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1 #HARLIER�!SSOCIATES�
#HARLIER�!SSOCIATES�)NC� Charlier Associates, Inc. 2511 31st StreetBoulder, CO 80301http://www.charlier.org NARROW LOCAL STREETS NARROW LOCAL STREETS CONOMICSAFETYNVIRONMENTUALITYIFE INTRODUCTION There is no planning, engineering or public policy basis for wide local streets. In fact, good engineering practice guides designers to minimize pavement and right-of-way widths in all functional classiÞ cations to the minimum needed. Residential Streets, a comprehensive street design guide jointly published by the American Society of Civil Engineers (ASCE), the National Association of Home Builders (NAHB), the Urban Land Institute (ULI), and the Institute of Transportation Engineers (ITE) summarizes:“The right-of-way width should be only as wide as necessary for the street pavement and other facilities and uses, including sidewalks, drainage, street trees, snow storage, and grading. Blanket requirements for right-of-way of 50 feet or more, often mandated by ordinances, are seldom justi ed for subcollectors and access streets...Residential street designers should select the minimum width that will reasonably satisfy all realistic needs, thereby minimizing construction and annual maintenance costs, while at the same time maximizing the livability of the community” (1).This document addresses residential commercial local streets. Local streets provide direct access to abutting land as well as connections to collector and arterial streets. The decision to design narrow streets in a community has an e ect on economics, safety, the environment, and overall quality of life. These topics are discussed in the following sections. Section 5 identiÞ es and dispels common myths used to justify overly wide local streets. References:Residential Streets 3rd Ed. American Society of Civil Engineers, 2001. 1 Charlier Associates, Inc. BENEFITS OF

2 NARROW STREETS ECONOMICS References:Res
NARROW STREETS ECONOMICS References:Residential Streets 3rd Ed. American Society of Civil Engineers, 2001. Several key components of road construction costs are a function of street width; therefore wide streets are more expensive to construct and maintain than narrow streets. Narrow streets reduce construction costs Generally, total roadway construction costs (T) can be expressed by the following formula: T = R + D + L + SR = Right-of-Way - 25 to 50 %L = Landscaping - 10 to 15 %S = Road base and Pavement - 25 to 50 %Road construction costs include some elements that are not a direct function of street width, such as landscaping, curb and gutter, utilities, sidewalks, and modern drainage (e.g. bioswales, retention). Elements of roadway construction costs that are a function of street width include right-of-way, drainage, and road base and pavement.Although there are right-of-way costs associated with road construction in greenÞ eld development, they mainly take the form of land use opportunity costs, as discussed below. Storm sewer capacity is a function of street width (i.e. wider streets require greater stormwater drainage capacity), although the relationship is not linear. The largest construction costs a ected by roadway width are the costs of the pavement and road base materials. The Residential Streets guide compares the cost of building two streets with widths of 24 and 36, as shown in Table 1 below. Table 1: Cost per 100’ of Street 24’ Wide36’ WideCost Di erence5-inch Asphalt Paving / 6-Inch Base$6,800$10,88060.0%Total Construction Costs$9,465$13,54543.1% Narrow streets reduce maintenance costs Because maintenance costs are a function of pavement surface area, the reduction in costs of maintaining a narrow street is proportional to the reduction in pavement square footage, as shown in Table 2 below. The same principle also applies to roadway resurfacing costs

3 .Table 2: Additional Pavement by Street
.Table 2: Additional Pavement by Street Width Street WidthFactor201.0261.3341.7422.1502.5 Narrow streets consume less land. This land can be used more productively and remains in Property taxes generated from urban land are a function of the amount of improved land and the value of that land. Land consumed for streets is not taxable. Reducing street width leaves more land available for productive use, and also permits development at a higher density without building up. 1 BENEFITS OF NARROW STREETS SAFETY Narrow streets encourage slower vehicle speeds ects of residential street widths on vehicle speeds found a strong positive correlation between street width and vehicle speeds. SpeciÞ cally, the highest speeds were observed on wide streets with low parking density, low tra c volumes, and long headways between vehicles (1). Slower vehicle speeds result in fewer crashesless severe injuries Several recent studies have found a direct relationship between vehicle speed and roadway safety. Two of these studies (Aarts and van Schagen, 2006 and Elvik, 2005) document three main e ects of speed on crashes and injuries, based on a summary of over 100 studies:The probability of a crash is proportional to the square of travel speed.In a crash, injury risk is proportional to the impact forces on a person, which are proportional to the square of the impact speed.The probability of a crash increases as a vehicles speed rises above the average travel speed of surrounding vehicles.A 1% decrease in travel speed reduces injury crashes by about 2%, serious injury crashes by about 3%, and fatal crashes by about 4%. On a roadway with an average travel speed of 40 mph, a reduction of 2 mph (a 5% decrease) reduces crashes by 10%, serious injury crashes by 14% and fatal crashes by 19% Similarly, a 2005 study by Eric Dumbaugh used crash data to compare the relative safet

4 y of two roadway segments. What the stu
y of two roadway segments. What the study called the livableŽ roadway section had 11 foot travel lanes, on-street parking, and roadside objects (set 1.5 to 2 feet from the curb) to bu er the pedestrian environment. The other segment had 12.5 foot lanes, did not include on-street parking, and had minimal roadside objects. Between 1999 and 2003, the livable segment had 11% fewer total, 31% fewer injurious, and 100% fewer fatal midblock crashes. A similar comparison was made for four other streets to conÞ rm these trends (4).The most signiÞ cant correlation tested in another study of local streets and safety in Longmont, Colorado was between street width and injury accidents per mile per year. The study concludes, As street widths widen, accidents per mile per year increases exponentially, and the safest residential street widths are the narrowest... References:(1) Daisa, James M., P.E. and John B. Peers, P.E. Narrow Residential Streets: Do They Really Slow Down Speeds? ITE 6th Annual Meeting Compendium of Technical Papers, 1997.(2) Aarts, L. and I. van Schagen. 2006. Driving speed and the risk of road crashes: A review. Accident Analysis and Prevention(3) Elvik, R. 2005. Speed and road safety: synthesis of evidence from evaluation studies. Transportation Research Record(4) Dumbaugh, Eric. 2005. “Safe Streets, Livable Streets.” Journal of the American Planning Association 71:3, pp. 283-298.(5) Swift, Peter, P.E., Dan Painter, AICP, and Matthew Goldstein. Residential Street Typology and Injury Accident Frequency. BENEFITS OF NARROW STREETS SAFETY References:(5) Leaf, W.A. and D.F. Preusser. Literature Review on Vehicle Travel Speeds and Pedestrian Injuries. (1999) Washington, DC: US Department of Transportation. (Publication no. DOT HS 809 021).(6) Homburger, Deakin, Bosselmann, Smith and Beukers. Residential Street Design and Tra c Control Institute of

5 Transportation Engineers. Higher vehicle
Transportation Engineers. Higher vehicle speeds are also associated with a greater likelihood of pedestrian crashes and more serious pedestrian injuries. Only Þ ve percent of pedestrians would be likely to die when struck by a vehicle traveling 20 mph or less, compared to fatality rates of 40, 80 and nearly 100 percent at speeds of 30, 40 and 50+ mph, respectively (5).This survival rate disparity is due to both the di erence in force of the impact and driver reaction times at varying speeds. An emergency stop made when traveling 35 mph requires a stopping distance of 115 to 165 feet for an average automobile (6). Adapted from: Literature Review on Vehicle Travel Speeds and Pedestrian Injuries BENEFITS OF NARROW STREETS References:(1) United States Environmental Protection Agency. Watersheds. http://www.epa.gov/weatherchannel/stormwater.html.(2) United States Environmental Protection Agency. Best Management Practices - Maintenance PracticesImpervious Surface Coe cients: A tool for environmental analysis and management. O ce of Environmental Health Hazard Assessment; Environmental Protection Agency (Sacramento/Oakland).(4) Bannerman, Roger; Owens, D.W.; Dodds, R.B.; HOrnewer, N.J. “Sources of Pollutants in Wisconsin Stormwater.” Water Science Technology. Vol. 28 No. 3-5: 241-259. 1993.(5) Pomerantz et al. The E ect of Pavements Temperatures on Air Temperatures in Large Cities. April, 2000. Narrow streets reduce impervious surface area Stormwater runo occurs when precipitation (rain or melting snow) occurs over impervious surfaces. Impervious surfaces such as streets, parking lots and driveways prevent stormwater runo from naturally soaking into the ground (1). Our transportation systems, namely roads and parking lots, account for nearly 70 percent of the total impervious surface in urban areas (2). Reducing impervious area allows more stormwate

6 r to inÞ ltrate and recharge aquifers.
r to inÞ ltrate and recharge aquifers. Recharging aquifers is critical to ensuring the continued availability and quality of groundwater. Narrow streets reduce stormwater volumewater pollution Increased impervious cover increases the volume and velocity of stormwater (3). A study of the runo from 12 erent types of surfaces found that streets create the highest pollutant load, contributing 54 percent of all runovolume in residential neighborhoods and 31 percent of runo in commercial neighborhoods (4). There is a negative correlation between the amount of impervious surface area and water quality within a watershed (3). Untreated stormwater runo transports chemicals, debris and other pollutants directly into lakes, rivers, wetlands and other bodies of water. These pollutants have a negative e ect on aquatic ecosystems and degrade the quality of water available for drinking, swimming and Þ shing. Narrow streets reduce impervious surface area, allowing greater inÞ ltration and natural Þ ltering to occur. Narrow streets reduce heat island e ects An urban heat island is a metropolitan area which is signiÞ cantly warmer than its surroundings. The main cause cation of the land surface by urban development. Horizontal surfaces such as rooftops and pavement absorb solar radiation, which is stored as heat. The air is then heated by these surfaces, which contributes to the urban heat island e ect (5). Reducing the amount of paved surface reduces local heat ects. Heat island e ects are further reduced when trees are planted along narrow streets to provide shade. Narrow streets with a planting strip allow snow storage and ltration during melting. Snow that is plowed from roadway travel lanes should be stored on a pervious surface (e.g. grass or dirt) to allow ltration during melting. A narrow street with a planting strip as a bu er between the street and pedestrian

7 walkway is the best design option. See
walkway is the best design option. See Myth #3 for additional discussion of snow removal and storage. 3 BENEFITS OF NARROW STREETS QUALITY OF LIFE References:(1) Chiras, Dan and Dave Wann. Superbia! 31 ways to create sustainable neighborhoods. Gabriola Island: New Society Publishers, 2000.(2) Picture This. . . The Results of Visual Preference Survey, A. Nelessen & Associates, Princeton, NJ, and Seattle, WA, 1993.(3) Chasin, Marshall, M.Sc., Aud(C), FAAA, Centre for Human Performance & Health, Ontario, Canada.(4) Cowan, James P. Handbook of Environmental Acoustics. John Wiley & Sons Inc., 1993.(5) Appleyard, Donald. Livable Streets. Berkeley, CA: University of California Press, 1981.(6) Brownson, Ross C.; Baker, Elizabeth A.; Housemann, Robyn A.; Brennan, Laura K.; Bacak, Stephen J. Environmental and Policy Determinants of Physical Activity in the United States. Am J Public Health. 2001 Dec; 91(12):1995…2003. Narrow streets contribute to neighborhood attractiveness Visual preference surveys consistently show that people prefer narrow streets, especially those that are lined with trees and on-street parking (1)(2). The majority of respondants also indicate a preference for streets that are no more than two lanes in width (1). Narrow streets slow vehicle travel speeds, reducing noise along the roadway. Noise increases with vehicle speed. A vehicle traveling 60 mph is perceived as being twice as loud as a vehicle traveling 30 mph (3), due to a 10 dB increase in sound, as shown in Table 3 below. Table 3: Vehicle Speed and Noise (from 50 feet) Vehicle Speed (mph)Noise (dB)3062406750706073Adapted from: Handbook of Environmental Acoustics Narrow streets contribute to community cohesion Donald Appleyards landmark study, Livable Streets, found that narrow streets with lower speeds and tra c volumes facilitated the greatest interaction among neighbors and the greates

8 t sense of ownership. The study found t
t sense of ownership. The study found that residents of narrow streets are more likely to consider the entire street their home, which generates greater surveillance, and therefore greater safety. Neighborhoods that have streets with more vehicle lanes, greater tra c volumes and higher posted speed limits have greater levels of noise and pollution, and lower levels of pedestrian activity and social capital (5). Narrow streets create a safer, more comfortable environment for Active Living Enhanced safety and quality of life resulting from narrow streets also help create more walkable and bikeable communities. Many of these beneÞ ts are a result of reduced vehicle speeds, which reduces the real and perceived threat pedestrians and bicyclists feel from motor vehicles. Other safety improvements, such as reduced crossing distances, also create a more pedestrian-friendly environment. However, the quality of life beneÞ ts discussed above (overall aesthetics, shade and noise) also have a signiÞ cant impact on utilitarian and recreational walking trips. Studies show that the presence of sidewalks and enjoyable scenery are positively associated with physical activity (6). BENEFITS OF NARROW STREETS COMMON MYTHS 1. Narrow streets save developers money.WHY THIS IS NOT TRUE: Although narrow streets are less expensive to construct, developers will ideally use these construction cost savings to increase connectivity within the development.Traditional neighborhood developments typically have about 25 percent more center-line miles than suburban developments. The cost of increased center-line miles and connectivity can be o set by making streets 25 percent narrower. 2. Wide local streets provide more capacity.WHY THIS IS NOT TRUE: A well-connected network of narrow streets is more e cient and has greater capacity than a limited network of wider streets. Intersections control the

9 capacity of any street network, and a de
capacity of any street network, and a dense network of small streets has more intersections than a limited network of wider streets. Additional intersections reduce the turning movement load at any given intersection; thus, the dense network of smaller streets can carry greater tra c volumes on the same amount of pavement (1). 3. Narrow streets are unsafe.WHY THIS IS NOT TRUE: Narrow streets encourage slower vehicle speeds. Slower vehicle speeds result in fewer crashes and less severe injuries for motorists and pedestrians (see the Safety section for additional details). 4. Wide streets are needed to accommodate snow removal and storage.WHY THIS IS NOT TRUE: Snow plowed from roadways should not be stored on the road, even in areas with wide streets. Snow should be stored on a pervious surface (e.g. grass or dirt) to allow inÞ ltration during melting. Permitting melting snow to inÞ ltrate reduces stormwater management requirements.Snow stored on the side of the road will often melt and re-freeze, which can create potentially unsafe conditions along the roadway.Snow stored along the side of the road limits or blocks on-street parking.Also, there is a larger quantity of snow to remove and store from a wider street. A narrow street with a planting er between the street and pedestrian walkway (illustrated below) is a better option. Pedestrian Supportive streetscape cross section and examples BENEFITS OF NARROW STREETS 5. Wider streets are better able to accommodate emergency vehicle access.WHY THIS IS NOT TRUE: Many communities believe wide streets are needed to accommodate emergency vehicles; however, a well-connected network of narrow streets can improve access to the site of an emergency by providing:MULTIPLE ROUTING OPTIONS: A well-connected network provides multiple routing options to reach a given destination. In a development with a single access point, if a roadwa

10 y is blocked due to an accident, fallen
y is blocked due to an accident, fallen power lines, etc., emergency responders will be delayed or unable to reach the site of the emergency. In a development with a well-connected street network, emergency responders would simply choose an alternate route (e.g. a parallel street). The same beneÞ t holds true in the event of tra c congestion on a major arterial leading to a poorly connected development. Building wider streets does not make up for the congestion that can occur as a result of a network with a limited number of connections and access points (2). MULTIPLE ACCESS POINTS: A well-connected network of smaller streets within a development provides multiple access points to a given location. For example, emergency responders would be able to access a building located in a development with a well-connected grid street network from up to four points (depending on block size and hose length for Þ re trucks).Many communities have successfully implemented narrow street width standards (3), yet continue to provide adequate emergency vehicle access, including:Birmingham, Michigan - 20 lanes with parking on one side; 26 lanes with parking on both sidesBoulder, Colorado - 22 lanes with parking on one side; 26 lanes with parking on both sidesPortland, Maine - 24 lanes with parking on one sidePortland, Oregon - 20 lanes with parking on one side; 26 lanes with parking on both sidesMissoula, Montana - 26 lanes with parking on both sidesCOMMON MYTHS Source: Arlington Community Planning and Housing Development References:(1) Kulash, Walter. Traditional Neighborhood Development: Will the Tra c Work? 11th Annual Pedestrian Conference. Bellevue, WA.Street Design and Emergency Response. Local Government Commission.(3) Cohen, A. Narrow Streets Database. Congress for the New Urbanism - http://www.sonic.net/abcaia/narrow.htm Well-connected and poorly connected street netw