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Context Sensitive DesignContext Sensitive Design

A.K.A.

The “Think” Method of Design

Howard Preston, P.E.

Senior Transportation Engineering

CH2M HILL

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AgendaAgenda

1. Definition of the “Think” Method of Design

2. Overview of the Design Process

3. Safety Issues

4. Questions and Answers

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The “Think” Method of DesignThe “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.

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The “Think” Method of Design (continued)

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.

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Flexibility vs. Absolute Requirements

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.”

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Flexibility vs. Absolute Requirements (cont.)

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…”

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Environmental & Community Considerations

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.”

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BalanceBalance

Designers are encouraged to find a “Reasonable” balance between design consistency, safety, aesthetics, environmental issues and community goals.

How do you find this point?

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Balance(continued)

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).

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Basic Design ProcessBasic Design ProcessS o lve th e P ro b le m !

D e fic ie n c ies

C o n s tru c tion

F in a l D e s ign

P re lim in ary D e s ign

C o n ecp t D e s ign

G o a ls & O b je c tives P u b lic Inp u t

B a s ic D e s ig n P ro ce ss

Develop Multiple Alternatives

Screen to Fewer Alternatives

The Build Alternative

C.S.D.

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Project Goals & ObjectivesProject Goals & Objectives

• Level of Service

• Traffic Operating Speed / Mobility

• Safety

• Accessibility

• Design Guidelines

Performance Measures

If you don’t measure performance, you can’t replicate what works or avoid what doesn’t.

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LOS DiagramLOS Diagram

Index of Congestion

• Not prescribed by FHWA.

• Absolutely necessary to link traffic volume and geometry to the quality of traffic operations

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MobilityMobility

• Metropolitan Council Guidance – 45 mph

• Mn/DOT Interregional Corridor Goals - 55 or 60 mph

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SafetySafety

• 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

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Identify DeficienciesIdentify Deficiencies

Compare Actual Conditions

To

Goals & Objectives

Deficiencies Countermeasures&

Strategies

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Geometric Design GuidelinesGeometric 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

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Geometric Design Guidelines(continued)

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.

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Geometric Design Guidelines(continued)

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.

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Variance for Design Speed / Vertical Curve

Variance for Design Speed / Vertical Curve

Existing Conditions

•Local Street / Residential Area

•Curvilinear Alignment – 20-25 mph operating speeds

25 mph design

East Gull Lake Example

Replace Existing Wooden Bridge w/Concrete Box Culvert

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East Gull Lake ExampleEast Gull Lake Example

Approach to bridge leading to island

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East Gull Lake ExampleEast Gull Lake Example

Top of bridge looking toward mainland

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East Gull Lake Example (cont.)East Gull Lake Example (cont.)

• 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

20 mph Crest VC

20 mph Sag VC

25 mph Sag VC

> 20 mph Crest VC

20 mph Sag VC

25 mph Sag VC

30 mph Design20 mph Design

Existing Conditions

Proposed Design

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Development of AlternativesDevelopment 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.

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Geometric Design Guidelines(continued)

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.

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Variance from Left Turn Lane and Taper Length

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

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Variance from Left Turn Lane and Taper Length

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

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Variance from Left Turn Lane and Taper Length

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%

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Process for Considering Variances From Recommended

Design Guides

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.

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Safety IssuesSafety 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

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Design – Safety ConnectionDesign – 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

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Design – Safety ConnectionDesign – Safety Connection

• Shoulders – paved shoulders on rural roads reduce single vehicle and total crash rates.

1.1

0.94

0.85

0.9

0.95

1

1.05

1.1

Gravel Paved

Crash Rate comparison of 2-Lane Rural Roadways with gravel shoulders and paved shoulders at least 4’ wide.

Source: A Comparison of Gravel & Bituminous Shoulders on 2 Lane Rural Roads, MnDOT 2/1978

Source: Accident Rates vs. Shoulder Width, California DOT

15% Reduction

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Design – Safety ConnectionDesign – Safety Connection

• 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.

Traffic Lanes

Shoulder

Shoulder Slope

R/W

Hennepin County Traffic Engineering

Workshop – April 2000

“Roadside Safety”

Clear Zone

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Design – Safety ConnectionDesign – Safety Connection• Turn Lanes – left turn lanes on urban arterials

reduce rear end and total crashes.

5.9

4.0 4.04.7

0.0

1.0

2.0

3.0

4.0

5.0

6.04-

Lan

eU

ndiv

ided

3-L

ane

4-L

ane

Div

ided

5-L

ane

No Left Turn Lanes Left Turn Lanes

Cra

sh R

ate

(per

MV

M)

Source: Traffic Safety Fundamentals Handbook

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Design – Safety ConnectionDesign – Safety Connection

• Access Density-

There is a positive

relationship between

access density and

crash rates => Higher

levels of Access

Density resulted in

Higher Crash RatesSource: Statistical Relationship Between Vehicular Crashes and Highway Access MnDOT Research Report No. 1998-27

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Safety Issues (cont.)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

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SummaryThe “Think” Method of Design

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.

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Summary(continued)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