Safe Streets, Livable Streets

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<ul><li><p>This article was downloaded by: [LMU Muenchen]On: 06 July 2014, At: 13:38Publisher: RoutledgeInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK</p><p>Journal of the American Planning AssociationPublication details, including instructions for authors and subscription information:</p><p>Safe Streets, Livable StreetsEric Dumbaugh &amp; J. L. GattisPublished online: 26 Nov 2007.</p><p>To cite this article: Eric Dumbaugh &amp; J. L. Gattis (2005) Safe Streets, Livable Streets, Journal of the American PlanningAssociation, 71:3, 283-300, DOI: 10.1080/01944360508976699</p><p>To link to this article:</p><p>PLEASE SCROLL DOWN FOR ARTICLE</p><p>Taylor &amp; Francis makes every effort to ensure the accuracy of all the information (the Content) containedin the publications on our platform. However, Taylor &amp; Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor &amp; Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.</p><p>This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms &amp; Conditions of access and use can be found at</p><p></p></li><li><p>Safe Streets, LivableStreets</p><p>Eric Dumbaugh</p><p>The danger in supplanting the real measure of safety (i.e., crash frequency andseverity) by surrogates arises when the link between the two is conjectural, when thelink remains unproven for long, and when the use of unproven surrogates becomesso habitual that the need to eventually speak in terms of crashes is forgotten. (Hauer, a, p. )</p><p>Beyond simply acting as thoroughfares for motor vehicles, urban streetsoften double as public spaces. Urban streets are places where people walk,shop, meet, and generally engage in the diverse array of social and recre-ational activities that, for many, are what makes urban living enjoyable. Andbeyond even these quality-of-life benefits, pedestrian-friendly urban streets havebeen increasingly linked to a host of highly desirable social outcomes, includingeconomic growth and innovation (Florida, ), improvements in air quality(Frank et al., ), and increased physical fitness and health (Frank et al., ),to name only a few. For these reasons, many groups and individuals encouragethe design of livable streets, or streets that seek to better integrate the needs ofpedestrians and local developmental objectives into a roadways design.</p><p>There has been a great deal of work describing the characteristics of livablestreets (see Duany et al., ; Ewing, ; Jacobs, ), and there is generalconsensus on their characteristics: livable streets, at a minimum, seek to enhancethe pedestrian character of the street by providing a continuous sidewalk networkand incorporating design features that minimize the negative impacts of motorvehicle use on pedestrians. Of particular importance is the role played by road-side features such as street trees and on-street parking, which serve to buffer thepedestrian realm from potentially hazardous oncoming traffic, and to providespatial definition to the public right-of-way. Indeed, many livability advocatesassert that trees, as much as any other single feature, can play a central role inenhancing a roadways livability (Duany et al., ; Jacobs, ).</p><p>While most would agree that the inclusion of trees and other streetscapefeatures enhances the aesthetic quality of a roadway, there is substantive disagree-ment about their safety effects (see Figure ). Conventional engineering practiceencourages the design of roadsides that will allow a vehicle leaving the travelwayto safely recover before encountering a potentially hazardous fixed object. Whenone considers the aggregate statistics on run-off-roadway crashes, there is indeed</p><p>Transportation safety is a highly con-tentious issue in the design of cities andcommunities. While urban designers,architects, and planners often encouragethe use of aesthetic streetscape treatmentsto enhance the livability of urban streets,conventional transportation safety practiceregards roadside features such as streettrees as fixed-object hazards and stronglydiscourages their use. In this study, Iexamine the subject of livable streetscapetreatments and find compelling evidencethat suggests they may actually enhancethe safety of urban roadways. Concernsabout their safety effects do not appearto be founded on empirical observationsof crash performance, but instead on adesign philosophy that discounts theimportant relationship between driverbehavior and safety. This study traces theorigin and evolution of this philosophy,and proposes an alternative that may bet-ter account for the dynamic relationshipsbetween road design, driver behavior, andtransportation safety.</p><p>Eric Dumbaugh is a doctoral candidatein the School of Civil and Environmen-tal Engineering at the Georgia Instituteof Technology. His research focuses oncontext-sensitive solutions, nonmotor-ized travel, and transportation systemsafety.</p><p>Journal of the American Planning Association,</p><p>Vol. , No. , Summer .</p><p> American Planning Association, Chicago, IL.</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>LM</p><p>U M</p><p>uenc</p><p>hen]</p><p> at 1</p><p>3:38</p><p> 06 </p><p>July</p><p> 201</p><p>4 </p></li><li><p>cause for concern. In alone, there were over ,fatalities involving roadside objects such as trees and utilitypoles on U.S. roadways, accounting for more than % ofthe total fatalities for that year (National Highway TrafficSafety Administration [NHTSA], n. d.). Correspondingly,designing livable streets is often more difficult than simplycounterbalancing the needs of motorists with those of pe-destrians. How is the transportation designer to conscien-tiously incorporate design elements that may result in theloss of life?</p><p>This study details existing design guidance and liter-ature, as well as the historical evolution of contemporarysafety practice, and reports the results of an empirical testof the professional assumptions that guide the currentapproach to addressing safety through design. It concludesby outlining an approach to urban roadway design thatmay better address the twin goals of safety and livability.</p><p>Considering the Literature onRoadside Safety</p><p>The initial motivation behind this research effort wasan attempt to understand the safety impacts of livablestreetscape treatments on urban roadways. On this issue,the design guidance is clear: for all types of highwayprojects, clear zones should be determined or identifiedand forgiving roadsides established (American Associationof State Highway and Transportation Officials [AASHTO],, p. ). In practice, this entails providing a clear road-side adjacent to the vehicle travelway, with a preferredwidth of feet. In terms of how to best accomplish thisgoal, AASHTOs () Roadside Design Guide, the centralauthority on the design of safe roadsides, is also clear:</p><p>Through decades of experience and research, theapplication of the forgiving roadside concept hasbeen refined to the point where roadside design is anintegral part of transportation design criteria. Designoptions for reducing roadside obstacles, in order ofpreference, are as follows:</p><p>1. Remove the obstacle.2. Redesign the obstacle so it can be safely traversed.3. Relocate the obstacle to a point where it is less likely</p><p>to be struck.4. Reduce impact severity by using an appropriate</p><p>breakaway device.5. Shield the obstacle with a longitudinal traffic barrier</p><p>designed for redirection or use a crash cushion.6. Delineate the obstacle if the above alternatives are</p><p>not appropriate. (pp. )</p><p> Journal of the American Planning Association, Summer , Vol. , No. </p><p>Figure . Livable streetscape treatments: urban amenities or roadsidehazards?</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>LM</p><p>U M</p><p>uenc</p><p>hen]</p><p> at 1</p><p>3:38</p><p> 06 </p><p>July</p><p> 201</p><p>4 </p></li><li><p>While the Roadside Design Guide cites decades ofexperience and research, there is very little information onthe use of aesthetic streetscape features, and much of theexisting literature on the application of clear zone policies inurban environments is problematic, at best. The definitivework on the subject is a study that describes the physicalcharacteristics of trees involved in crashes within the Citylimits of Huntsville, Alabama (Turner &amp; Mansfield, ).This study found that most crashes involving trees occurwithin feet of the roadway, that % of the reportedcrashes involved trees with a caliper width inches or more,and that almost % occurred on a horizontal curve. Whilesuch information is useful for understanding the character-istics of tree-related crashes, it does not lead to the conclu-sion that eliminating trees with any or even all of thesecharacteristics will have any effect on a roadways safety.Such conclusions can only be made by examining the actualcrash performance of eliminating trees in urban areas, asmeasured by changes in crash frequency and severity.</p><p>Indeed, there is a growing body of evidence suggestingthat the inclusion of trees and other streetscape features inthe roadside environment may actually reduce crashes andinjuries on urban roadways. Naderi () examined thesafety impacts of aesthetic streetscape enhancements placedalong the roadside and medians of five arterial roadways indowntown Toronto. Using a quasi-experimental design,the author found that the inclusion of features such as treesand concrete planters along the roadside resulted in statis-tically significant reductions in the number of mid-blockcrashes along all five roadways, with the number of crashesdecreasing from between and % as a result of the street-scape improvements. While the cause for these reductionsis not clear, the author suggests that the presence of a welldefined roadside edge may be leading drivers to exercisegreater caution.</p><p>Ossenbruggen, Pendharkar, and Ivan () examinedsites with urban, suburban, and residential characteristicsin New Hampshire and hypothesized that the urban vil-lage areas, with greater traffic volumes and more pedes-trian activity, would be associated with higher numbers ofcrashes and injuries. Instead, they found the opposite: thevillage areas, which had on-street parking and pedestrian-friendly roadside treatments, were two times less likely toexperience a crash event than the comparison sites. Theauthors associate these crash reductions with the character-istics of the roadside environment, which included side-walks, mixed land uses, and other pedestrian-friendlyroadside features. The authors also attributed the safetyperformance to reduced speeds, noting that since nospeed limit signs are erected at village sites, it suggests[speeds] are self regulating (p. ).</p><p>A study of two-lane roadways by Ivan, Pasupathy, andOssenbruggen () found that while shoulder widthswere associated with reductions in single-vehicle, fixed-object crashes, they were also associated with a statisticallysignificant increase in total crashes, with multiple-vehiclecrashes offsetting safety gains achieved through reductionsin fixed-object crashes. The authors comment that the pos-itive coefficient on right shoulder width is troubling; onenormally expects a wider shoulder to be a safety feature(p. ).</p><p>Finally, Lee and Mannering () examined run-off-roadway crashes along a -km section of an arterial road-way in Washington State that traveled through both urbanand rural environments. Using a negative binomial model,the authors sought to associate crash frequencies with thecharacteristics of the roadside environment. While theirmodel for rural areas performed as expected, with trees andother features being associated with a statistically significantincrease in the number of roadside crashes, their model forurban areas produced radically different results (see Table). Not only were trees not associated with crash increases,but the model coefficients entered negatively at a statisti-cally significant level, indicating that the presence of treesin urban areas was associated with a decrease in the proba-bility that a run-off-roadway crash would occur.</p><p>The authors attribute these unexpected crash reduc-tions to the fact that there are fewer trees in urban areasthan in rural ones, but this begs the question: even if thereare fewer trees in urban areas, which suggests that theirpresence would violate driver expectancy, why are theyassociated with statistically significant crash reductions?</p><p>Other roadside features proved to be statisticallyrelated to crash reductions as well. The number of signsupports was associated with crash reductions, as was thepresence of miscellaneous fixed objects, a variable thatincluded such roadside features as mailboxes. Further,wider lanes and shoulders were associated with statisticallysignificant increases in crash frequencies.</p><p>Interestingly, clear zones are not the only design fea-ture for which such safety anomalies appear. Hauer (a)reexamined the literature on lane widths and found thatthere was little evidence to support the assertion thatwidening lanes beyond feet enhances safety. Instead, theliterature has almost uniformly reported that the safetybenefit of widening lanes stops once lanes reach a width ofroughly feet, with crash frequencies increasing as lanesapproach and exceed the more common -foot standard.</p><p>Further, in a series of broad-sweeping and profoundlyimportant studies, Noland (, ) and Noland andOh () consistently found that when one controls forintervening factors such as time-series effects, seat belt use,</p><p>Dumbaugh: Safe Streets, Livable Streets </p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>LM</p><p>U M</p><p>uenc</p><p>hen]</p><p> at 1</p><p>3:38</p><p> 06 </p><p>July</p><p> 201</p><p>4 </p></li><li><p>and the demographic characteristics of the population,conventional design improvements result in increases incrashes and fatalities.</p><p>Indeed, there are a host of safety anomalies in theexisting design literature, but as Noland and Oh ()stated, the problem is that</p><p>Studies that find unexpected or unconventional resultstend to dismiss these results as aberrations and havenot examined them in further detail. . . . The resultsof many of these studies lead us to conclude that theimpact of various inf...</p></li></ul>