Download - Smart growth 2011 street network and safety
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Traffic Safety Traffic Safety and the Smart Growth Street Networkand the Smart Growth Street Network
Norman W. GarrickUniversity of Connecticut
Wesley MarshallUniversity of Colorado - Denver
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What Does a Smart Growth Street Network look Like?
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Or this?
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Or this?
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Or this?
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Characterizing Street Networks
Streets networks are devilishly complicated
• The possible variations are infinite and even subtle differences can have a huge impacts
• Street networks simultaneously operate at numerous geographic scales while serving many different – some times conflicting - functions
• The common descriptors are not all that good for illuminating the differences between networks
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California Cities Study of Street NetworksDoes the Street Network Matter?
Twenty-four Cities
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Davis, CA14 % of people ride to work
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Davis, CA
Road Fatality Rate: 1 per 100,000
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Road Fatality Rate for All 157 California Cities Over 40,000
number per 100,000 population
0.0
17.3
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Road Fatality Rate for All 157 California Cities Over 40,000
number per 100,000 population
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Risk of Fatality
(Fatalities as % of Injuries) California Cities of 40,000 to 120,000
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Chance of Pedestrian Fatality vs. Impact Speed
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Adapted from Stephen Marshall
Pre-1950’s Post-1950’s
Evolution of the Street Network
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FHA Technical Bulletin No. 7 (1938)Planning Profitable Neighborhoods
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According to the FHA the grid layout was
MonotonousHad Little CharacterUneconomicalPosed Safety Concerns
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Research
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Evolution of the Street Network
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Characterizing the the Street Network
Shape and Configuration
Street Network Scale
Street Network Connectivity
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Citywide Street NetworkN
eig
hb
orh
ood
Str
eet
Netw
ork
Linear Tree GridTributary Radial
Gri
dTre
e
Adapted from Stephen Marshall, Streets & Patterns
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550 Intersections per Square Mile
Network Scale
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110 Intersections per Square Mile
Network Scale
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45 Intersections per Square Mile
Network Scale
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Link-to-Node Ratio = 1.61
Link-to-Node Ratio = 1.13
Link-to-Node Ratio = 1.16
Network Connectivity
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Street Network Properties
Street Design Properties
Average Total Number of LanesAverage Outside Shoulder WidthRaised MedianPainted MedianOn-Street ParkingBike LanesRaised Curbs
Travel and Activity Level
Distance from City Center
Income
Mix of Land Use
Variables included in Our Safety and Travel Choice Models
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Safety Analysis Based on Geo-coding 230,000 Accident Recordsin 24 California Cities
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Safety and Travel Choice Analysis done for 1040 Census Block Groups 24 California Cities
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versus
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versus
Risk of Severe Injury or Fatality*
Chance of being Severely Injured
30% Higher
Chance of being Killed
50% Higher
*Given that an injury occurred
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Odds of Dying in a Road Accident based on Intersection Density*
1 in 200
1 in 500
*Given that an injury occurred
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Does Street Network Matter for Safety?
The street network plays a huge role in terms of traffic safety
1. Street density seems to be the most important factor2. Street network configuration is also important3. The results with respect to ‘connectivity’ are a bit more complicated
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What About Connectivity?
Street pattern types that are more connected are safer
Safer Less Safe
Within each type of street pattern increasing connectivity – measured by link-to-node ratio - was not correlated with improved safety
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Clipping the Grid in BerkeleyVery high connectivity can be a mixed blessing
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Why Does Street Network Matter?Why does street network matter for safety?Understanding this issue is important for helping us to understand how to develop a
new approach for designing street network
Street network affects• Speed control• Travel mode choice and VMT• Traffic distribution
Speed control reduces accident severity
More diverse mode choice and good traffic distribution reduces the demand for road capacity and means that we are more likely to keep the streets to a human scale
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Design for Safety
The two most important characteristics of the street network in designing for safety are
1. Dense (Fine Grained) Fabric2. Functional Connection for walking within and between
neighborhoods
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Connected?Yes… based on convention matrices
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Functionally Connected for Walking?No…. Each neighborhood is an isolated pod
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Traffic Safety and Smart Growth
The same street design features that support traffic safety also– Promote use of active transportation– Reduce VMT– Create value
In other words, a safe street network is also a smart growth street network
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Percentage of People Walking, Biking or Taking Transit
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Percentage of People Walking, Biking or Taking TransitEffect of Intersection Density for Cul-de-sac Network
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Percentage of People Walking, Biking or Taking TransitEffect of Intersection Density for Gridded Network
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Street Networks for Smart Growth
There is no one type of ‘smart growth street network’
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Smart Growth Street Network?
Yes
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Smart Growth Street Network?
No
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What is a Smart Growth Street Network?
No
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Smart Growth Street Network?
Yes
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Connectivity Standards
A handful of jurisdictions have enacted connectivity standards
Connectivity standards by themselves will not necessarily produce safer communities or smart growth communities
We need more focus on the issues of street network scale and functional connectivity
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After years of ignoring the role of street network on traffic safety we are beginning to build a body of knowledge – we need to embrace the complexities of the issues
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Great cities are built on the foundation of a Great Street Network