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
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).
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.
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:
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.
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
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: email@example.com
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
|Location and Street Type
|< 300' long
|< 9 units per acre
* 56' with sidewalks
|Location & Street Type
|< 3 units per acre
|3-10 units per acre
|Location & Street Type
|Serves 2-4 dwellings
|Serves 5-15 dwellings
San Jose, CA
|Location & Street Type
** Narrows to 24' at tree planters forming parking bays. (Used in Evergreen Planned
|Location & Street Type
*** 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
- 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
[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
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, firstname.lastname@example.org
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