Section 3-01 General Concepts, Design Standards and Design
Transcript Of Section 3-01 General Concepts, Design Standards and Design
Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
Section 3-01 General Concepts, Design Standards and Design Exceptions
TABLE OF CONTENTS
GENERAL CONCEPTS .........................................................................................2 General .........................................................................................................2 Design Vehicle.............................................................................................2 Driver Expectancy .......................................................................................2 Design Speed ...............................................................................................2 Sight Distances.............................................................................................3 Exhibit 1, Stopping Sight Distance ..............................................................4 Exhibit 2, Passing Sight Distance ................................................................5 Traffic Volumes ...........................................................................................5 Level of Service ...........................................................................................6 Capacity Analysis ........................................................................................7 Access Control .............................................................................................7
DESIGN STANDARDS ..........................................................................................8 General .........................................................................................................8 Functional Classification .............................................................................8 Exhibit 3, National Highway System Map ..................................................9 Exhibit 4, Design Functional Classification Map ......................................10 Project Types .............................................................................................11 Controlling Design Criteria........................................................................12
DESIGN EXCEPTIONS .......................................................................................16 General .......................................................................................................16 Design Exception Request .........................................................................17 Design Documentation...............................................................................17 Sample Exception Request Letter ..............................................................18
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
GENERAL CONCEPTS
General: This section is intended to provide a basic overview on a variety of general subjects associated with geometric design of highways. For detailed design information, consult the following references: AASHTO A Policy on Geometric Design of Highways and Streets, WYDOT Design Guides, WYDOT Standard Plans, WYDOT Standard Specifications for Road and Bridge Construction, the AASHTO Roadside Design Guide, Highway Capacity Manual, and other Transportation Department design manuals and AASHTO publications.
Design Vehicle: A design vehicle is a motor vehicle of standard dimensions that is used in establishing controls for geometric design. Design vehicles are grouped into three main classifications: passenger vehicles, buses/recreational vehicles, and trucks. Their dimensions and turning radii are used in setting roadway widths, turning radii, vertical clearances, etc.
The design vehicle is selected by the Traffic Program in accordance with criteria set forth in AASHTO A Policy on Geometric Design of Highways and Streets. In highway design, the largest standard design vehicle likely to use the highway frequently is used to design such critical features as corner radii at intersections and radii of turning roadways.
Driver Expectancy: Driver expectancy refers to the expectation, by the driver, of standard geometric, operational, and route characteristics. Departure from common design features or abrupt changes in their character may result in driver error, or increased response time, which may result in an increased risk of a crash.
An example of driver expectancy is the expectation that an off-ramp from a highway will be located on the right side of the road. Adequate informational signing may help offset the confusion a left-hand exit would cause, but wherever possible the normal and expected right-hand exit should be used. Another example of design features that violate driver expectancy is when abrupt changes are made in design speed or horizontal alignment. Such changes should be made gradually to allow the driver to make smooth transitions in driving actions.
Design Speed: Design speed is a selected speed used to determine various geometric features of the roadway. The selection of a design speed is influenced by topography, adjacent land use, and the functional classification. Design speed should be selected to attain a desired combination of safety, mobility, and
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions efficiency, within the constraints of environmental quality, economics, aesthetics, and social or political impacts.
Sight Distances: A driver’s ability to see ahead is needed for safe and efficient operation of a vehicle on a highway.
Stopping Sight Distance Drivers need to be able to see far enough ahead to react to hazardous objects on the roadway and to be able to stop before impact. Therefore, the design of the roadway must provide adequate stopping sight distance for the driver at all locations. AASHTO's definition of stopping sight distance consists of two components: the distance traveled by the vehicle during the driver's brake reaction time and the actual braking distance. A brake reaction time of 2.5 seconds is used for all drivers and for all conditions, but does not account for situations requiring complex decisions by the driver. Braking distance is based on a conservative deceleration rate of 11.2 ft/sec2 that accounts for wet pavements, worn tires, etc., but not for icy roads. Stopping sight distances, according the 2011 AASHTO policy, are shown in EXHIBIT 1, Stopping Sight Distance on Level Roadways.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
EXHIBIT 1
STOPPING SIGHT DISTANCE ON LEVEL ROADWAYS
Drivers sometimes do not drive slower on wet or icy pavement. Therefore, providing longer sight distances when possible adds to the safety of the roadway under adverse conditions.
Passing Sight Distance Passing sight distance is needed to safely provide for the passing maneuver on two-lane roadways where the faster vehicle must use the lane of opposing traffic. Because of the nature of the terrain in which the roads are constructed, it is not always possible or economically feasible to supply continuous passing sight distance. WYDOT attempts to fulfill the requirements for safe passing sight distance whenever practical. The more passing sight distance that can be provided for a roadway, the better and safer it will operate. See EXHIBIT 2, Passing Sight Distance For Design of Two-Lane Highways. Detailed sight distance descriptions can be found in the AASHTO A Policy on Geometric Design of Highways and Streets including discussion on the effect of grades on stopping sight distance, decision sight distance, and how it applies to trucks.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
EXHIBIT 2
PASSING SIGHT DISTANCE FOR DESIGN OF TWO-LANE HIGHWAYS
Traffic Volumes: Traffic volumes are a measure of the traffic usage for a particular segment of a road or a highway. Different derived traffic volumes are used in design for selection of typical sections, design speeds, surfacing thickness, etc. For the purposes of highway design and planning, WYDOT commonly uses projected 20-year traffic volumes from when the project is first initiated.
ADT (Average Daily Traffic) Derived from the total volume for a given time period divided by the number of days in that time period, and expressed in vpd (vehicles per day).
AADT (Annual Average Daily Traffic) Total number of vehicles traversing a section of roadway in a year divided by 365, the number of days in a year. The AADT are the traffic volumes employed in selecting design criteria such as number of lanes, shoulder widths, design speed, and expressed in vpd (vehicles per day).
DHV (Design Hourly Volume) Generally taken as the 30th-highest hourly volume of the year, or approximately 15 percent of the AADT, and is used to determine
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
what level of capacity or service a road should be designed to accommodate, and expressed in a percentage of the ADT. This value balances the difference between adequacy of design and economy of design.
DDHV (Directional Design Hourly Volume) Used in situations where traffic patterns, in different directions, vary significantly depending on time of day, such as high volume commuter traffic to and from a business center.
% of Trucks The percentage of the traffic stream that is comprised of buses, single-unit trucks and truck combinations. Percentage of trucks does not include light delivery trucks.
Traffic volumes and design volume projections are collected and/or prepared by the Planning Program - Transportation Surveys Section.
For projects with long design duration, the traffic count and projections should be updated prior to final design, if the ones used to select design parameters are somewhat outdated. The design should be reviewed against the updated traffic counts for compliance. If any aspect of design would be changed by the updated counts, the Project Development Engineer should be advised.
Level of Service: Level of service (LOS) is the concept used to describe operational conditions within a traffic stream and their perception by motorists. The designated level of service compares the existing or proposed roadway to the "ideal" conditions for that type of roadway. Level of service is defined by a letter designation from A to F, with A representing the best operating conditions and F the worst operating conditions. This definition describes the conditions encountered in terms of such factors as speed, travel time, maneuverability, traffic interruptions, comfort, convenience, and safety.
The AASHTO A Policy on Geometric Design of Highways and Streets defines the general operating conditions for levels of service as follows:
A - free flow, with low volumes and high speeds. B - reasonably free flow, but speeds beginning to be restricted by traffic
conditions. C - stable flow, but most drivers restricted in freedom to select their own
speed.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
D - approaching unstable flow, drivers have little freedom to maneuver. E - unstable flow, may be short stoppages. F - stop and go operation.
WYDOT uses a design year Level-of-Service (LOS) C as the appropriate LOS to warrant capacity improvements for Interstate mainline sections; a design year Level-of-Service (LOS) D has been selected as the appropriate LOS to warrant capacity improvements for Interstate interchange ramps and intersecting roads. A design year Level-of-Service (LOS) D has been selected as the appropriate LOS to warrant capacity improvements for NHS Arterial & Non-NHS State Highway rural and urban mainline sections. Achieving a specified LOS level in some situations may not be feasible, in such cases, a lower LOS may be acceptable. Further information on level of service can be found in the Highway Capacity Manual.
Capacity Analysis: A detailed discussion of capacity analysis can be found in the Highway Capacity Manual. The Highway Capacity Manual is defined as- "a set of procedures used to estimate the traffic carrying ability of facilities over a range of defined operational conditions. It provides tools for the analysis and improvement of existing facilities, and for the planning and design of future facilities." The Traffic Program will usually do the detailed capacity analysis required on a project.
Access Control: Access control is the regulation of ingress and egress from streets and highways by state or local authorities. Access ranges from fully controlled access on interstates and freeways, where preference is given to the mobility of through traffic, to partially controlled access on lower functional classification roads and highways, where the access needs of adjacent landowners receive nearly as much priority as mobility of the traveling public.
In general, access to state highways is regulated in accordance with the WYDOT Access Manual, and in accordance with WYDOT Operating Policy. All access points should be evaluated during the design. Access on non-interstate highways is permitted by the District and Traffic Program, and access on interstate highways may be allowed only with FHWA approval.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
DESIGN STANDARDS
General: Design standards to be applied to the various combinations of roadway
classification and project types (Preservation, Rehabilitation, Reconstruction) are the current editions of:
AASHTO A Policy of Geometric Design of Highways and Streets WYDOT Design Guide, Interstate Highways WYDOT Design Guide, NHS Arterial (Non-Interstate) WYDOT Design Guide, Non-NHS State Highways WYDOT Road Design Manual WYDOT County Road Fund Manual
The WYDOT Design Guides are specific to the roadway’s functional classification and project type. It is important that the applicable book and section be referenced to ensure that correct design standards are being applied.
Functional Classification: Functional classification of roads and highways is a system by which roads are grouped according to the general function they provide. Functions provided by roads and highways, both urban and rural, include main (through) movement, transition (e.g. freeway ramps), distribution, collection, access, and termination. For the purposes of design, WYDOT recognizes the following functional classifications for both urban and rural facilities:
Interstate NHS Arterial (Non-Interstate) Non-NHS State Highways (arterial, collector, or local)
Definitions of the functional classifications for both urban and rural can be found in the AASHTO A Policy on Geometric Design of Highways and Streets. Functional classifications are established for the roadway by the Planning Program and are documented in the Reconnaissance Inspection Report. See EXHIBIT 3 for the National Highway System: Wyoming Map, and EXHIBIT 4 for the Design Functional Classification Map.
The website link to the National Highway System: Wyoming Map is:
http://www.fhwa.dot.gov/planning/national_highway_system/nhs_maps/wyoming/
The Design Functional Classification Map can be found as a downloadable PDF file on the Road Design Manual webpage.
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EXHIBIT 3
NATIONAL HIGHWAY SYSTEM: WYOMING
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EXHIBIT 4
DESIGN FUNCTIONAL CLASSIFICATION
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Section 3-01 General Concepts, Design Standards and Design Exceptions
TABLE OF CONTENTS
GENERAL CONCEPTS .........................................................................................2 General .........................................................................................................2 Design Vehicle.............................................................................................2 Driver Expectancy .......................................................................................2 Design Speed ...............................................................................................2 Sight Distances.............................................................................................3 Exhibit 1, Stopping Sight Distance ..............................................................4 Exhibit 2, Passing Sight Distance ................................................................5 Traffic Volumes ...........................................................................................5 Level of Service ...........................................................................................6 Capacity Analysis ........................................................................................7 Access Control .............................................................................................7
DESIGN STANDARDS ..........................................................................................8 General .........................................................................................................8 Functional Classification .............................................................................8 Exhibit 3, National Highway System Map ..................................................9 Exhibit 4, Design Functional Classification Map ......................................10 Project Types .............................................................................................11 Controlling Design Criteria........................................................................12
DESIGN EXCEPTIONS .......................................................................................16 General .......................................................................................................16 Design Exception Request .........................................................................17 Design Documentation...............................................................................17 Sample Exception Request Letter ..............................................................18
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
GENERAL CONCEPTS
General: This section is intended to provide a basic overview on a variety of general subjects associated with geometric design of highways. For detailed design information, consult the following references: AASHTO A Policy on Geometric Design of Highways and Streets, WYDOT Design Guides, WYDOT Standard Plans, WYDOT Standard Specifications for Road and Bridge Construction, the AASHTO Roadside Design Guide, Highway Capacity Manual, and other Transportation Department design manuals and AASHTO publications.
Design Vehicle: A design vehicle is a motor vehicle of standard dimensions that is used in establishing controls for geometric design. Design vehicles are grouped into three main classifications: passenger vehicles, buses/recreational vehicles, and trucks. Their dimensions and turning radii are used in setting roadway widths, turning radii, vertical clearances, etc.
The design vehicle is selected by the Traffic Program in accordance with criteria set forth in AASHTO A Policy on Geometric Design of Highways and Streets. In highway design, the largest standard design vehicle likely to use the highway frequently is used to design such critical features as corner radii at intersections and radii of turning roadways.
Driver Expectancy: Driver expectancy refers to the expectation, by the driver, of standard geometric, operational, and route characteristics. Departure from common design features or abrupt changes in their character may result in driver error, or increased response time, which may result in an increased risk of a crash.
An example of driver expectancy is the expectation that an off-ramp from a highway will be located on the right side of the road. Adequate informational signing may help offset the confusion a left-hand exit would cause, but wherever possible the normal and expected right-hand exit should be used. Another example of design features that violate driver expectancy is when abrupt changes are made in design speed or horizontal alignment. Such changes should be made gradually to allow the driver to make smooth transitions in driving actions.
Design Speed: Design speed is a selected speed used to determine various geometric features of the roadway. The selection of a design speed is influenced by topography, adjacent land use, and the functional classification. Design speed should be selected to attain a desired combination of safety, mobility, and
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions efficiency, within the constraints of environmental quality, economics, aesthetics, and social or political impacts.
Sight Distances: A driver’s ability to see ahead is needed for safe and efficient operation of a vehicle on a highway.
Stopping Sight Distance Drivers need to be able to see far enough ahead to react to hazardous objects on the roadway and to be able to stop before impact. Therefore, the design of the roadway must provide adequate stopping sight distance for the driver at all locations. AASHTO's definition of stopping sight distance consists of two components: the distance traveled by the vehicle during the driver's brake reaction time and the actual braking distance. A brake reaction time of 2.5 seconds is used for all drivers and for all conditions, but does not account for situations requiring complex decisions by the driver. Braking distance is based on a conservative deceleration rate of 11.2 ft/sec2 that accounts for wet pavements, worn tires, etc., but not for icy roads. Stopping sight distances, according the 2011 AASHTO policy, are shown in EXHIBIT 1, Stopping Sight Distance on Level Roadways.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
EXHIBIT 1
STOPPING SIGHT DISTANCE ON LEVEL ROADWAYS
Drivers sometimes do not drive slower on wet or icy pavement. Therefore, providing longer sight distances when possible adds to the safety of the roadway under adverse conditions.
Passing Sight Distance Passing sight distance is needed to safely provide for the passing maneuver on two-lane roadways where the faster vehicle must use the lane of opposing traffic. Because of the nature of the terrain in which the roads are constructed, it is not always possible or economically feasible to supply continuous passing sight distance. WYDOT attempts to fulfill the requirements for safe passing sight distance whenever practical. The more passing sight distance that can be provided for a roadway, the better and safer it will operate. See EXHIBIT 2, Passing Sight Distance For Design of Two-Lane Highways. Detailed sight distance descriptions can be found in the AASHTO A Policy on Geometric Design of Highways and Streets including discussion on the effect of grades on stopping sight distance, decision sight distance, and how it applies to trucks.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
EXHIBIT 2
PASSING SIGHT DISTANCE FOR DESIGN OF TWO-LANE HIGHWAYS
Traffic Volumes: Traffic volumes are a measure of the traffic usage for a particular segment of a road or a highway. Different derived traffic volumes are used in design for selection of typical sections, design speeds, surfacing thickness, etc. For the purposes of highway design and planning, WYDOT commonly uses projected 20-year traffic volumes from when the project is first initiated.
ADT (Average Daily Traffic) Derived from the total volume for a given time period divided by the number of days in that time period, and expressed in vpd (vehicles per day).
AADT (Annual Average Daily Traffic) Total number of vehicles traversing a section of roadway in a year divided by 365, the number of days in a year. The AADT are the traffic volumes employed in selecting design criteria such as number of lanes, shoulder widths, design speed, and expressed in vpd (vehicles per day).
DHV (Design Hourly Volume) Generally taken as the 30th-highest hourly volume of the year, or approximately 15 percent of the AADT, and is used to determine
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
what level of capacity or service a road should be designed to accommodate, and expressed in a percentage of the ADT. This value balances the difference between adequacy of design and economy of design.
DDHV (Directional Design Hourly Volume) Used in situations where traffic patterns, in different directions, vary significantly depending on time of day, such as high volume commuter traffic to and from a business center.
% of Trucks The percentage of the traffic stream that is comprised of buses, single-unit trucks and truck combinations. Percentage of trucks does not include light delivery trucks.
Traffic volumes and design volume projections are collected and/or prepared by the Planning Program - Transportation Surveys Section.
For projects with long design duration, the traffic count and projections should be updated prior to final design, if the ones used to select design parameters are somewhat outdated. The design should be reviewed against the updated traffic counts for compliance. If any aspect of design would be changed by the updated counts, the Project Development Engineer should be advised.
Level of Service: Level of service (LOS) is the concept used to describe operational conditions within a traffic stream and their perception by motorists. The designated level of service compares the existing or proposed roadway to the "ideal" conditions for that type of roadway. Level of service is defined by a letter designation from A to F, with A representing the best operating conditions and F the worst operating conditions. This definition describes the conditions encountered in terms of such factors as speed, travel time, maneuverability, traffic interruptions, comfort, convenience, and safety.
The AASHTO A Policy on Geometric Design of Highways and Streets defines the general operating conditions for levels of service as follows:
A - free flow, with low volumes and high speeds. B - reasonably free flow, but speeds beginning to be restricted by traffic
conditions. C - stable flow, but most drivers restricted in freedom to select their own
speed.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
D - approaching unstable flow, drivers have little freedom to maneuver. E - unstable flow, may be short stoppages. F - stop and go operation.
WYDOT uses a design year Level-of-Service (LOS) C as the appropriate LOS to warrant capacity improvements for Interstate mainline sections; a design year Level-of-Service (LOS) D has been selected as the appropriate LOS to warrant capacity improvements for Interstate interchange ramps and intersecting roads. A design year Level-of-Service (LOS) D has been selected as the appropriate LOS to warrant capacity improvements for NHS Arterial & Non-NHS State Highway rural and urban mainline sections. Achieving a specified LOS level in some situations may not be feasible, in such cases, a lower LOS may be acceptable. Further information on level of service can be found in the Highway Capacity Manual.
Capacity Analysis: A detailed discussion of capacity analysis can be found in the Highway Capacity Manual. The Highway Capacity Manual is defined as- "a set of procedures used to estimate the traffic carrying ability of facilities over a range of defined operational conditions. It provides tools for the analysis and improvement of existing facilities, and for the planning and design of future facilities." The Traffic Program will usually do the detailed capacity analysis required on a project.
Access Control: Access control is the regulation of ingress and egress from streets and highways by state or local authorities. Access ranges from fully controlled access on interstates and freeways, where preference is given to the mobility of through traffic, to partially controlled access on lower functional classification roads and highways, where the access needs of adjacent landowners receive nearly as much priority as mobility of the traveling public.
In general, access to state highways is regulated in accordance with the WYDOT Access Manual, and in accordance with WYDOT Operating Policy. All access points should be evaluated during the design. Access on non-interstate highways is permitted by the District and Traffic Program, and access on interstate highways may be allowed only with FHWA approval.
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
DESIGN STANDARDS
General: Design standards to be applied to the various combinations of roadway
classification and project types (Preservation, Rehabilitation, Reconstruction) are the current editions of:
AASHTO A Policy of Geometric Design of Highways and Streets WYDOT Design Guide, Interstate Highways WYDOT Design Guide, NHS Arterial (Non-Interstate) WYDOT Design Guide, Non-NHS State Highways WYDOT Road Design Manual WYDOT County Road Fund Manual
The WYDOT Design Guides are specific to the roadway’s functional classification and project type. It is important that the applicable book and section be referenced to ensure that correct design standards are being applied.
Functional Classification: Functional classification of roads and highways is a system by which roads are grouped according to the general function they provide. Functions provided by roads and highways, both urban and rural, include main (through) movement, transition (e.g. freeway ramps), distribution, collection, access, and termination. For the purposes of design, WYDOT recognizes the following functional classifications for both urban and rural facilities:
Interstate NHS Arterial (Non-Interstate) Non-NHS State Highways (arterial, collector, or local)
Definitions of the functional classifications for both urban and rural can be found in the AASHTO A Policy on Geometric Design of Highways and Streets. Functional classifications are established for the roadway by the Planning Program and are documented in the Reconnaissance Inspection Report. See EXHIBIT 3 for the National Highway System: Wyoming Map, and EXHIBIT 4 for the Design Functional Classification Map.
The website link to the National Highway System: Wyoming Map is:
http://www.fhwa.dot.gov/planning/national_highway_system/nhs_maps/wyoming/
The Design Functional Classification Map can be found as a downloadable PDF file on the Road Design Manual webpage.
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EXHIBIT 3
NATIONAL HIGHWAY SYSTEM: WYOMING
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Chapter 3 - Section 3-01 General Concepts, Design Standards, Design Exceptions
EXHIBIT 4
DESIGN FUNCTIONAL CLASSIFICATION
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