Context Sensitive Design

1. Context Sensitive Design A.K.A. The “Think” Method of Design Howard Preston, P.E. Senior Transportation Engineering CH2M HILL

2. Agenda • Definition of the “Think” Method of Design • Overview of the Design Process • Safety Issues • Questions and Answers

3. The “Think” Method of Design Is based on: • The design guides in AASHTO documents allow engineers a great degree of flexibility and were not intended to be absolute requirements. • Road improvements are an attempt to mitigate some kind of deficiency in the transportation system. • In order to develop and evaluate alternative mitigation strategies, need to define the characteristics of the deficiencies.

4. The “Think” Method of Design (continued) • Purpose and need for a project. • Design / Safety relationship – currently lack the definitive tool to assess / quantify safety effects, but some help is available. • A reasonable balance between design consistency, safety, aesthetics, environment and community goals and objectives.

5. Flexibility vs. Absolute Requirements The Forward to the AASHTO Green Book encourages designers to be flexible and to develop solutions tailored to particular situations. “Unique combinations of requirements that are often conflicting result in unique solutions to the design problems.”

6. Flexibility vs. Absolute Requirements (cont.) “Sufficient flexibility is permitted to encourage independent designs tailored to particular situations.” The Forward also suggests that the design concepts in the Green Book should be considered as guidance as opposed to standards. “These guidelines are intended to provide…”

7. Environmental & Community Considerations The Green Book also encourages designers to be aware of and sensitive to environmental issues. “…highway engineers strive to provide for the needs of highway users (safety & efficiency) while maintaining the integrity of the environment.” “These design concepts were also developed with consideration for environmental quality. The effects of environmental impacts can and should be mitigated by thoughtful design processes.”

8. Balance Designers are encouraged to find a “Reasonable” balance between design consistency, safety, aesthetics, environmental issues and community goals. How do you find this point? Aesthetics Safety Environmental Issues Design Considerations Community Goals

9. Balance(continued) Consider the primary function of the road – the balance point on a road classified as an Arterial (Primary Function = mobility) would likely be different than on a collector or local street (Primary Function = access).

10. Basic Design Process Develop Multiple Alternatives Screen to Fewer Alternatives The Build Alternative C.S.D.

11. Project Goals & Objectives Performance Measures • Level of Service • Traffic Operating Speed / Mobility • Safety • Accessibility • Design Guidelines If you don’t measure performance, you can’t replicate what works or avoid what doesn’t.

12. LOS Diagram • Not prescribed by FHWA. • Absolutely necessary to link traffic volume and geometry to the quality of traffic operations Index of Congestion

13. Mobility • Metropolitan Council Guidance – 45 mph • Mn/DOT Interregional Corridor Goals - 55 or 60 mph

14. Safety • Achieve a level of traffic safety in the study area that results in crash rates at intersections and along roadway segments that are at, or below appropriate averages for similar facilities. Intersections Crash Rates Unsignalized 0.4 Crashes/MV Signalized 0.8 Crashes/MV Segments Crash Rates Rural Two-Lane 1.0 Crashes/MVM Urban Four-Lane Divided 4.0 Crashes/MVM Source: Mn/DOT Traffic Safety Fundamentals Handbook

15. Identify Deficiencies Compare Actual Conditions To Goals & Objectives Deficiencies Countermeasures & Strategies

16. Geometric Design Guidelines Always start with and document the recommended values for each by design feature. • Design Speed • Horizontal Alignment • Vertical Alignment • Lane / Shoulder Widths • Turn Lane / Taper Lengths • Super-elevation Rates

17. Geometric Design Guidelines(continued) Documenting an understanding of the recommended guidelines is a critical step in bringing any future decisions (to vary from the guidelines) under an umbrella of immunity. In addition to documenting the recommended guidelines for each design feature, also identify a project specific objective for each key feature.

18. Geometric Design Guidelines(continued) For Example: • Design Speed Guidance – Minimum Recommended Design Speed = 30 to 40 mph for Off –System Bridges Objective – Provide a design speed that is consistent with roadway function, other features along the same segment of roadway and with the roadway environment.

19. Variance for Design Speed / Vertical Curve East Gull Lake Example • Existing Conditions • Local Street / Residential Area • Curvilinear Alignment – 20-25 mph operating speeds Replace Existing Wooden Bridge w/Concrete Box Culvert 25 mph design

20. East Gull Lake Example Approach to bridge leading to island

21. East Gull Lake Example Top of bridge looking toward mainland

22. East Gull Lake Example (cont.) 20 mph Crest VC > 20 mph Crest VC 20 mph Sag VC • Design Guidelines – Design Speed of 30-40 mph minimum • Project Objectives – Replace the bridge, provide a consistent design speed and minimize environmental impacts • Rejected 30 mph design – required fill in the lake and inconsistent with the rest of the roadway • Implemented 20 mph design – no fill in the lake and consistent with Project Objectives • Additional Mitigations – Warning signs and street lights 25 mph Sag VC 20 mph Sag VC Existing Conditions 25 mph Sag VC 20 mph Design Proposed Design 30 mph Design

23. Development of Alternatives • Solve the Problem / Consistent with Project Goals & Objectives. • Consider a range of alternatives. • Match the magnitude of the solution to the magnitude of the problem. • Step back and look beyond the edge of the pavement.

24. Geometric Design Guidelines(continued) • Turn Lane Length Guidance – 300 feet of full width and 180 feet of taper Objective – Provide sufficient length to accommodate deceleration and storage.

25. Variance from Left Turn Lane and Taper Length TH 61 in Hastings • Before Condition • 4-lane undivided • High Crash Rate – 13.8 crashes/MVM • High frequency of rear end (left turn) crashes

26. Variance from Left Turn Lane and Taper Length TH 61 in Hastings • Alternative 1 • 4-lane Divided / Raised median • 300 foot Left Turn Lanes & 180 foot Tapers • Required closing access to every other city street • This alternative was REJECTED and MnDOT asked to leave town

27. Variance from Left Turn Lane and Taper Length TH 61 in Hastings • Alternative 2 • 4-lane Divided / Raised median • 125 foot Left Turn Lanes & 60 foot Tapers • All public street intersections remained open • Project was APPROVED and constructed • The raised median and exclusive Left Turn Lanes reduced crashes by 44%

28. Process for Considering Variances From Recommended Design Guides • Start with and document the recommended values. • Identify project goals and objectives. • Document the consequences of implementing the recommended design values. • Identify the design alternatives, advantages / disadvantages and any safety consequences. • If there are safety consequences, identify and consider potential mitigation strategies – additional warning devices, street lights, guardrail, etc. • Document the entire evaluation process.

29. Safety Issues Always consider / document safety issues – what you don’t know could be used against you later. Understand the design – safety connection. Research has established a relationship between some design features and crash rates. • Alignment • Shoulders • Clear Road Sides • Turn Lanes • Access Density

30. Design – Safety Connection For Example: • Alignment – curvilinear alignments have higher crash rates and a higher frequency of run off the road crashes. Source: Traffic Safety Fundamentals Handbook

31. Design – Safety Connection • Shoulders – paved shoulders on rural roads reduce single vehicle and total crash rates. Crash Rate comparison of 2-Lane Rural Roadways with gravel shoulders and paved shoulders at least 4’ wide. 15% Reduction Source: A Comparison of Gravel & Bituminous Shoulders on 2 Lane Rural Roads, MnDOT 2/1978 Source: Accident Rates vs. Shoulder Width, California DOT

32. R/W Traffic Lanes Shoulder Slope Shoulder Design – Safety Connection Clear Zone • Single vehicle crashes are the most common type on rural roads (21%). • Single vehicle crashes account for 67% of all rural fatalities and 33% of all fatal crashes. • Trees cause more deaths than any other fixed object. Hennepin County Traffic Engineering Workshop – April 2000 “Roadside Safety”

34. Design – Safety Connection • Turn Lanes – left turn lanes on urban arterials reduce rear end and total crashes. Crash Rate (per MVM) Source: Traffic Safety Fundamentals Handbook

35. Design – Safety Connection • Access Density- There is a positive relationship between access density and crash rates => Higher levels of Access Density resulted in Higher Crash Rates Source: Statistical Relationship Between Vehicular Crashes and Highway Access MnDOT Research Report No. 1998-27

36. Safety Issues (cont.) If you must consider variances from the design guides, document the expected effect on safety and evaluate additional safety strategies. For example, if you cannot provide a clear roadside, consider additional ways to help vehicles stay on the road: • Paved shoulders • Shoulder rumble strips • Durable pavement markings • Delineators • Street lights

37. SummaryThe “Think” Method of Design • If you are concerned that the phrase “Context Sensitive Design” is too trendy, take comfort in the fact that the theories behind it are firmly rooted in the “Think” Method of Design. • A.A.S.H.T.O. design documents clearly support the Think Method of Design and encourage designers to solve highway problems in a way that balances design, safety, environmental and community goals and objectives. • Understanding the magnitude and characteristics of the problem is a critical first step in developing project specific alternative mitigation strategies. • Always consider the issue of safety and attempt to understand the design – safety connection.

38. Summary(continued) • Thoroughly document the project development process. -Goals & Objectives -Deficiencies -Alternatives -Evaluating Criteria -Selected Improvement Strategy – Variances from the Design Guides • Be aware of but not overly concerned about tort liability. Community Involvement