Geometric Design Guidelines for Very Low-Volume Local Roads

1. Local Government Section • Welcome • Marty Andersen • ODOT Local Government Section • 355 Capitol Street NE, Rm. 326 • Salem, Oregon 97301 • Ph: 503-986-3640 • E-mail: martin.e.andersen@odot.state.or.us

2. Geometric Design Guidelines for Very Low-Volume Local Roads (< 400 ADT)

3. Course Objective • History • Scope of Low Volume Roads • Frame work for Design Guidelines • Design Philosophy • Design Guidelines • Design examples

4. 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

5. History NCHRP Reports • NCHRP Reports: • 362 Roadway Widths for Low-Traffic-Volume Roads • 383 Intersection Sight Distance • 400 Determination of Stopping Sight Distance

6. 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

7. 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

8. 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

9. Design Guidelines • design guidelines are stratified by: • functional subclasses • design/operating speed • traffic volume

10. 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

11. Functional Classification SUBCLASSES OF URBAN VERY LOW-VOLUME LOCAL ROADS • urban major access streets • urban residential streets • urban industrial/commercial access streets

12. Collectors • major access road • urban major access street

13. Design/Operating Speed • Low speed – 0 to 45 mph • High speed – 50 mph or over

14. Traffic Volumes • 100 veh/day or less • 100 to 250 veh/day • 250 to 400 veh/day • Consideration of Vehicles is important

15. 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

16. 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”

17. New Construction vs. Existing Roads NEW CONSTRUCTION • 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

18. 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

19. 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

20. 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

21. Additional risk is site specific • ‘Substandard’ radius horizontal curve • Location with ‘substandard’ clear zone • Substandard lane or shoulder width

22. 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 0.135. Difference is 0.135 - 0.105 = 0.03 crashes per 5 years, or 1 additional crash per 165 years of operation at 250 vpd

23. Geometric Design Criteria • cross section • bridge width • horizontal alignment • stopping sight distance • intersection sight distance • roadside design • unpaved roads • two-way single lane roads

24. 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

25. Cross Section New Construction Rural

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. Horizontal Alignment (example) 55 50 0.140 925 1190 AASHTO 0.130

33. 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.

34. 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

35. 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

36. 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

37. 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

38. 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

39. Consider Barrier?

40. 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

41. 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

42. 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

43. 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.

44. THANK YOU