptable, and consistent with good engineering practice, to
develop a design that will lower the operating speed.
Generally, the design speed selected for traffic calming elements
should be consistent with the target speed for the corridor as a
whole. The traffic calming elements should not result in operating
speed substantially lower than the target speed at certain points
along the corridor and higher speed elsewhere. Selection of a
reasonable design speed for traffic calming elements, selection of
type of elements, and the spacing of traffic calming elements can
help achieve the desired uniform reduction in operating speed along
a roadway. Great care must be taken to ensure that the proposed
design will actually reduce the operating speed to levels
consistent with the design. The burden is on the individual
designer of a traffic-calming feature to document a reasonable
expectation that the proposed measures will reduce the operating
speed. Once traffic calming has been implemented, monitoring of the
performance of the project should be undertaken to assure that
speed has indeed been reduced, and to provide valuable lessons for
future traffic-calming.
6. Sight Distance
Sight distance is the length of roadway ahead that is visible to the roadway user. In most cases, specific sight distance measures apply to motor vehicles and cyclists. The following aspects are commonly discussed for motor vehicle sight distance:
Stopping Sight
Distance
The provision of adequate stopping sight distance (SSD) is a
critical sight distance consideration for design and is described
in the more detail below.
Motor Vehicle Stopping Sight
Distance
Stopping sight distance is the distance necessary for a vehicle
travelling at the design speed to stop before reaching a stationary
object in its path. The sight distance at every point along a
roadway should be at least the stopping sight distance. The motor
vehicle stopping sight distance is given in Table 1.
Passing Sight
Distance
For two-lane highways, passing manoeuvers in which faster vehicles
move ahead of slower vehicle must be accomplished on lanes
regularly used by opposing traffic. If passing is to be
accomplished safely, passing sight distance is necessary to allow
the passing driver to see a sufficient distance ahead, clear of
traffic, to complete the passing manoeuvers without cutting off the
passed vehicle and before meeting an opposing vehicle that appears
during the manoeuver.
Decision Sight
Distance
Decision sight distance adds a dimension of time to stopping sight
distance to allow a driver to detect and react to an unexpected
condition along a roadway. Decision sight distance is suggested
when there is evidence that it would be prudent to provide longer
sight distance, such as when complex decisions are needed or when
information is difficult to perceive. It is the distance needed for
a driver to detect an unexpected or otherwise difficult-to-perceive
information source or condition in a roadway environment that may
be visually cluttered, recognize the condition or its potential
threat, select an appropriate speed and path, and initiate and
complete and manoeuvre safely and efficiently.
2. Design principles of Intersections
Intersection design is an important task. They need to be designed carefully after considering a number of factors. Some of the main design principles are mentioned below-
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