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Local Government Section
Welcome Marty Andersen ODOT Local Government Section 355 Capitol Street NE, Rm. 326 Salem, Oregon 97301 Ph:
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Geometric Design Guidelines for Very Low-Volume Local Roads (< 400 ADT)
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Course Objective History Scope of Low Volume Roads
Frame work for Design Guidelines Design Philosophy Design Guidelines Design examples
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History The National Cooperative Highway Research Program Studies (NCHRP) Guideline document prepared in NCHRP Project 20-7(108) T.R. Neuman CH2M Hill Guidelines were published in 2001 Included by reference in 2004 AASHTO
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History NCHRP Reports NCHRP Reports:
362 Roadway Widths for Low-Traffic-Volume Roads 383 Intersection Sight Distance 400 Determination of Stopping Sight Distance
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Scope A very low-volume local road is a road that is functionally classified as a local road and has an ADT of 400 vpd or less Local refers to functional class, not to type of highway agency Both rural and urban roads included Low-volume collectors may be included
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An Important Perspective-- The Very Low Volume Local Road Operating Environment
Thousands of miles to construct, reconstruct and maintain Environmental impacts Local (functionally) means familiar drivers For very low volume roads (<400 vpd) the concept of “high accident” locations is meaningless Limited resources Concerns about design process, design decisions and tort liability
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Intent of Very Low Volume Local Road Criteria
Provide some basis for design guidance Provide greater flexibility to designers Reflect cost-effectiveness, with the sole focus being costs and measurable safety (crash frequency and severity
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Design Guidelines design guidelines are stratified by:
functional subclasses design/operating speed traffic volume
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Functional Classification
SUBCLASSES OF RURAL VERY LOW-VOLUME LOCAL ROADS major access roads minor access roads industrial/commercial access roads agricultural access roads recreational and scenic roads resource recovery roads
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Functional Classification
SUBCLASSES OF URBAN VERY LOW-VOLUME LOCAL ROADS urban major access streets urban residential streets urban industrial/commercial access streets
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Collectors major access road urban major access street
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Design/Operating Speed
Low speed – 0 to 45 mph High speed – 50 mph or over
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Traffic Volumes 100 veh/day or less 100 to 250 veh/day
Consideration of Vehicles is important
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Design Philosophy Less restrictive design criteria can be used on very low-volume local roads because: lower traffic volumes present substantially reduced opportunities for multiple-vehicle collisions most drivers are familiar with the roadway an are less likely to be surprised by geometric elements
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Design Philosophy Implied by AASHTO Criteria
Provide a “Margin of Safety” Wide ranges in potential traffic conditions, drivers and vehicles Reflect safety, traffic operational quality (“comfort and convenience”), constructability and maintainability Not strictly based on cost-effectiveness Provide consistent, minimal “quality”
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New Construction vs. Existing Roads
Design to very low-volume local road criteria based on risk assessment Designer has great flexibility in exercising judgment to deviate from numerical design criteria
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New Construction vs. Existing Roads
Includes reconstruction, rehabilitation, restoration, and resurfacing Retain current geometrics unless there is evidence of a site-specific safety problem If there is an identified safety problem, upgrade at least to very low-volume local road criteria There is flexibility in upgrading criteria as well
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What is Evidence of a Site-Specific Safety Problem?
adverse accident history (5 to 10 years) skid marks or roadside damage noted in field reviews speeds higher than design speed concerns raised by police, fire or local residents
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Risk Assessment Urban Low-speed Facilities
1 additional crash per mile every 4 to 6 years based on proposed project is acceptable Rural or Urban High Speed Facilities 1 additional crash per mile every 6 to 9 years based on proposed project is acceptable
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Additional risk is site specific
‘Substandard’ radius horizontal curve Location with ‘substandard’ clear zone Substandard lane or shoulder width
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Illustration of Risk Assessment -- Horizontal Curve Radius
A given curve on a roadway with 250 vpd is designed according to Green Book criteria for a design speed of 50 mph, with a central angle of 20 degrees. What is the estimated safety risk of accepting a smaller radius curve equivalent to a speed of 40 mph per Green Book criteria? Per Zegeer’s model for horizontal curves (FHWA research), the expected 5-year crash frequency for an 50 mph curve is 0.105; and for a 40 mph curve is Difference is = 0.03 crashes per 5 years, or 1 additional crash per 165 years of operation at 250 vpd
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Geometric Design Criteria
cross section bridge width horizontal alignment stopping sight distance intersection sight distance roadside design unpaved roads two-way single lane roads
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Cross Section New Construction Rural
Based on the functional subclass of the road Varies from 18 to 26 feet. Is based on total width not on lane width & shoulder Designer has flexibility to use discretion for choosing widths There is a table for this on page 18 of the Low volume guide book Vehicle type needs to be considered
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Cross Section New Construction Rural
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Cross Section New Construction Urban
Follows the same functional subclass except for Residential Parking will have a greater impact on urban sections Speed typically are lower Designer has flexibility to use discretion for choosing widths Urban residential is based on development density
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Bridge Widths New Construction
Bridge should be equal to roadway plus 2 feet If shoulder is paved than match the total width Bridges longer than 100 feet should be evaluated to determine the width Type of vehicle should be considered One lane bridges can be considered for <100 ADT One lane bridges need turn outs One lane width between 15 & 16 feet
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Bridge Widths Existing Bridges
Remain the same width unless there is a evidence of a safety problem Replacement of existing bridge can remain the same width unless there is evidence of a safety problem Even if there is a shift in the alignment Should consider the vehicle types with replacement bridges
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Horizontal Alignment Radius of curves are based on friction factor, vehicle speed and super elevation The guide uses the concept that the AASHTO method was developed for driver comfort and there is substantial safety factors built in Radius is determined based on functional subclass, design speed and ADT < or > 250
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Horizontal Alignment Radius of curves are based on friction factor, vehicle speed and super elevation The guide uses the concept that the AASHTO method was developed for driver comfort and there is substantial safety factors built in Radius is determined based on functional subclass, design speed, Super elevation and ADT < or > 250
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Horizontal Alignment Guide uses a concept of reduced design speed and friction factor for selection of the radius Reductions are from 0 to 10 MPH depending on the sub classification and ADT Super elevation transitions are base on AASHTO Policy on Geometric Design of Highways and Streets
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Horizontal Alignment (example)
55 50 0.140 AASHTO 0.130
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Sight Distance Sight distance used is dependent on the ADT and risk of location Maneuver sight distance is use to calculate for the lower volume roads with low safety risk Low risk are locations away from intersections, narrow bridges, railway grade crossings sharp curves and steep grades.
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Sight Distance Use the design site distance chart on page 34 to determine minimums Two curve tables one for Horizontal curves one for crest vertical curves Sag curves are to be design from the AASHTO Policy on Geometric Design of Highways and Streets Existing roads do not require site distance improvements unless safety problem is identified
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Intersection Sight Distance
Establish clear sight triangles Approach sight triangles and departure sight triangles If any leg of an intersection is over 400 ADT use AASHTO Policy on Geometric Design of Highways and Streets Traffic control methods and speed will determine the sight distance needed
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Roadside Design There are two key aspects to roadside design
Clear zone Traffic barrier warrants It has been found that on low volume roads it is not generally cost effective to provide clear zones or traffic barriers
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Clear Zones Provide 6 ft. or more at low cost, minimal social/environmental impacts Provide 0- 6 ft. where there are constraints such as cost, terrain, right of way or social/environmental impacts Designer must use judgment and consider the conditions and existing safety problems
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Barriers Barriers are a road side obstacle
Vehicle impacts with barriers produce injures Engineer may use judgment in the placement of barriers Barriers should be considered where departure would likely be extremely severe
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Consider Barrier?
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Unpaved Roads Crash rates are higher for unpaved roads
Crash rates increase at high volumes and there is some evidence at 300 ADT paving should be considered Narrower unpaved roads have lower crash rates Design speeds should generally not exceed 45 mph
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Unpaved Roads Can function find for all functional subclasses at low volumes Geometric design needs have a through review if paving an existing unpaved road because of anticipated higher speeds
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Design Examples There are eight design examples in the back of the Guide starting on page 53 When using this guide as the design parameters the designer should include something similar to these examples as part of the documentation
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Conclusion Very low volume roads have low safety risk even though the accident rates may be higher than other roads Only low cost safety elements should be considered unless evidence of a problem Designers have flexibility with in the guidelines Considerable savings in cost and impacts result when this guide is used for low volume roads.
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THANK YOU
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