National Academies Press: OpenBook

Roundabouts: An Informational Guide – Second Edition (2010)

Chapter: Chapter 9 - Landscaping

« Previous: Chapter 8 - Illumination
Page 300
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 300
Page 301
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 301
Page 302
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 302
Page 303
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 303
Page 304
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 304
Page 305
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 305
Page 306
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 306
Page 307
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 307
Page 308
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 308
Page 309
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 309
Page 310
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 310
Page 311
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 311
Page 312
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 312
Page 313
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 313
Page 314
Suggested Citation:"Chapter 9 - Landscaping." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
×
Page 314

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.

Chapter 9/Landscaping Page 9-1 Roundabouts: An Informational Guide CHAPTER 9 LANDSCAPING CONTENTS 9.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-3 9.2 PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-7 9.3 CENTRAL ISLAND LANDSCAPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8 9.4 SPLITTER ISLAND AND APPROACH LANDSCAPING . . . . . . . . . . . . 9-13 9.5 MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-13 9.6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-15

Roundabouts: An Informational Guide Page 9-2 Chapter 9/Landscaping LIST OF EXHIBITS Exhibit 9-1 Examples of Landscaping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-3 Exhibit 9-2 Summary of Roundabout Landscaping Zones . . . . . . . . . . . . . . . . . . 9-7 Exhibit 9-3 Landscaping Considerations as a Function of Diameter . . . . . . . . . . 9-8 Exhibit 9-4 Central Island Landscaping Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-9 Exhibit 9-5 Example of Central Island Landscaping . . . . . . . . . . . . . . . . . . . . . . . 9-9 Exhibit 9-6 Examples of Central Island Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-10 Exhibit 9-7 Landscaping Trade-Offs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-12 Exhibit 9-8 Example of Splitter Island Landscaping Encroaching on Sight Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-13 Exhibit 9-9 Maintenance of Landscaping in Central Island . . . . . . . . . . . . . . . . 9-14

Chapter 9/Landscaping Page 9-3 Roundabouts: An Informational Guide 9.1 INTRODUCTION Landscaping is one of the distinguishing features that give roundabouts an aesthetic advantage over traditional intersections. Landscaping in the central island, splitter islands (where appropriate), and along the approaches can benefit both public safety and community enhancement. In addition to landscaping, some agencies use the central island of a roundabout as an opportunity to display local art or other gateway features. To determine the type and quantity of landscaping or other material to incorporate into a roundabout design, maintenance, sight dis- tance, and the available planting zones should all be considered. The primary objectives and considerations of incorporating landscaping or art into a roundabout design are to: • Make the central island more conspicuous, thus improving safety; • Improve the aesthetics of the area while complementing surrounding streetscapes as much as possible; • Make decisions regarding placement of fixed objects (e.g., trees, poles, walls, guide rail, statues, or large rocks) that are sensitive to the speed environment in which the roundabout is located; • Avoid obscuring the form of the roundabout or the signing to the driver; • Maintain adequate sight distances, as discussed in Chapter 6; • Clearly indicate to drivers that they cannot pass straight through the intersection; • Discourage pedestrian traffic through the central island; and • Help pedestrians who are visually impaired locate sidewalks and crosswalks. Exhibit 9-1 provides a variety of landscaping examples. Maintenance, sighting distance, and available planting zones should be considered when designing landscaping. (a) Carson City, Nevada Exhibit 9-1 Examples of Landscaping

Roundabouts: An Informational Guide Page 9-4 Chapter 9/Landscaping (b) Davis, California (c) Monroe, Washington (d) Denver, Colorado Exhibit 9-1 (cont.) Examples of Landscaping

Chapter 9/Landscaping Page 9-5 Roundabouts: An Informational Guide (e) Coralville, Iowa (f) Parkville, Missouri (g) Perth, Western Australia, Australia Exhibit 9-1 (cont.) Examples of Landscaping

Roundabouts: An Informational Guide Page 9-6 Chapter 9/Landscaping (h) Towson, Maryland (i) Reno, Nevada (j) Anchorage, Alaska Exhibit 9-1 (cont.) Examples of Landscaping

Chapter 9/Landscaping Page 9-7 Roundabouts: An Informational Guide 9.2 PRINCIPLES Landscaping should be designed to ensure that vehicles can observe the signing and shape of the roundabout as they approach and have adequate visi- bility for making decisions within the roundabout. As described in Chapter 6, the sight distance requirements at the roundabout dictate the size and types of landscaping materials appropriate for the various areas within and adjacent to the roundabout. Landscaping within the critical visibility areas must be limited to a height of 2 ft (0.6 m) to ensure adequate sight distance. The appropriate planting zones within a roundabout and the types of landscaping for each zone are described below. The overall speed environment of the roadway is another important consid- eration when selecting plant material and other landscape features. Within lower-speed urban environments [typically 35 mph (55 km/h) or less], there is generally more flexibility than in higher-speed suburban and rural environ- ments [typically 40 mph (65 km/h) or greater] where drivers are traveling at greater speeds upstream of the roundabout. Therefore, the types and location of landscape features are dependent on operating environment and the potential risk. Exhibit 9-2 illustrates the typical landscaping zones within a roundabout. Exhibit 9-2 Summary of Roundabout Landscaping Zones

Roundabouts: An Informational Guide Page 9-8 Chapter 9/Landscaping 9.3 CENTRAL ISLAND LANDSCAPING The landscaping of the central island can enhance the safety of the intersection by making the intersection a focal point, by promoting lower speeds, and by breaking the headlight glare of oncoming vehicles. Landscaping elements should be selected so that sight distance (discussed in Chapter 6) is maintained where required. Conversely, the landscaping should also be strategically located to limit the amount of excess sight distance to help encourage slow speeds. This typically results in different types of landscaping being considered for the inner and outer portion of the central island, as described below. Landscaping plans must give consideration of future maintenance requirements to ensure adequate sight distance for the life of the project It is desirable to create a domed or mounded central island to increase the visibility of the intersection on the approach. The Wisconsin Department of Transportation Facilities Development Manual recommends a minimum elevation of 3.5 ft (1.0 m) and a maximum elevation of 6 ft (1.8 m) for the domed area on the central island (1). In addition, the slope of the central island should not exceed a horizontal-to-vertical ratio of 6:1 in order to enable errant vehicles to recover (2). The size of the roundabout can influence the type and location of landscaping. Large and small diameter roundabouts have unique landscaping trade-offs that should be considered, as seen in Exhibit 9-3. Exhibit 9-3 Landscaping Considerations as a Function of Diameter Large Diameter Small Diameter There is more surface area for landscaping features. A greater focal point for visibility is available as drivers approach the intersection. There is greater opportunity to create a gateway feature for community enhancement. A greater amount of landscaping is required, which requires initial installation cost and ongoing maintenance. State and city agencies often cannot provide ongoing maintenance of roundabouts; therefore, an agreement with a local civic group and/or garden club may be necessary. If limited maintenance is desired, hardscape features may be installed. Central island landscaping features (trees, gateway features, hardscape) can create a potential fixed- object conflict, particularly for the high-speed approaching vehicles. There is less surface area for landscaping features. The limited surface area would likely require a lower initial installation cost and less ongoing maintenance. Central island landscaping is likely not feasible, and the focus should be on the perimeter of the roundabout. Perimeter landscaping does not typically provide the same visibility benefits to drivers approaching the roundabout. A small central island provides less opportunity for gateway features in the center of the roundabout. Less concern for fixed-object conflicts exists when trees and gateway features are not placed within the central island. Care is needed when considering landscaping that introduces fixed objects within the central island, particularly in environments with higher approach speeds. While it is important to provide features that increase the visibility of the roundabout to approaching drivers, fixed objects such as trees, poles, walls, guide rail, statues, and large rocks can introduce potential safety concerns for errant vehicles. In most cases, fixed objects should be minimized, particularly in the

Chapter 9/Landscaping Page 9-9 Roundabouts: An Informational Guide perimeter area of the central island. If used, fixed objects should preferably be placed in a location where the geometry of the roundabout deflects approaching vehicles away from the object. In some cases, trees, shrubs, statues, and other larger items can be placed on the inner central island to help obscure the line of sight straight through the round- about to provide drivers an indication that they cannot pass straight through the intersection. In addition, landscaping in this planting zone can make the round- about more visible at night with the vehicle headlights illuminating the central island (3). The perimeter portion of the central island can be landscaped with low-level shrubs, grass, or groundcover to ensure that stopping sight distance requirements are maintained for vehicles within the circulatory roadway and at the entrance line of the roundabout. The planting zone width around the perimeter of the cen- tral island will vary depending on the size of the roundabout and the required sight triangles. Exhibit 9-4 illustrates the two potential landscaping zones and possible landscape features within the central island. Exhibit 9-5 shows an example of proper landscaping within the central island. Exhibit 9-4 Central Island Landscaping Profile Avon, Colorado Exhibit 9-5 Example of Central Island Landscaping

Roundabouts: An Informational Guide Page 9-10 Chapter 9/Landscaping In northern climates, the salt tolerance of any plant material should be consid- ered, as well as snow storage and removal practices. In addition, landscaping that requires watering may increase the likelihood of wet and potentially slippery pavement. Landscaping within the central island should discourage pedestrian traffic to and through the central island. Street furniture that may attract pedestrian traffic to the central island, such as benches or monuments with small text, should be avoided. Communities commonly desire to place public art or other large aesthetic objects within the central island, including statues, fountains, monuments, and other gateway features for community enhancement. This type of landscaping is acceptable provided that the objects are located outside the sight triangles and minimize the likelihood of a fixed-object conflict for errant vehicles. In addition, the central island features should not impact the vehicles circulating the round- about. For example, fountains in windy areas can generate water spray that impacts drivers’ visibility through the intersection. In some areas, a roundabout design can help define a community, township, or region by displaying a piece of art that represents local heritage. This is particu- larly the case in European countries, where it is an honor for an artist to have art displayed in the central island of a roundabout, and communities look for ways to display their cultural characteristics through roundabout art (4). Exhibit 9-6 illustrates examples of central island art. Exhibit 9-7 discusses the trade-offs of landscaping roundabouts. Uplighting is an additional feature that some agencies use, particularly for trees or aesthetic features on the central island. While uplighting can provide an aesthetic nighttime feature by illuminating the trees or art, other agencies do not use uplighting due to its impact on the natural night sky. Exhibit 9-6 Examples of Central Island Art (a) Federal Way, Washington

Exhibit 9-6 (cont.) Examples of Central Island Art Chapter 9/Landscaping Page 9-11 Roundabouts: An Informational Guide (b) Bend, Oregon (c) Des Moines, Iowa (d) Pemberton, British Columbia, Canada

Roundabouts: An Informational Guide Page 9-12 Chapter 9/Landscaping Exhibit 9-7 Landscaping Trade-Offs Example: Landscaping Trade-Offs Scenario A roundabout has been designed on a state highway on the eastern edge of a city. The east approach has a posted speed of 50 mph, and all other approaches have a posted speed of 35 mph. The state highway serves as the primary route through the city; therefore, the city would like the roundabout to serve as a community enhancement with aesthetic gateway features. The intersection has a crash history involving high-speed vehicles on the east approach; therefore, the city would like the roundabout to increase driver awareness and potentially reduce speeds of vehicles approaching the intersection. Question What are the trade-offs in installing landscaping? Principles The primary considerations for developing a landscaping plan for a roundabout include: • Ensure visibility and sight distance for vehicles approaching and traveling through the roundabout, • Identify potential speed-reduction measures for the east approach, • Identify potential for fixed-object conflicts on the high-speed approach and review the recommended clear-zone and offset distances, • Develop maintenance agreement with state and city agencies, and • Create a gateway feature for the community. Alternative 1: Install landscaping at the roundabout • Creates opportunity for community enhancement through gateway features and aesthetics. • Requires the development of a maintenance program. • Requires additional construction cost to install landscaping. • Provides visibility for drivers approaching the roundabout. • Creates funneling effect at the roundabout entries. • Encourages proper use of pedestrian walkways. • Provides the opportunity for speed reduction and increased driver awareness on each approach by introducing changes in the roadway environment. Alternative 2: Do not install landscaping at the roundabout • Minimizes and even eliminates the need for maintenance. • Reduces the construction costs. • Provides less community enhancement. • Does not provide as much visibility as drivers approach the roundabout. • Creates the need for other visibility features to ensure that drivers do not pass straight through the intersection. − Additional approach signing. − Mounding the central island. • Creates potential for improper pedestrian crossing. • Reduces the concern for fixed-object conflicts. • May require additional mitigation for high speeds on the east approach to reduce approach speeds. − Lengthen the splitter island. − Install speed-reduction treatments, such as rumble strips. − Dynamic warning signs. − Transverse pavement markings.

Chapter 9/Landscaping Page 9-13 Roundabouts: An Informational Guide 9.4 SPLITTER ISLAND AND APPROACH LANDSCAPING When designing landscaping for the splitter islands and along the outside edges of the approach, care should be taken with the landscaping to avoid obstructing sight distance since the splitter islands are usually located within the critical sight triangles (see Chapter 6). Exhibit 9-8 gives an example where the vegetation in the splitter island is beginning to encroach on driver sight lines. In addition, landscaping should not obscure the form of the roundabout or signing to an approaching vehicle. Therefore, the size of the splitter islands and location of the roundabout are determining factors in assessing whether to provide landscap- ing within the splitter islands (1). Exhibit 9-8 Example of Splitter Island Landscaping Encroaching on Sight Lines San Diego, California Landscaping on the approaches to the roundabout can enhance safety by mak- ing the intersection a focal point and by reducing the perception of a high-speed through-traffic movement. Plant material in the splitter islands (where appropriate) and on the right and left side of the approaches can help to create a funneling effect and induce drivers to slow down when approaching the roundabout. Landscaping between the sidewalk and the circulatory roadway will help to channelize pedestri- ans to the crosswalk areas and discourage pedestrian crossing to the central island. Because a portion of the splitter island and the area between the sidewalk and the circulatory roadway are typically situated within the critical sight triangles, the landscaping in these areas may be constructed with low-growth plants or grass. Grass or low shrubs are also desirable due to their ability to blend well with nearby streetscapes and the fact that they require only limited maintenance. Splitter islands should generally not contain trees, planter boxes, or light poles. Hardscape treat- ments like a simple patterned concrete or paver surface may be used on splitter islands in lieu of landscaping. 9.5 MAINTENANCE A realistic maintenance program should be considered in the design of the landscape features of a roundabout. Prior to developing a landscaping plan for a roundabout, the responsible party for future maintenance, water supply, drainage,

Exhibit 9-9 Maintenance of Landscaping in Central Island Roundabouts: An Informational Guide Page 9-14 Chapter 9/Landscaping and expected growth of the plantings should be addressed. Exhibit 9-9(a) shows an example of landscaping being maintained within the central island. The agency or group responsible for maintaining the landscaping should be identified. It is generally necessary for local governments to assume maintenance responsibilities for the roundabout landscaping to provide enhancements to their communities. However, it may be unrealistic to expect a typical highway agency to maintain a complex planting plan. In these cases, formal agreements may be devel- oped with local civic groups and garden clubs for maintenance. Liability issues should be considered in writing these agreements. Where there is no interest in maintaining the proposed enhancements, the landscape design should consist of simple plant materials or hardscape items that require little or no maintenance. A water supply that is accessible to service vehicles should be provided on the central island or adjacent to the intersection. Landscaping that requires frequent watering may require installation of a sprinkler system. Proper drainage for the watering system should be provided and should minimize the water runoff onto the circulatory roadway. Watering systems with a mist-type spray should be avoided as water spray onto windshields could create safety concerns (1). In addi- tion, proper access for maintenance vehicles to the central island and splitter islands should be considered. Potential stoppage or pullout areas for maintenance vehicles (a) Central island maintenance (Decatur, Georgia) (b) Maintenance pullout in central island (Bend, Oregon)

Chapter 9/Landscaping Page 9-15 Roundabouts: An Informational Guide should be located such that visibility and access for vehicles and pedestrians is preserved (5). Exhibit 9-9(b) shows an example of a pullout area for maintenance vehicles. It is important that the plants and trees within the roundabout do not interfere with the users’ visibility within the roundabout. Therefore, the expected growth of specific plant and tree species included in a landscape plan should be consid- ered. In addition, grass, trees, and shrubs should be regularly trimmed or pruned to prevent obstruction of the sight triangles and to maintain the aesthetics of the intersection (1). 9.6 REFERENCES 1. Facilities Development Manual. Wisconsin Department of Transportation, 2009. 2. Roadside Design Guide. AASHTO, Washington, D.C., 2006. 3. Kansas Roundabout Guide. Kansas Department of Transportation, 2005. 4. Institute of Transportation Engineers. Enhancing Intersection Safety through Roundabouts: An ITE Informational Report. Unpublished. July 2008 Draft. 5. The Design of Interurban Intersections on Major Roads: At-Grade Intersections. Sétra (Service d’Études Techniques des Routes et Autoroutes), Bagneux, France, 1998.

Next: Chapter 10 - Construction and Maintenance »
Roundabouts: An Informational Guide – Second Edition 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 672: Roundabouts: An Informational Guide – Second Edition explores the planning, design, construction, maintenance, and operation of roundabouts. The report also addresses issues that may be useful in helping to explain the trade-offs associated with roundabouts.

This report updates the U.S. Federal Highway Administration’s Roundabouts: An Informational Guide, based on experience gained in the United States since that guide was published in 2000.

Errata

Equation 6-3 on page 6-58 incorrectly contains an addition sign (+) as an operator. The correct operator should be a subtraction sign (-).

Errata #2

Exhibit 5-23 on page 5-29 of NCHRP Report 672 contains an incorrect calculation of estimated injury crashes.

There is a summary document, Paths to Practice, available.

  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!