1. Local Government Section
Welcome
Marty Andersen
ODOT Local Government Section
355 Capitol Street NE, Rm. 326
Salem, Oregon 97301
Ph:

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
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
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
Difference is = 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
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