Sierra Club Home Page   Environmental Update  
chapter button
Explore, enjoy and protect the planet
Click here to visit the Member Center.         
Search
Take Action
Get Outdoors
Join or Give
Inside Sierra Club
Press Room
Politics & Issues
Sierra Magazine
Sierra Club Books
Apparel and Other Merchandise
Contact Us

Join the Sierra ClubWhy become a member?

Stopping Sprawl campaign home page - click here.
Get an overview. Sign up for an e-newsletter. Find out what you can do to help.
Backtrack
Environmental Update Main
Sprawl Main
In This Section
Sprawl Overview
Reports & Factsheets
Activist Resources
Get Involved!
Communities
Transportation
Articles & Research
Population and Sprawl

Get The Sierra Club Insider
Environmental news, green living tips, and ways to take action: Subscribe to the Sierra Club Insider!

Subscribe!

Stop Sprawl
Streets And Traffic

Recent studies have shown that narrow streets slow traffic and reduce vehicular crashes, increasing neighborhood safety. Following is a summary of a study by Peter Swift, and an email from Patrick Siegman.

(More information available on Peter Swift's website.)

Swift's studies were on residential streets where high mileage provides more records than rural streets. But these studies could have significant implications to any area with distractions such as scenery or pedestrians. The study supports the thesis that narrower streets and/or winding roadways (or traffic lanes) slow traffic, reducing the frequency of accidents, and the severity of any crashes that do occur. While not tested, these conclusions may even apply to interstates and freeways. --John Holtzclaw


Residential Street Typology and Injury Accident Frequency

by Peter Swift

ABSTRACT

Communities all across the U.S. are concerned about the safety of their residential streets. Although this concern is nearly universal, the literature offers few precedents and little information on the relative safety of common residential street typologies. This study offers a method for analyzing the theory that the physical design of streets impacts safety. Through research, systematic observation, and statistical analysis, this study attempts to identify the safest residential street form with respect to several physical characteristics. These findings expose issues that need to be addressed by practitioners and policy-makers, and encourage further study of related topics.

Approximately 20,000 police accident reports from the City of Longmont, Colorado were reviewed and compared against five criteria that would signify the probability that the street design contributed to the accidents. Once catalogued and entered into a database, each accident location was mapped and described by thirteen physical characteristics. Comparing injury accidents per mile per year against other factors, several correlations were explored. The most significant relationships to injury accidents were found to be street width and street curvature. The analysis illustrates that as street width widens, accidents per mile per year increases exponentially, and that the safest residential street width is 24 feet (curb face).

I. Introduction
II. Data Gathering and Mapping
III. Accident Location Observation
IV. Statistical analysis and Correlations

Using this regression, a typical 36 foot wide residential street has 1.21 a/m/y (Ed: accidents/mile-year) as opposed to 0.32 for a 24 foot wide street, the street with the least a/m/y. This is about a 400 percent increase in accident rates. The a/m/y for a 30 foot wide street is 0.36. It appears that the group of streets with the safest results occur between 22 and 30 feet in curb face width.

There was another interesting observation made concerning this study. Figure 2 is a graph of street width vs. ADT (Ed: average daily traffic). It shows a clustering of accidents below 1,000 ADT and between street widths of 36 to 44 feet. The most intense portion of accident frequency lies below an ADT of 500. This indicates that more accidents occur on wide streets that have low daily volumes.

It also appears that a greater number of accidents occur on straight, rather than curvilinear streets. There seems to be a nexus with these observations and Farouki's conclusion (see note 1, below) that there is a greater number of accidents with an increase in vehicular speed. It is logical to assume that vehicular speeds increase with straight, wide streets of low ADT contributing to more severe accidents.

It should be mentioned here that it is not the conclusion of the authors that straight streets in and of themselves have a greater accident rate. If a straight street is narrow and includes parking on both sides, the opportunity for decreasing a/m/y is enhanced.

V. Conclusion

This study indicates a clear relationship between accident frequency and street width and curvature. The findings support the theory that narrower, so called "skinny" streets, are safer than standard width residential streets. Since municipal code generally mandates a minimum of 36 foot wide residential streets (planned unit developments may be an exception), the results from this study indicate that current street design standards are directly contributing to automobile accidents.

This brings up a larger question of public safety issues concerning fire apparatus and emergency vehicle access with narrow streets. The service reports from the Fire Department of the City of Longmont were evaluated. There were no fire related injuries or accidents during the eight year period of the study. There were fires in the older part of town during the study period that have alley access and narrow streets, but no injuries were attributed to those fires. It is suggested, therefore, that the municipal or county government look at the larger picture of public safety issues and ask if it is better to reduce dozens of potential vehicular accidents, injuries and deaths or provide wide streets for no apparent benefit to fire related injuries or deaths.

Fire and EMT response times were not part of the evaluation for this paper. The authors, however, are in the process of evaluating several towns and cities in the Denver metropolitan area to see if there are any correlations of that nature in the older portions of the municipalities that include narrow streets.

Lastly, we encourage additional research to verify these results. This is a relatively untouched area of potential research. In a very limited search of the literature, two studies stand out. The first indicates that the mean free speed of cars in suburban roads increases linearly with the roadway width, particularly between 17 and 37 feet. The second paper by Giese, et al, suggests that spatial enclosure, sight distance and [width] constriction techniques influence vehicle speeds.

This study supports the hypothesis that injury accidents are related to vehicular speed and vehicular speed is a function of street width, ADT and alignment. Further, since posted parking did not have statistical significance in a/m/y, accident mitigation should include narrower streets and on-street parking.

Notes: 1. Omar Farouki and William Nixon. 1976. "The Effect of the Width of Suburban Roads on the Mean Free Speed of Cars". Traffic Engineering and Control 17,2: 508-9.

2. Joni Giese, Gary Davis and Robert Sykes. 1997. "The Relationship Between Residential Street Design and Pedestrian Safety". Institute of Transportation Engineers Compendium of Technical Papers on CD-ROM.


Patrick Siegman

I. "In Search of Cheap and Skinny Streets",

was written by Terence L. Bray and Victor F. Rhodes, and published in the Summer 1997 issue of Places. It was in a special issue devoted entirely to streets, titled "Streets: Old Paradigm, New Investment " , and is really worth tracking down.


II. Thanks to the fine work of Alan B. Cohen of the CNU Transportation Task Force, and Randy S. McCourt of DKS Associates in Portland, Oregon, there are now two databases available on the Web of cities which have adopted reduced width street standards. Both are works in progress, but together contain some 25 examples, complete with contact names and phone numbers.

1) Alan B. Cohen's 'Narrow Streets Database' is the most accessible and easy to read. It can be found at: www.sonic.net/abcaia/narrow.htm

2) Randy McCourt's survey results are part of a much larger survey on neighborhood traffic management, sent to over 1000 Institute of Transportation Engineers (ITE) members. It covers both narrow streets and many other traffic calming measures. Of particular note: the 120 agencies who responded to the survey reported that they encounter well over 1,500 lawsuits a year regarding various transportation related issues. Only 6 lawsuits were identified in this survey group to be associated with their traffic calming devices, and the 15 agencies using narrow street standards reported that there were no lawsuits at all associated with their narrower street standards.

Alan Cohen would appreciate hearing from all those with more information to contribute. Please write him at: abcaia@sonic.net


III. I put together the following quotes from J. Kevin Keck's "Caught in the Middle: The Fight for Narrow Residential Streets" [Proceedings of the ITE 14th International Conference (1998)] for a client seeking relief from overly wide street standards. They briefly summarize some of the latest narrow street standards and research from around the country. Included are some measured effects on driver's speeds; narrowing existing streets (with no landscape strip) by adding landscape strips between the sidewalk and the street; and the relationship between street width and housing costs.

Street Width Standards and Research from Around the Country

1) Table 1: Example Residential Street Width Standards (California) Location and Street Type ROW Width Pave Width Parking Direction

Portland, Oregon

Location and Street Type ROW Width Pave Width Parking Direction
< 300' long 35' 18' None 2-way
< 9 units per acre 35' 20' 1-side 2-way
Standard Residential 40'* 24' 2-sides 2-way

* 56' with sidewalks

Madison, WI

Location & Street Type ROW Width Pave Width Parking Direction
< 3 units per acre 40' 27' 2-sides 2-way
3-10 units per acre 56' 28' 2-sides 2-way

Novato, CA

Location & Street Type ROW Width Pave Width Parking Direction
Serves 2-4 dwellings 25' 20' 2-sides 2-way
Serves 5-15 dwellings 40' 28' 2-sides 2-way

San Jose, CA

Location & Street Type ROW Width Pave Width Parking Direction
  50' 24'-36' ** 2-sides 2-way

** Narrows to 24' at tree planters forming parking bays. (Used in Evergreen Planned Residential Community.)

Dublin, CA

Location & Street Type ROW Width Pave Width Parking Direction
  50' 24'-36' *** 2-sides 2-way

*** Two opposing five foot wide tree planters located every 100' reduce the effective street width by 10'. (Used in Kaufman & Broad Creekside Residential Development.)

[Source: J. Kevin Keck, "Caught in the Middle: The Fight for Narrow Residential Streets", Proceedings of the ITE 14th International Conference (1998). The table above summarizes several tables in Mr. Keck's paper.]

2) Street Widths and Traffic Speeds: Phoenix AZ

Just as wider streets encourage higher speeds, narrower street clearances have been shown to effect a reduction in average vehicle speeds. Residential streets with parked vehicles have been observed to experience lower average vehicle speeds. In Phoenix, it was reported that the most effective traffic calming measure was a narrowing of neighborhood streets to 28 feet from 32 feet, adding a landscaped strip between the sidewalk and curb.

Neighborhood traffic calming includes lots of tools. Many focus on somehow narrowing the width of the street. "The wider you go," observed, a Phoenix traffic engineering supervisor, "the more you're encouraging fast traffic."

[Ingley Kathleen, "Calming The Mean Streets: Strategy To Slow Drivers Brings Neighbors Peace", Arizona Republic, March 31, 1997, Pg. Al]

3) Street Width vs. Housing Cost & Supply

A report by the University of Wisconsin-MadisonCenter for Urban Land Economics Research indicated that in rapidly growing Waukesha County (immediately west of Milwaukee County), overly generous street width requirements have served to reduce the prevalence of $75,000-and-under houses from 16% of the average subdivision in 1990 to "virtually none" today. The report surmised that communities do not realize how much land they use up by requiring wider streets, and found that each 10 feet of required street width reduces the supply of homes by 3 to 4 percentage points.

[University of Wisconsin-Madison Center for Urban Land Economics Research report by Richard K. Green, cited by Derus, Michelle, "Zoning can curb lower-cost housing: UW study of Waukesha County blames wide streets, broad lots", Milwaukee Journal Sentinel, September 21, 1997 Sunday, Business Pg. 1]

4) Smaller Residential Streets Reduce Speeds

In the San Francisco area, an extensive survey of residential streets was conducted with magnetic imaging counters that were able to collect a broader range of data. In addition to the data obtained from the device, parking density information was also collected concurrently. The analysis indicated the following results:

  • Wider residential streets experience higher speeds for both the average and 85th percentile speeds.
  • On street parking density significantly affects speeds.
  • Traffic volume and vehicle headways affect speeds.
  • Significant reductions in effective street width are required to dramatically reduce speeds.

[Daisa James M. and John B. Peers, Narrow Residential Streets: Do They Really Slow Down Speeds?, ITE 6th Annual Meeting Compendium of Technical Papers (1997)]

5) Factors Affecting Residential Street Speeds

One of the most interesting aspects of the research is the multidisciplinary approach that some have taken to resolve the issues of narrow streets. Traffic engineers, planners, landscape architects, demographers have worked together to re-examine previous assumptions. In Minneapolis/St. Paul a multidisciplinary team explored the influence of streetscape's "spatial enclosure" on vehicle speeds. Such constrictions may include lateral constraints such as reduced street widths, as well as vista terminations such as those at the ends of a short street.

[Giese Joni L. Gary A. Davis, and Robert D. Sykes, The Relationship Between Residential Street Design and Pedestrian Safety, ITE 6th Annual Meeting Compendium of Technical Papers (1997)]


IV. The California-based Local Government Commission has released a new guide entitled "Street Design Guidelines for Healthy Neighborhoods", which includes, for example, recommendations for 26' curb-to-curb residential streets, with parking allowed on both sides. The focus of this guide is how to build healthy neighborhoods and streets, and provides health practitioners, political leaders, planners, engineers, architects, landscape architects and developers appropriate street dimensions to build low speed, low traffic neighborhoods. This 52 page comprehensive guide sells for $25 and has ample illustrations. It is written for the lay person but serves the working professional. For further details or ordering contact information is below. Easiest way to order is to go to the LGC website.

The Local Government Commission. Karen Cole, Publications Coordinator, pubs@lgc.org


Up to Top | Printer-friendly version of this page