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DESIGN GUIDELINES & SAFETY 8 Kentucky Transportation Center, 1999

Flexibility in Design: 

Flexibility in Design

Learning Objectives: 

Learning Objectives Safety and roadway design Design principles

Old Excuse: 

Old Excuse

Basic Tenet: 

Basic Tenet “Well-designed” roads should Handle traffic efficiently and safely Fit nicely into their surroundings Address community’s needs and desires

In Other Words: 

In Other Words A project is successful if It works It’s safe People like it It was done on time and within budget

Safety & Roadway Design: 

Safety & Roadway Design Each design assumes a safety level Section 109, Title 23, USC Need for balancing of safety, mobility, economic considerations, protection and enhancement of natural environment, and preservation of community values Safety is not paramount over other considerations Tradeoffs among design, cost, and safety Aim to achieve a “reasonable” level of safety & service

Safety Levels: 

Safety Levels Nominal safety Measured as compliance to standards, warrants and guidelines Yes/No Substantive safety Measured by crash frequency and severity Continuous scale

Fix the Safety Problem…: 

Fix the Safety Problem…

Whose Safety?: 

Whose Safety?

A Safe Roadway?: 

A Safe Roadway? In the space below, please list what are the top three items that make a roadway safe

A Safe Roadway?: 

A Safe Roadway? Wide roads with medians Grade separated intersections Smooth roads Proper signing Driver expectancy Proper vert & horiz design values Good drivers Road with no cars High design standards

Geometric Features & Safety--1: 

Lane Width, 2L2W Rural Roads Geometric Features & Safety--1

Geometric Features & Safety--2 : 

Shoulder Width, 2L2W Rural Roads Geometric Features & Safety--2

Geometric Features & Safety--3: 

Addition of turning lanes/bays Traffic islands Roadway improvements & Safety Report KTC-96-13 Geometric Features & Safety--3

Traffic Islands: 

Traffic Islands

Traffic Control & Safety-1: 

Traffic Control & Safety-1

Traffic Control & Safety-2: 

Traffic Control & Safety-2







Hard Choices: 

Hard Choices

Alternative Solutions: 

Alternative Solutions Avoid Minimize Mitigate Enhance And if none of these work...

Design Exception: 

Design Exception Uses in lane & shoulder widths, clear zones, design speeds, median widths and types, right-of-way… Could be avoided with the use of creative design

Creative Design: 

Creative Design Alternative design speeds Design consistency Radii & superelevation combinations Slopes for clear zones Innovative traffic alternatives Horizontal & vertical alignment as a unit

Left-turn Treatments: 

Median U-turns Left-turn Treatments

2+1 Lanes: 

2+1 Lanes

3-D Alignment: 

3-D Alignment

Case Study: 

Case Study Given the “design commitments” and your area of concern: Determine possible safety concerns within the project; & Identify actions to resolve these concerns. Q1

Design Choices: 

Functional class Design speed Corridor Cross section Design Choices

Functional Class: 

Functional Class Arterial Collectors Locals

Functional Class--1: 

Functional Class--1 UPA

Functional Class--2: 

Functional Class--2 UPA

Functional Class--3: 

Functional Class--3 UMA

Access Management: 

Access Management Concerns with Safety Capacity Delays Environmental impacts

Design Speed: 

Design Speed A selected speed used to determine the various geometric features of the roadway… as high a design speed as practical to attain a desired degree of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts Source: A Policy on Geometric Design of Highways and Streets, AASHTO 2001

Design Speeds (mph): 

Design Speeds (mph) Source: AASHTO Policy on Geometric Design of Highways and Streets, 2001

Speed Limit: 

Speed Limit Design speed NOT Speed limit Speed Limit approximately 85th-percentile operating speed Local policies

Design speed-Speed Limit: 

Design speed-Speed Limit 35

Operating Speed: 

Operating Speed Desired speed for driver Design speed = Operating speed Design Consistency

Design/Operating Speeds: 

Design/Operating Speeds UPA

Cross Section: 

Cross Section Number of lanes Width of lanes Median Shoulder Clear zones Sidewalks/Bikeways Transit/Parking

Cross Section Tradeoffs: 

Cross Section Tradeoffs

Level of Service--1: 

Level of Service--1 Rating of acceptable level of congestion LOS definitions A: Free flow, low traffic B: Stable flow, noticeable traffic C: Stable flow, traffic interactions D: High density, movement restrictions E: Maximum flow, capacity, little freedom F: Unstable flow, no freedom, stop & go

Level of Service--2: 

Rural Urban Arterial B/C C Collector C/D D Local D D Level of Service--2 Function of Facility type Location Desirable design values

Level of Service--3: 

Level of Service--3

Self-Enforcing, Self-Explaining Road: 

Self-Enforcing, Self-Explaining Road

Demand vs Capacity: 

Demand vs Capacity

Only Autos?: 

Only Autos?

Cross Section Tradeoffs-2: 

Cross Section Tradeoffs-2 TO

Public Input: 

Public Input

Existing Conditions: 

Existing Conditions


Summary Flexibility in applying guidelines “Bigger” not always safer Design exception process Innovative traffic solutions Technology advancements

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