National Academies Press: OpenBook

Safe and Aesthetic Design of Urban Roadside Treatments (2008)

Chapter: Chapter 4 - Conclusions and Suggested Research

« Previous: Chapter 3 - Findings and Applications
Page 50
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." 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 50
Page 51
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." 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 51

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

50 Conclusions The urban roadside environment is complex. Due to the constrained nature of this built environment, it is difficult for a designer to achieve an acceptable clear zone, free of objects. As a result, a lateral offset that enhances roadway operations may be used, but this offset does not represent a safe place- ment for rigid roadside objects. This research identified known safety characteristics and placement strategies for urban roadside objects by means of a comprehensive literature review. Following this state-of-the-practice identification, the research team further evaluated roadside safety conditions using two approaches. First, the research team videotaped over 241 km (150 mi) of urban corridors and compared their 6-year crash history and crash locations to the various roadside features observed on these corridors. The result of this assessment was the iden- tification of several potential urban control zones. These locations are shown to have a greater likelihood of crashes and, as a result, should be kept free of rigid objects whenever possible. These urban control zones include locations with the following: • Obstacles in close lateral proximity to the curb face or lane edge; • Roadside objects placed near lane merge points; • Lateral offsets not appropriately adjusted for auxiliary lane treatments; • Objects placed inappropriately in sidewalk buffer treatments; • Driveways that interrupt positive guidance and have ob- jects placed near them; • Three kinds of fixed-object placement at intersections; • Unique roadside configurations associated with high crash occurrence; and • Roadside configurations commonly known to be hazardous. Each of these urban control zones is reviewed in detail in Chapter 3. The recommendations that are the result of this research effort are the following: • Where possible at curb locations, provide a lateral offset to rigid objects of at least 1.8 m (6 ft) from the face of the curb and maintain a minimum lateral offset of 1.2 m (4 ft). • At lane merge locations, do not place rigid objects in an area that is 3.0 m (10 ft) longitudinally from the taper point. This will result in a 6.1-m (20-ft), object-free length at the taper point. The lateral offset for this 6.1-m (20-ft) section should be consistent with the lane width, typically 3.7 m (12 ft). • Although many auxiliary lanes, such as bus lanes or bicy- cle lanes, have low volumes and may be included as part of a clear zone in the urban environment, higher speed aux- iliary lane locations, such as extended length right-turn lanes, are common locations for run-off-road crashes. A lateral offset of 1.8 m (6 ft) from the curb face to rigid ob- jects is preferred, and 1.2-m (4-ft) minimum lateral offset should be maintained. • At locations where a sidewalk buffer is present, rigid ob- jects should not be located in the buffer area when it has a width of 0.9 m (3 ft) or less. For buffer widths greater than 0.9 m (3 ft), lateral offsets from the curb face to rigid objects should be maintained with a minimum offset of 1.2 m (4 ft). At these wider buffer locations, other frangi- ble objects can be strategically located to help shield any rigid objects. • Rigid objects should not be located in the proximity of driveways, and care should be taken to avoid placing rigid objects on the immediate far side of a driveway. In addi- tion, objects should not be located within the required sight triangle for a driveway. A second component of this research included a case study assessment for roadside enhancement or beautification C H A P T E R 4 Conclusions and Suggested Research

51 projects. At these locations, the governing agencies incor- porated a variety of urban roadside changes to improve the aesthetic quality of the roadside and enhance the functional use of the space, often with particular emphasis on pedes- trian facilities. Although the findings of this task were in- conclusive, the individual case studies can be used by agencies to help determine general safety trends for similar future projects. Suggested Research This research effort creates a foundation for better under- standing on how urban roadside configurations can influence safety. As with any such effort, the questions answered by this research also help to identify knowledge gaps. The gaps could substantially benefit from future research efforts. Specifically, the research team identified five specific issues that merit additional research. The first issue is the influence of positive guidance at driveway and intersection locations. This issue appeared to contribute to crash conditions; how- ever, the disproportionate number of sites where positive guidance in the form of a white edge line was not present pro- hibited the researchers from drawing definitive conclusions regarding this issue. The second issue of interest is the evaluation of auxiliary lanes and their role in roadside safety. In some instances, the inclusion of a bicycle lane provided an additional offset to roadside objects. At these locations, the number of roadside crashes appeared to be reduced. This observation suggests that the bicycle lane can be included as part of the available clear zone and that at locations where this occurs, the white stripe that separates the motor vehicle lane from the bicycle lane could serve as the edge of the clear zone. Alternatively, some of the sites studied included what appeared to be an almost continuous right-turn lane (referred to in the report as an extended right-turn lane). At these locations, turn movements were channelized by pavement markings only. A large number of crashes occurred when these auxiliary turn lanes functioned similarly to through lanes, yet lateral offsets were not increased. At locations where upstream or down- stream lateral offsets were approximately 4.3 m (14 ft), these offsets were reduced to less than 0.6 m (2 ft) adjacent to the turn lanes. The number of roadside crashes dramatically in- creased as a result. Future research should investigate when an auxiliary lane should be treated as another motor vehicle lane. For example, could a short right-turn pocket be treated like a bicycle lane and this width be included in the clear zone or should any lane designed for motor vehicles, regardless of its function, be treated similarly? A third issue for future research is the definition of a haz- ardous tree. Although this study identified some recommended tree placement strategies, the concept of a tree as a rigid object requires further definition. Historically, a tree with a caliper width of 4 in. or more has been considered a rigid object, but the literature review indicated that this dimension was based on wooden pole crash tests. The influence of tree type (soft wood versus hard wood), tree size, root system configuration, and similar issues merits further consideration. A fourth issue for further research resulting from this eval- uation is the influence of moving light standards farther from the travel lane and how this change in location might affect nighttime visibility. At the study corridors, numerous light standards located close to the road were hit by vehicles, so the recommendation to move these lights closer to the near side of the sidewalk or even to the far side of the sidewalk would probably improve safety as it relates to roadside haz- ards. The effect of this relocation of the street lights on safety as it relates to visibility merits further evaluation for light pedestals that do not include mast arm configurations that can be easily lengthened. The fifth issue for further research is roadside improvements at intersections where pedestrian access ramps appear to direct an errant motor vehicle toward a rigid object (often a signal pole). It seems like a minor issue to shift the pole so that this conflict is minimized; however, often a pedestrian button is located on the pole, and this relocation could adversely affect operations for the pedestrian. As a result, the placement of traffic signal poles in relation to access ramps, roadside safety, and pedestrian usability should be assessed. Finally, the research team identified one additional item that is not included in the five research issues but appears to warrant further evaluation. For the corridor analysis task, the research team attempted to identify corridors with relatively high operating speeds (as higher speed crashes generally result in greater injury severity). A few of these corridors transi- tioned into lower speed corridors with on-street parking and curb extensions. The curb extensions generally were posi- tioned to help define intersections and to enable shorter pedestrian crossing distances. Since the number of sites with this lower speed configuration was limited, the research team could not comprehensively evaluate these curb extension locations. However, at the few locations where the research team did observe these extensions, roadside crashes appeared to peak during nighttime hours, presumably when on-street parking was limited. Due to the small sample size, the re- search team could not draw any definitive conclusions; there- fore, the team strongly recommends that roadside safety at curb extensions be the subject of future research.

Next: References »
Safe and Aesthetic Design of Urban Roadside Treatments Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!