Safe and Aesthetic Design of Urban Roadside Treatments (2008) / Chapter Skim
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Pages 5-36
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From page 5... ...
Urban roadside environments can range from dense downtown zones with on-street parking to high-speed zones with motor vehicle operational priorities. Given the importance of the roadside environment to the quality of urban life, it is unsurprising that urban residents and stakeholders often seek to have the roadside designed in a manner that enhances the quality of the urban environment.
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From page 6... ...
This literature review summarizes known roadside design safety guidance for roadways in urban areas. There is often little substantive knowledge on the safety impacts of various design treatments, leaving the definition of what constitutes a "safe" facility open to question.
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From page 7... ...
In addition, this chapter reviews the following: • Roadside crash statistics, in an effort to identify the specific nature of roadside crashes in urban areas; • The various strategies currently in use in urban environments to keep vehicles from leaving the travelway; and • General information, safety research, and proposed safety strategies for a variety of potential roadside objects common to the urban environment. Although this review targets the design of roadsides in urban areas, much of the literature on roadside design has been based on studies of rural environments.
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From page 8... ...
Another secondary strategy that is commonly employed to alert drivers before their vehicle leaves the travelway is to place rumble strips on the shoulder of the roadway. Delineate Potentially Hazardous Roadside Environments A common practice in minimizing run-off-road crashes is to use signs to delineate potentially hazardous roadside conditions.
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From page 9... ...
In rural and select urban environments where roadways do not have curb lines, a common method for lane edge delineation is edge striping (using reflective paint for low-volume roads and thermoplastic stripes for more densely traveled facilities)
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From page 10... ...
; thus, the ability of rumble strips to alert drivers 10 Photo reprinted from "New Focus for Highway Safety." (119) Figure 1.
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From page 11... ...
shoulder rumble strips assumes the existence of a level, paved shoulder. In many urban environments, raised curb is used in lieu of shoulders.
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From page 12... ...
The first is to incorporate frangible roadside objects and hardware into the 12 Features Immediately Adjacent to the Travelway Safety Barriers Curbs Barriers and Guardrails Shoulders Bridge Railings Channelization Crash Cushions and End Terminals Medians Roadside Grading Static Roadside Objects Dynamic Roadside Features Mailboxes Bicycle Facilities Landscaping, Trees, and Shrubs Parking Street Furniture Sidewalks and Pedestrian Facilities Utility Poles, Luminaires, and Sign Posts/Hardware Table 5. Common urban roadside features.
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From page 13... ...
Much of the rural research regarding pavement edge treatments evaluates the influence of graded or paved shoulders on safety performance at the time a vehicle enters the roadside environment. In an urban environment, very few road edge treatments include roadway shoulders as a transition from the travel lanes to the adjacent roadside environment.
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From page 14... ...
A variety of strategies have been proposed, applied, and/or tested for safe application of curb treatments. Common strategies are as follows: Purpose Strategy Prevent curb from vault- • Use appropriate curb heights with ing vehicles known influences on vehicle trajectories (P)
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From page 15... ...
Safety research. The research regarding shoulder safety has been generally divided into three categories -- safe shoulder width, pavement edge treatments, and safety of paved versus graded shoulders.
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From page 16... ...
Currently, the Federal Highway Administration promotes a pavement edge treatment called the safety edge that uses a similar 45-deg angle with construction standards that permit compaction to provide pavement edge stability. • Safety of paved versus graded shoulder.
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From page 17... ...
The issue of landscaping and the specific evaluation of tree placement are further discussed in the landscaping section; however, a recent three-phase study performed at California Polytechnic State University (53) specifically evaluated the placement of large trees in raised medians on urban and suburban highways.
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From page 18... ...
Mailboxes should not obstruct intersection sight distance, nor should they 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. Where such conditions exist, the Roadside Design Guide recommends the use of a 2.4-m (8-ft)
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From page 19... ...
These treatments may include the placement of shrubs, street trees, or alternative treatments such as landscape berms. In addition to the concern of traversability in the event that an errant vehicle encounters roadside landscaping, a common safety issue of adjacent landscape treatments is sight distance and the impact landscape treatments may have for intersection, driveway, and stopping sight distance considerations.
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From page 20... ...
With the landscaping placement close to the driveway, the driver of an exiting vehicle cannot detect approaching vehicles without edging into the active travel lane; therefore, these roadside treatments encroach on the required sight distance. As can be viewed in the photo, the adjacent property owner also positioned large "ornamental" rocks at the corner, thereby adding a rigid obstacle in the immediate vicinity of the roadway.
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From page 21... ...
Figure 6. Landscape berm that blocks horizontal sight distance.
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From page 22... ...
recommends plant layering, an approach in which plants are grouped according to height, as depicted in Figure 7. This plant layering approach permits the use of roadside landscaping and, as indicated in the guide, will do the following: – Allow wider clear zones for rigid objects, – Permit the inclusion of large trees in the roadside design, – Allow appropriate sight distance, and – Permit visually appealing plant compositions.
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From page 23... ...
Table 8 depicts the distance to roadside hazards for fatalities that occurred during the study period for speed zones of 80 km/h (50 mph) or less.
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From page 24... ...
and very little direction regarding appropriate placement of trees in medians. As a result, Phases 2 and 3 of the study were focused on evaluating urban street trees in curbed urban and suburban highway medians with a variety of median widths, including narrow medians.
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From page 25... ...
Common street furniture safety strategies are as follows: Purpose Strategy Minimize likelihood of • Locate street furniture as far crash from street as possible (P) • Restrict street furniture placement to avoid sight distance issues for road user (P)
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From page 26... ...
In general, utility poles should be placed in the following locations: – As far as possible from the active travel lanes, – Away from access points where the pole may restrict sight distance, – Inside a sharp horizontal curve (as errant vehicles tend to continue straight toward the outside of curves) , and – On only one side of the road (66, 77)
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From page 27... ...
. 27 Graphic reprinted from Roadside Design Guide (1)
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From page 28... ...
The use of barriers in an urban environment can be to shield roadside obstacles (such as rigid utility poles) , separate motorized and nonmotorized traffic, and provide a physical separation between the active travel lanes and pedestrian activity.
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From page 29... ...
Wide shoulders and bicycle lanes provide an additional "clear" area adjacent to the travelway, so these features could potentially provide a secondary safety benefit for motorists, provided bicycle volumes are low. These bicycle facilities will also further separate the motor vehicle from any roadside obstructions and improve the resulting sight distance for motor vehicle drivers at intersecting driveways and streets.
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From page 30... ...
For locations with head-in or reverse-in parking, the crash severity likelihood is increased as the moving vehicle may impact a vehicle in reverse. Proper sight distance and separation of parked vehicles from the active travel lane (often by the use of a bulbout at the intersection)
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From page 31... ...
Buffer strips may be either planted or paved. The Green Book supports the use of buffer strips on urban arterials, collectors, and local streets (37)
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From page 32... ...
Common strategies for eliminating or minimizing motor vehicle–pedestrian crashes at roadside locations are as follows: Purpose Strategy Reduce motor vehicle- • Provide continuous pedestrian pedestrian crash facilities (P) likelihood at roadside • Install pedestrian refuge medians locations or channelized islands (see previous section on medians and islands)
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From page 33... ...
Most of the research regarding gateway treatments focuses on their influence on operating speed or road users' perceptions of gateway treatments and their understanding that they are, in fact, transitioning into a different and slower speed environment. In a 1997 study in the United Kingdom, researchers at the Transport Research Laboratory performed a before-after evaluation of a series of traffic calming strategies on major roads (112)
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From page 34... ...
. The researchers evaluated the gateway transition from rural to urban environments in two phases: • From the "Traffic Calming Ahead" sign to the "Do Not Pass" sign and • From the "Do Not Pass" sign to the gateway treatments in the form of raised islands.
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From page 35... ...
. Graphic reprinted from "Raised Traffic Islands at City Limits -- Their Effect on Speed" (113)
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From page 36... ...
The operational offset concept for curbed urban roadways should, therefore, not be mistaken as a safety standard, and urban setback policies should be considered for future adoption. Although they are not the focus of this research, the strategies used in urban environments to help prevent errant vehicles from leaving the travelway and encountering a fixed roadside object are briefly reviewed in this report.
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