National Academies Press: OpenBook

Safe and Aesthetic Design of Urban Roadside Treatments (2008)

Chapter: Appendix C - Toolkit for Urban Roadside Design

« Previous: Appendix B - Case Study Reports
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Suggested Citation:"Appendix C - Toolkit for Urban Roadside Design." National Academies of Sciences, Engineering, and Medicine. 2008. Safe and Aesthetic Design of Urban Roadside Treatments. Washington, DC: The National Academies Press. doi: 10.17226/14171.
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Suggested Citation:"Appendix C - Toolkit for Urban Roadside Design." National Academies of Sciences, Engineering, and Medicine. 2008. Safe and Aesthetic Design of Urban Roadside Treatments. Washington, DC: The National Academies Press. doi: 10.17226/14171.
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Page 60
Suggested Citation:"Appendix C - Toolkit for Urban Roadside Design." National Academies of Sciences, Engineering, and Medicine. 2008. Safe and Aesthetic Design of Urban Roadside Treatments. Washington, DC: The National Academies Press. doi: 10.17226/14171.
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Page 60
Page 61
Suggested Citation:"Appendix C - Toolkit for Urban Roadside Design." National Academies of Sciences, Engineering, and Medicine. 2008. Safe and Aesthetic Design of Urban Roadside Treatments. Washington, DC: The National Academies Press. doi: 10.17226/14171.
×
Page 61
Page 62
Suggested Citation:"Appendix C - Toolkit for Urban Roadside Design." National Academies of Sciences, Engineering, and Medicine. 2008. Safe and Aesthetic Design of Urban Roadside Treatments. Washington, DC: The National Academies Press. doi: 10.17226/14171.
×
Page 62
Page 63
Suggested Citation:"Appendix C - Toolkit for Urban Roadside Design." National Academies of Sciences, Engineering, and Medicine. 2008. Safe and Aesthetic Design of Urban Roadside Treatments. Washington, DC: The National Academies Press. doi: 10.17226/14171.
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Page 63

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58 Introduction It is a challenge to design an urban roadside environment that balances the often conflicting demands of land owners, road users, and local jurisdictions to effectively use this valu- able space without dramatically compromising safety. Though there is still much to learn about how the urban roadside con- figuration influences the functional operation and safety of a roadway corridor, some basic design concepts can help assure the placement of roadside objects that minimize potentially hazardous conditions. This toolkit provides placement strate- gies, referred to as urban control zones, for a variety of urban conditions. The primary focus of this summary is roadside object placement for high speed urban roads. Roads charac- terized by twenty-four hour on-street parking or low speed local roads are not directly addressed in this toolkit. Follow- ing the urban control zone section are treatment details for known objects common to the urban roadside environment. Urban Control Zones An urban control zone is a roadside location that can be shown to pose a greater hazard for errant vehicles and as such should be given special attention regarding object placement strategies. Key urban control zones include lateral placement strategies, lane merge locations, driveways, intersections, and sidewalk configurations. In addition high crash locations and common roadside crash locations can also be identified as potential urban control zones for roadside safety. Lateral Placement of Objects Description. Where possible, achieve the clear zone as recommended by the AASHTO Roadside Design Guide. In constrained urban areas where clear zones are not feasible, the recommended lateral offset to roadside objects may vary depending on specific road features. Object placement strategies. Object placement strategies include the following: • Roads on tangent with vertical curb – recommend lateral offset of 6 ft (absolute minimum of 4 ft) from curb face to rigid objects [these values also apply to median locations]. Frangible objects should be positioned no closer than 1.5 ft to curb face. • Roads at horizontal curves with vertical curb – recommend lateral offsets to objects a minimum of 6 ft from the curb face on the outside of curve face and a recommended lat- eral offset of 4 ft for the inside of curve locations (absolute minimum of 1.5 ft for frangible items only) from curb face [these values also apply to median locations]. For sharp curvature locations, determine an object free zone based on sight distance criteria (see Figure C-1). • Roads on tangent or curve with shoulder (no curb) – adhere to clear zone guidelines where possible. If infeasible, then locate objects immediately adjacent to the right-of-way boundary to maximize lateral offset. • Auxiliary lanes that function as higher speed lanes such as extended right-turn lanes must meet the tangent and curve criteria. • Auxiliary lanes such as bicycle lanes can include the width of the bicycle lane in the clear zone; however, it is still recommended that lateral offset from the curb face exceed 1.5 ft where possible at these locations. Lane Merge Locations Description. Often acceleration lanes, lane merges, and bus bay exit points transition to the through travel lane at a taper point. At this location, the driver of the vehicle needs to focus on merging into the active traffic stream. If the driver does not judge the merge correctly, he or she may run off the road at this location. As a result, lane merge locations should be free of roadside objects where possible. A P P E N D I X C Toolkit for Urban Roadside Design

59 Object placement strategies. Object placement strate- gies include the following: • Lateral offset of rigid objects should be as large as possible. Since the presumption is that a vehicle unable to traverse the lane merge will continue along its current path, a lateral off- set equivalent to a standard lane width should be kept free of rigid objects. Where feasible, therefore, objects should be placed at least 12 ft from the curb face so that errant vehicles unable to merge and that continue straight will not impact the object. Breakaway objects should be located 4 to 6 ft from the curb face as a minimum at the taper point locations. • Longitudinal placement of rigid objects should not occur within 10 ft upstream or downstream of the taper point for a total length of 20 ft where feasible (see Figure C-2). Where this placement is infeasible, priority should be given to keeping the upstream roadside area object free. Driveways Description. The placement of roadside objects in the vicinity of driveways should occur in such a way as not to compromise available sight distance or provide a clear path for errant vehicles to impact a rigid object on the far side of a driveway entry. Object placement strategies. Object placement strate- gies include the following: • Downstream (far side) placement of objects should be located 10 to 15 ft from the driveway throat edge (see Figure C-3). • Upstream (near side) placement of objects should be lo- cated so as to provide adequate sight distance for drivers of exiting vehicles. Intersections Description. Though intersections are common crash locations for multiple vehicle collisions, numerous single vehicle roadside crashes can also be expected at intersections. These collisions can occur because of the use of small islands Required Sight Distance along Drivers' Line of Sight Curb Face 4' 6' 4' 4' 4' Lateral Offset Lateral Offset at Inside of Curve Std. Recommended Lateral Offset LEGEND Figure C-1. Lateral placement of objects at horizontal curves. Lateral Offset configuration applies to Lane Merges, Acceleration Lanes, and Bus Bay Returns Curb Face 4' 4' 12' Lateral Offset at Taper Point Std. Recommended Lateral Offset LEGEND 10' 4' Offset extended to Intersect where feasible 10' Figure C-2. Object-free zone at merge points.

60 that are not noticeable to drivers, objects located too close to the curb in the curb return region, and objects located directly aligned with pedestrian access ramps. Object placement strategies. Object placement strategies include the following: • Research shows that curbs can provide a positive (visual) guidance but have very little re-directional ability; there- fore, curbs should be used at raised channelization islands to assist with providing positive guidance to the driver. • Since research shows that sloping curbs can be traversable, their use at channelization islands as a means of restricting vehicle access is not recommended. • For intersection channelization islands (also known as corner islands), the island design should adhere to the AASHTO Geometric Design of Highways and Streets criteria (see their Exhibit 9-37 and 9-38). The island should be suf- ficiently designed so as to be conspicuous to approaching drivers and should not encroach on vehicle paths. Simi- larly, median noses should be conspicuous and designed so as not to impede normal traffic operations. At both the corner islands and the median noses, the placement of rigid objects should be avoided completely. Only breakaway devices should be constructed at these locations. • Often a turning vehicle does not successfully navigate the designated turn path and strays onto the adjacent curb re- turn or shoulder. This situation often occurs for truck turning movements. Object placement at the inside edge of intersection turning movements should be as far as practi- cal from the curb face or lane edge. Similarly, for locations without curb these values should be as far as possible from the edge of lane as these locations do not have a curb to help the driver realize that the vehicle has strayed from the designated path. • Many urban intersections with curb include directional pedestrian access ramps at the intersection corners. For these locations, rigid objects should not be positioned so that errant vehicles are directed towards them along the path of the access ramp. As a result, placement of pedes- trian buttons should either be located on a breakaway pedestal pole adjacent to the directional ramp where pos- sible rather than on a rigid traffic signal pole. This will enable the traffic signal pole placement to occur further away from the curb return region. Sidewalk Configurations Description. In urban environments, sidewalks are often attached directly to the curb. When this occurs, all fixed road- side objects should be located beyond the sidewalk. Another common sidewalk configuration includes a buffer strip be- tween the curb and the sidewalk edge. For these locations, ob- jects are often located within this buffer strip. Care should be taken to assure that objects placed in the buffer strip area do not become roadside hazards. Object placement strategies. Object placement strate- gies include the following: • For a buffer strip 3 to 4 ft wide, rigid objects should not be constructed. Only frangible items such as breakaway signs or forgiving landscaping treatments are appropriate for use in these narrow buffer strips. Curb Face 4' 4' Lateral Offset due to Driveway Std. Recommended Lateral Offset LEGEND 10' to 15' Offset at Far Side of Driveway Drivers' Line of Sight Drivers' Line of Sight 15' 10' to Figure C-3. Roadside object-free zones at driveways.

• Buffer strips that are 5 ft wide or greater should include smaller roadside items such as forgiving landscaping treat- ments or ornamental trees (with canopies that do not impede on sight distance) in the region adjacent to the curb but be- yond recommended lateral offsets. If it is infeasible to locate more rigid objects such as light standards or utility poles beyond the sidewalk, then their placement should be imme- diately adjacent to the sidewalk so as to place them as far from the active travelway as possible. Under no circumstances, however, should these objects be located within the sidewalk boundaries as this space must be kept completely object free so that pedestrians can remain on their designated path. High Crash Locations and Common Urban Roadside Crash Locations Description. Many urban corridors are characterized by unique physical features that may directly contribute to a roadside crash. Though the urban control zones previously identified capture most of the high roadside crash locations, a specific design or operational characteristic for a road may also be a location that merits roadside crash mitigation. These locations can be identified by creating spot maps that demon- strate cluster crash locations that do not fall within the bounds of the previously identified urban control zones. Object placement strategies. Each high crash location that fits the above description will have roadside safety improve- ment strategies unique to the specific feature contributing to the high crash numbers. As a result, placement strategies can in- clude increased lateral offset, shielding, or reconstruction for extreme cases. This must be evaluated on a case by case basis. Roadside Treatment Details Several roadside treatments common to an urban environ- ment can become roadside hazards if not properly positioned. The following summaries identify these common urban road- side features and placement strategies that may help enhance safety at these locations. Landscaping, Trees, Shrubs, and Plant Layering General information. Several types of roadside land- scaping are commonly employed to enhance the aesthetics of roadside environments. These treatments may include the placement of shrubs, street trees, or alternative treatments such as landscape berms. In addition to the concern of tra- versability in the event that an errant vehicle encounters roadside landscaping, a common issue regarding the safety of adjacent landscape treatments is sight distance and the impact landscape treatments may have for intersection, driveway, and stopping sight distance considerations. Strategy summary. The placement criteria, in some cases, is based on the functional purpose or posted speed limits of adjacent roads. Common landscape placement issues include the following: • Avoid placement in proximity to intersections as discussed in the urban control zone section. • Avoid placement in proximity to driveways as discussed in the urban control zone section. • At locations with isolated hazardous trees, consider re- moving these trees. • At locations with known hazardous trees that cannot be relocated, shield the trees with safety barrier where possible. • Lateral offset placement of trees and landscaping as dis- cussed in the urban control zone section. Where practical, use plant layering in front of the more rigid items. In the event of an errant vehicle, this initial landscaping will func- tion as an energy dissipation device and slow down the vehicle prior to impact with the more rigid tree. • Implement median planting strategies as discussed in the lateral placement urban control zone section. • Maintaining a clear vision space, which is a space above ground that preserves the lines of sight for drivers, bicy- clists, and pedestrians. In general, this space should extend vertically 1 to 3 m [3.3 to 10 ft]. These dimensions will as- sure clear sight distance for drivers in low-riding sports cars as well as drivers in high trucks and buses. The “clear vision space” then is essentially the space above shrub growth and below tree overhang. A low tree overhang can also create an obstacle for pedestrian access. • Longitudinal placement of trees and landscaping will help keep landscaping growth from encroaching on other func- tions of the roadside environment. In addition to longitu- dinal placement strategies discussed in the urban control zone section, it is advisable to prohibit landscape place- ment at a variety of other locations. One jurisdiction, for example, recommends that these placement strategies could include the separation of trees from underground utility lines by 1.5 m [5 ft] and a placement a minimum of 3.0 m [10 ft] from utility poles with 4.6 m [15 ft] recommended. In addition, trees could be separated from street lights by 6.1 m [20 ft], from fire hydrants and alleys a distance of 3.0 m [10 ft], and 1.5 m [5 ft] from water meters or utility vaults. Additional longitudinal placement strategies may be implemented to try and achieve uniform tree spacing. This spacing will depend on the specific tree characteristics but could range from 7.6 to 15.2 m [25 to 50 ft]. Tree canopies should not be positioned under service wires. 61

• The strategic placement of landscaping to influence the vi- sual perception of a driver is a relatively new technique. Landscaping can be used to help visually delineate the downstream road and geometric features of that road. The use of landscaping for visual perception purposes can also help create visual narrowing of the driver’s field by gradu- ally tapering a tree line towards the road. Utility Poles, Posts, Light Standards General information. Utility poles, posts, light poles, and similar vertical roadside treatments are some of the most common urban roadside hazards. The urban environment, by its very nature, can be expected to include these common roadside objects. Strategy summary (for utility poles). Several potential strategies can be considered for addressing roadside safety for utility pole placement. These include the following: • Place utilities completely underground and remove the haz- ardous poles. The removal of all poles in the urban roadside environment may not be practical, but the placement of util- ities underground, where feasible, will minimize this hazard. • Place poles as far as possible from the active travel lanes. Rec- ommended goals include specific pole lateral clearance based on speed limits. One jurisdiction suggested a pole offset strat- egy with a target goal of 3.6 m [12 ft] from face of curb to face of pole for all locations where possible. For speed limits greater than 56 km/h [35 mph] but not exceeding 72 km/h [45 mph], a lateral clearance of 2.4 m [8 ft] is acceptable. For roads with posted speed limits less than or equal to 56 km/h [35 mph], a lateral clearance of 1.8 [6 ft] is acceptable. A sec- ond jurisdiction recommends an offset greater than 2.4 m [8 ft] for roads with speed limits of 40 to 55 km/h [25 to 35 mph] and a lateral offset of 4.3 m [14 ft] for roads with speed limits of 65 to 70 km/h [40 to 45 mph]. • Locate poles away from access points where the pole may restrict sight distance or be easily impacted. • Place poles on the inside of sharp horizontal curves (as errant vehicles tend to continue straight towards the out- side of curves), but be sure pole placement conforms with the urban control zone recommendations previously shown. • Locate poles on only one side of the road and place shared utilities on poles where possible. • Use breakaway poles at select hazardous locations or shield them with safety barrier. Mailboxes General information. The placement of mailboxes in urban environments can result in new hazardous roadside objects if jurisdictions do not enforce guidelines about mailbox type and placement. There are several crashworthy mailboxes that have been tested including standard boxes mounted on a 100 mm by 100 mm [4 in by 4 in] wooden post or a 38 mm [11/2 in] light-gauge pipe for mounting mailboxes, with these posts embedded no deeper than 600 mm [24 in] into the ground. Mailboxes should further be mounted to their supports to prevent the mailbox from separating from the post during a crash event. Standard cluster mailboxes (as approved by U.S. Postal Service Standards) can also be used in urban regions. Many of the larger mail collection boxes fail safety requirements and should be placed outside of clear recovery areas. Strategy summary. While making mailboxes crashwor- thy will satisfy safety associated with mailbox-related crashes, it is important to recognize that the placement of mailboxes may have an important impact on the overall safety of the roadway. The following recommendations detail appropriate placement of mailboxes: • Mailboxes should not obstruct intersection sight distance. • Mailboxes should not be located directly on higher-speed roadways, where stopping associated with mail delivery and collection can lead to substantial speed differentials between vehicles on the travelway, thereby increasing the possibility of a rear-end collision. For higher-speed urban roads without curb where mailboxes are present, one option is to provide a 2.4 m [8 ft] mailbox turnout lane adjacent to the travelway that will permit vehicles to leave the trav- elway for mail collection and delivery purposes. Alterna- tively, a minimum shoulder width for these higher-speed roads of 1.8 m [6 ft] should be maintained at these locations. • At curbed residential locations, mailboxes should be posi- tioned so that the minimum distance from the roadside face of the mailbox to the face of the curb is 150 mm [6 in], with a preferred offset ranging from 200 to 300 mm [8 to 12 in]. • Mailbox placement at driveways should be compatible with the urban control zones previously defined. • Shield rigid mailboxes. • Add reflective object markers to improve nighttime visi- bility of mailboxes. Safety Barriers General information. Roadside barriers are subject to NCHRP Report 350 testing criteria. There are several types of safety barriers that may be present in an urban environment. These include the following: • Barriers (flexible, semi-rigid, and rigid), • Bridge Railings, and • End Treatments (crash cushions and end terminals). 62

63 Generally, most of the research on safety barriers has been oriented towards the design of barriers and their placement to shield vehicles from hazardous roadside conditions. The Fed- eral Highway Administration maintains a roadside hardware website that provides information about specific roadside hard- ware that has been tested. This information is available at: http:// safety. fhwa.got.gov/roadway_dept/road_hardware/index.htm. Strategy summary. In the urban environment, it may be challenging to construct roadside barriers in the confined roadside space available. In locations with bicycle activity, for example, safety barriers located immediately adjacent to the road may expose cyclists to unnecessary risks as the barrier may result in a sensation of “squashing” the cyclist between the barrier and an adjacent motor vehicle. Similarly, barriers immediately adjacent to motor vehicle lanes cause vehicles to shy away from the barrier, thereby adversely impacting traf- fic operations. Finally, traffic barrier restricts pedestrian ac- tivity in an urban environment and requires careful design of openings for pedestrian crossings that include crashworthy barrier end treatments. Due to the wide variety of potential safety barriers that may be selected for use, the two references identified above should be consulted for specific applications for each barrier type. Street Furniture General information. In many urban areas the use of street furniture is a common approach to improving the aes- thetic and functional quality of a street. Street furniture includes items placed adjacent to the road that are there to improve the adjacent land use or to improve transportation operations. In some jurisdictions, street lights and signs are included in the category of street furniture; however, for the purposes of this review street furniture is considered to be supplemental items such as benches, public art, trash recep- tacles, phone booths, planters, bollards, fountains, kiosks, transit shelters, bicycle stands, etc. Often the placement of these devices can obscure sight distance, so their location should not occur in the sight triangles of intersections or driveways. Many street furniture items are placed along the right-of-way by the property-owners themselves, as in the case of the placement of a sidewalk cafe in front of a restau- rant, and are thus largely outside the engineer’s control. Strategy summary. Little is known about the safe place- ment of street furniture. The following general recommen- dations should enhance roadside safety in these locations: • While maintaining its functional purpose, locate street fur- niture as far from the street as possible. • Restrict street furniture placement to avoid sight distance issues for road users. The Urban Control Zones previously identified should be applied to all street furniture. • Where possible, deploy street furniture that meets basic crashworthy standards; however, concern for pedestrians has led to the use of fixed supports in some urban areas. Examples of sites where breakaway supports may be imprudent are sites adjacent to bus shelters or in areas of extensive pedestrian concentration.

Next: Appendix D - Draft Chapter 10 for AASHTO Roadside Design Guide »
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TRB's National Cooperative Highway Research Program (NCHRP) Report 612: Safe and Aesthetic Design of Urban Roadside Treatments explores recommended design guidelines for safe and aesthetic roadside treatments in urban areas. The report also examines a toolbox of roadside treatments designed to balance pedestrian, bicyclist, and motorist safety and mobility.

NCHRP Report 612 includes four appendices, three of which are available online. The fourth, Appendix C, is included with the report.

Appendix A includes detailed information about the urban control zone corridor sites.

Appendix B includes the summary statistics for the report's case study sites.

Appendix C includes an urban roadside design toolbox, and

Appendix D provides draft language for the urban chapter in the AASHTO Roadside Design Guide.

Tables 19 and 20 on p. 43 of NCHRP Report 612 include incorrect information. The corrected tables are available online.

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