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11 to hazardous conditions is largely restricted to the vibration possibilities for improving roadside safety in urban areas. the rumble strips produce. This vibration is nevertheless a Placing a mid-lane rumble strip in the outside travel lane can substantial cue for increasing the driver's awareness of the be used to produce the same effect on the vehicle as a shoulder- roadway environment. Several studies of the effectiveness of based treatment (i.e., sound and vibration) without necessi- rumble strips have determined that placement of rumble tating a roadside treatment. strips can decrease the number of run-off-road crashes be- The use of a mid-lane treatment raises two potentially im- tween 30 and 85 percent (25, 26). portant questions. First, what is the appropriate location of such a treatment for a curbed urban roadway? Second, what are the impacts of such a treatment on motorcyclists? In the Applicability of Shoulder Rumble Strips case of roadways where the shoulders are curbed, or where to Low-Speed Urban Roadways there is a limited operational offset, the mid-lane rumble strip While shoulder-based rumble strips have proven effective can be oriented to correspond to the expected location of in reducing run-off-road crashes on interstates and freeways the left tire of the roadway's design vehicle. In these cases, the (particularly in rural environments), their applicability to narrowest vehicle--a passenger vehicle--is the appropriate lower-speed roadways may be limited. The use of physical design vehicle for the treatment. Thus, the left tire of passen- (grooved) shoulder rumble strips assumes the existence of a ger vehicles will be used to delineate the appropriate position level, paved shoulder. In many urban environments, raised of mid-lane rumble strip treatments (or the right tire, assum- curb is used in lieu of shoulders. This prevents the possibility ing a treatment oriented toward preventing a crash into the of introducing physical rumble strips as a potential treatment median). While such an application will do little to address for eliminating run-off-road urban crashes; however, ther- the safety needs of larger design vehicles, it should, nevertheless, moplastic rumble strips may be used in the urban setting to have an effect on decreasing the rates of passenger-vehicle achieve a similar result. run-off-road crashes. Another issue affecting use of shoulder rumble strips in Addressing the needs of motorcyclists is more difficult. urban areas is that when a shoulder is available in urban areas, While it has been demonstrated that motorcyclists can safely in many cases it serves as a travelway for bicyclists (27). Beyond navigate rumble strips (24), the vibration associated with the physical unpleasantness that rumble strips may pose for rumble strips can create discomfort when the rider is forced the bicyclist, the application of rumble strips can potentially to travel over them for prolonged periods. An assumed min- result in the loss of control of the bicycle (28). Given the imum motorcycle tire width of approximately 13 cm (5 in.) potentially negative influence on bicycle use in urban areas, can reasonably be accommodated with a left- or right-tire as well as the frequent use of raised curb, the use of shoulder offset on a 3-m (10-ft) travel lane. Nevertheless, such an appli- rumble strips is typically not appropriate on low-speed urban cation should be further researched before being employed in roadways. practice. Finally, a common complaint about rumble strips in urban While the use of mid-lane rumble strips seems promising, environments is that the noise they generate disrupts the it is important to consider the longer-term behavioral impacts peaceful environment of the adjacent land and the residents that may result from a widespread use of mid-lane rumble of the area (particularly during the quieter night hours). This strips. In urban areas, travel is often characterized by frequent perceived adverse affect on adjacent property owners result- lane changing. Where mid-lane rumble strips are common, ing from the use of rumble strips also limits their use in urban drivers may become acclimated to the sound and vibration areas. they produce and cease to treat them as special events that re- quire increased attention to the driving task. In addition, the placement of mid-lane rumble strips should not occur at Mid-Lane Rumble Strips locations of heavy pedestrian activity, such as mid-block A potential rumble strip treatment that may be more appli- pedestrian crossings. Both the raised and grooved rumble cable to urban roadways--particularly urban arterials--is the strips create a potential tripping hazard for pedestrians by in- use of mid-lane rumble strips. In this treatment, rather than troducing an uneven walking surface. applying rumble strips to the shoulder of the road, rumble strips are placed in the center of the vehicle travel lane. In this Safety of Urban Roadside Elements application, as vehicles leave their travel lane, their tires cross over the rumble strips, thereby producing the sound and An urban environment is characterized by many potential vibration associated with shoulder rumble strips, without re- roadside hazards. To improve roadside safety, many of these quiring a shoulder-based treatment (25). While mid-lane objects can be removed or relocated; however, it is probable rumble strips are largely untested, they nevertheless present that numerous prospective roadside hazards must be retained
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12 to facilitate the needs of the community or the road users. As Under current urban roadside design guidelines, engineers a result, this chapter reviews known roadside objects and are provided with a special designation, the operational offset, strategies that may help improve the safety of their place- which effectively permits the location of fixed objects 0.5-m ment. Table 5 provides an overview of common urban road- (1.5-ft) from the curb face (1, 30). This offset value is a min- side features and features often sought by local stakeholders imum suggested distance associated with avoiding such op- to increase the aesthetic quality of urban roadsides. Each of erational issues as car-door and vehicle mirror conflicts with these items is reviewed in greater detail in this chapter. roadside objects and minimizing the impact to traffic opera- tions; it is not provided for safety purposes (31). The opera- tional offset should not be considered as an acceptable clear Removal/Relocation/Placement zone, but simply as a minimum value to ensure elimination of Roadside Objects of traffic operational conflicts. Where a clear zone cannot be Engineers are encouraged to identify potentially danger- achieved, the individual road should be tailored for the con- ous objects adjacent to the travelway and remove them, ditions at a specific site. The influence of supplemental factors ideally through the use of a clear zone. The recommended such as crash history, future traffic, and heavy vehicle pres- standard practice for higher-speed roadways is the provi- ence should be included in the decision process. sion of a lateral clear zone that will enable at least 80 per- For evaluation of changes to the roadside such as removal cent of errant vehicles to stop or return to their travel lane of potential hazards, an engineer must determine whether safely. The appropriate width of clear zones is ultimately the benefits associated with relocating a hazardous object based on the slope of the roadside, daily traffic volumes, outweigh the cost of doing so. The "cost" may take many and speed (1). forms, such as societal impacts or actual removal dollars, so The opportunities for providing a clear zone in urban areas elaborate cost-benefit methodologies have been developed are often limited due to the restricted width of the existing to estimate the relative benefits of removing these objects right-of-way and the density of adjacent roadside develop- (29, 32, 33, 34, 35). ment. Use of the available right-of-way includes many com- If a potentially hazardous object must be located adja- peting demands. Further, many communities seek to provide cent to the travelway, the principal means of addressing a physical buffer zone adjacent to the travelway to encour- run-off-road crashes where adequate clear zones cannot be age pedestrian activity or to enhance the aesthetic quality of provided is to ensure that any object placed in the clear the roadway. Often, this involves the planting of trees or in- zone is "crashworthy," that is, any object located in the clusion of landscaping in a buffer area between the sidewalk clear zone is designed to minimize the severity of a potential and the vehicle travelway. Placement of mature street trees in crash. NCHRP Report 350 (36) provides specific standards close proximity to the road can present a hazard to the and test conditions, such as soil and vehicle specifications, motorist. Minor departures from the travelway under these that are used for evaluating the crashworthiness of roadside conditions can result in a potentially serious fixed-object fixtures such as guardrails, utility poles, and light supports. crash, particularly at high speeds. Often, a configuration with The reader is referred to NCHRP Report 350 for a full con- rigid objects located immediately adjacent to the travelway is a sideration of the test specifications used in the evaluation result of a road-widening project where the only way to of crashworthiness. accommodate increasing vehicle capacity demands within the Two strategies exist, both of which are subject to the test constraints of the current transportation infrastructure was conditions contained in NCHRP Report 350. The first is to by further encroaching on the existing roadside (29). incorporate frangible roadside objects and hardware into the Table 5. Common urban roadside features. 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
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13 design of the roadside environment, and the second is to shield or cushion potentially hazardous objects and environments. A more detailed description of known safety strategies for (A) (B) the various urban roadside elements previously identified in Sample Vertical Curbs Table 5 is presented in the following sections. Features Immediately Adjacent to the Travelway (C) (D) (E) Physical features immediately adjacent to the travelway are the first objects encountered when an errant vehicle exits the Sample Sloping Curbs Graphic adapted from A Policy on Geometric Design of Highways and Streets, road. These features can include curbs, shoulders, channelized 4th ed. (37). islands, medians, and roadside grading. This section reviews each feature and known safety issues regarding each item. Figure 2. AASHTO example curbs. Curbs currently provides the following recommendations for the design of vertical curbs (1, p. 107): General information. Much of the rural research re- garding pavement edge treatments evaluates the influence of At speeds over 40 km/h (25 mph), a vehicle can mount the graded or paved shoulders on safety performance at the time curb at relatively flat angles. Consequently, when sidewalks or bicycle paths are adjacent to the traveled way of high-speed a vehicle enters the roadside environment. In an urban envi- facilities, some provision other than curbing may need to be ronment, very few road edge treatments include roadway made for the safety of pedestrians and bicyclists. shoulders as a transition from the travel lanes to the adjacent roadside environment. Instead, curb is commonly used in The Roadside Design Guide further suggests provision of a urban environments as it can help direct storm drainage minimum horizontal clearance of 0.5 m (1.5 ft) beyond the (thereby reducing the need for roadside ditches and wider face of curbs to any obstructions. This distance is the opera- right-of-ways) and provides visual channelization to help tional offset previously discussed. delineate the pavement edge. There is, therefore, a need to The Green Book recommends the use of curbing on road- understand the safety of curbs in the urban environment. ways with speeds of approximately 73 km/h (45 mph) or less An important issue of concern for addressing roadside (37). The Green Book further notes that when vertical curbs safety at curbed locations is the influence that various curb are used on these lower-speed roadways, placement of verti- types may have in causing vehicles to trip or launch during a cal curb will preferably be offset 0.3 to 0.6 m (1 to 2 ft) from run-off-road event where the "first harmful" object the vehi- the edge of the travelway. The Green Book recommends cle encounters is the roadside curb. The vertical curb has an against the use of curbs along high-speed arterials such as almost vertical face and is generally between 150 to 225 mm freeways, but indicates that when used on these facilities, (6 to 9 in.) in height above the driving surface of the adjacent a curb "should be of the sloping type and should not be pavement. The vertical curb is used as a means for discour- located closer to the travelway than the outer edge of the aging motorists from intentionally leaving the roadway. shoulder." (37, p. 322) A sloping curb has an angled surface, a height of 150 mm (6 in.) A Guide for Achieving Flexibility in Highway Design (31) or less, and is designed for use on higher-speed roadways also indicates that vertical face curbs at low-speed (40 km/h (greater than 70 km/h [approximately 45 mph]) or at loca- [25 mph] or less) locations have limited redirectional capa- tions where a vehicle may need to leave the roadway for emer- bilities for errant vehicles. For speeds above 40 km/h (25 mph), gency purposes. The sloping curb is designed so that a vehicle the curb can influence driver behavior, but does not provide can traverse the curb without damaging the vehicle (30). a vehicle redirection function. Curbs "A" and "B" in Figure 2 depict example vertical curbs, while Curbs "C" through "E" represent various sloping curb Safety research. The research supporting the statements configurations. summarized above and found in common design guidelines The AASHTO publications A Policy on Geometric Design of spans curb crash testing and computer modeling over a period Highways and Streets (referred to hereafter as the Green Book, of many years. However, crash testing standards, computer ) and the Roadside Design Guide (1) both indicate that modeling capabilities, and typical study vehicles have changed a vertical curb, struck at higher speeds, may cause an errant during this period. In general, researchers have performed test- vehicle to mount and/or launch. The Roadside Design Guide ing on vertical curbs, sloping curbs, and curbs with adjacent
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14 barriers such as guard rails. In 1972, Dunlap et al. (38) per- or substantial vertical displacement of the vehicle, nor were formed several roadside curb evaluations including tests for the events shown to result in more than minor damage to the five standard curbs and eight curb/guardrail combinations. vehicle. In 1974, Olson et al. (39) evaluated curbs using crash testing The AASHTO Highway Safety Design and Operations combined with computer simulation. These two research Guide (30) indicates that the potential for vehicle vaulting or studies were among the first to suggest the following commonly rollover for curbs higher than 100 mm (4 in.) is a factor of the accepted concepts regarding curb safety: vehicle's weight, speed, suspension system, angle of impact, and vehicle lane tracking. As a result, small cars are generally · Curbs 150 mm (6 in.) tall or less do not redirect vehicles at overrepresented in serious curb-related crashes. The poten- speeds above 73 km/h (45 mph) and should therefore not tial for a vehicle to vault precludes the exclusive use of a curb be used for high-speed roads, as sufficient protection for pedestrian facilities or roadside · Impacting curbs 150 mm (6 in.) tall or less will generally elements. result in either no injury or minor injuries only, and In 2005, Plaxico and colleagues published NCHRP Report · Combinations of lower speeds and small approach angles 537: Recommended Guidelines for Curb and Curb-Barrier In- produce the greatest effect on vehicle path correction. stallations in which they evaluated roads with operating speeds of 60 km/h (40 mph) or greater and the potential influence of A study performed in the 1970s at the Texas Transportation curb or curb-barrier combinations at these locations (43). Institute evaluated curb placement in conjunction with traffic They determined that the most significant factor influencing barriers and sloped medians (40). The researchers concluded vehicle trajectory is curb height. As a result, shorter curbs with that the traffic barriers should not be immediately adjacent to flatter sloping faces should be used at higher speed locations. curbs as vehicles may vault or underride the barrier. They also They also determined that a lateral distance of approximately concluded that grading the median or roadside level with the 2.5 m (8.2 ft) is needed for a traversing vehicle to return to top of the curb will help reduce problems with barriers and its predeparture vehicle suspension state. As a result, guard- guardrail interactions near curbs. rails should not be placed closer than 2.5 m (8.2 ft) behind An evaluation performed for the Nebraska Department of curbs on roads where vehicle speeds are greater than 60 km/h Roads (NDOR) included crash tests as well as simulations of (40 mph). As the research performed by Plaxico and col- sloping curbs and curb-guardrail combinations (41). The re- leagues did not focus on low-speed roads, the placement searchers' evaluation included various degrees of impact and of guardrails behind curbs for speeds lower than 60 km/h vehicle trajectory. They concluded that the three sloping (40 mph) is not known. curbs tested (two NDOR standard curbs and one AASHTO In summary, curbs can provide positive (visual) guidance standard curb) were traversable for a wide range of impact for drivers, but curbs do not have the ability to redirect errant conditions and had very little likelihood of causing vehicle vehicles upon impact (unless the vehicle speed is quite low and rollovers. The researchers further determined that the chance the vehicle impact angle is extremely small). If an errant vehi- that a vehicle could underride a guardrail was slight, and the cle approaches the curb at a small deflection angle, the impact chance that a vehicle would be vaulted by the curb-guardrail of the curb is unlikely to be the cause of serious injury to the combination was greatest when the barrier was located any- vehicle occupants; however, the curb may affect a vehicle's tra- where from 0.45 to 3.7 m (1.5 to 12.1 ft) behind the curb. This jectory, resulting in impact with a second, more substantial range of offset values applied to both a small and a large test roadside hazard. A barrier or guardrail must be placed behind vehicle. the curb in such a way as to avoid vaulting the errant vehicle. A report commissioned by the Florida Department of Trans- portation (42) simulated the trajectories of three design vehi- Strategy summary. A variety of strategies have been pro- cles hitting sloped (125 mm [5 in.] tall) and vertical (150 mm posed, applied, and/or tested for safe application of curb [6 in.] tall) curbs at approach speeds of approximately 57, 73, treatments. Common strategies are as follows: and 90 km/h (35, 45 and 55 mph) and impact angles rang- ing from 3 to 15 deg. The model results found that the verti- Purpose Strategy cal curbs would deflect the Ford Festiva test vehicle for all Prevent curb from vault- · Use appropriate curb heights with approach speeds at angles of impact up to 12 deg. For a Chevy ing vehicles known influences on vehicle trajectories (P) C2500 pickup, the vertical curb would deflect vehicles oper- · Locate barriers behind curbs an ating at 90 km/h (55 mph), but only when the angle of impact appropriate distance to improve was 3 deg or less. The sloping curb was not shown to redirect curb-barrier interactions (P) the vehicle under any combination of approach speed or · Grade adjacent terrain flush with angle. None of the impacts were shown to result in a rollover the top of the curb (P)
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15 Shoulders There are many research studies that have evaluated the safety benefits of shoulders and companion shoulder widths. General information. The common edge treatment for Several of these studies are included in the safety research sec- urban roads is a curb or curb with gutter; however, many tion that follows. roads exist in urban environments with a graded or paved shoulder instead of a curb located immediately adjacent to the Safety research. The research regarding shoulder safety travelway. The purpose of a shoulder is to provide a smooth has been generally divided into three categories--safe shoulder transition from the travelway to the adjacent roadside while width, pavement edge treatments, and safety of paved versus facilitating drainage and promoting various other shoulder graded shoulders. The research regarding these three areas of functions (as listed in Table 6). The shoulder width is included shoulder safety is summarized in the following: as part of the clear zone width; therefore, the values shown in Table 6 should not be confused with clear zone requirements. · Safe shoulder width. Much of the research into the appro- There are many recommendations regarding appropriate priate width of shoulders focuses on the high-speed rural shoulder widths for lower speed roads. These widths vary condition. Early research indicated that crash frequency depending on the function of the shoulders as well as the tended to increase with shoulder width. For example, available right-of-way. Table 6 shows suggested shoulder Belmont published a paper on rural shoulder widths in 1954 widths from the AASHTO publication A Guide for Achieving and a subsequent paper in 1956 (extending the study to Flexibility in Highway Design (31). This information was first lower volume rural roads) and suggested that wider shoul- compiled for a 1982 NCHRP study (44). These widths are ders for higher speed, high-volume rural roads resulted in recommended for shoulder functional use and do not reflect increased crash rates, while the trend appeared reversed for identified widths for safety purposes. lower volume, high-speed roads (45, 46). Subsequent to Because right-of-way costs are high in urban environments, these early studies, numerous researchers have studied the the use of paved or graded shoulders in these environments shoulder width question. In an unpublished critical review often is the result of previously rural roads being incorpo- of research in this area (47), Hauer re-evaluated many of the rated into urbanized land use without the companion road- original shoulder width studies using the original data and way improvements. Often the road with a shoulder will have concluded that shoulder width safety is a sum of several a drainage ditch located parallel to the road, so care must be opposing tendencies. These can be summarized as follows: taken to maintain traversable conditions in the event that an The shoulder is even and obstacle free and available for errant vehicle exits the road, travels across the shoulder, and drivers of errant vehicles to use to regain control of their then encounters the roadside grading. vehicles, correct for their error, and resume normal travel; Table 6. Acceptable shoulder widths for shoulder functions. Functional Classification Arterial Collector and Local Shoulder Function m (ft) m (ft) Drainage of Roadway and Shoulder 0.3 (1) 0.3 (1) Lateral Support of Pavement 0.45 (1.5) 0.3 (1) Encroachment of Wide Vehicles 0.6 (2) 0.6 (2) Off-tracking of Wide Vehicles 0.6 (2) 0.6 (2) Errant Vehicles (Run-off-road) 0.9 (3) 0.6 (2) Bicycles 1.2 (4) 1.2 (4) Pedestrians 1.2 (4) 1.2 (4) Emergency Stopping 1.8 (6) 1.8 (6) Emergency Vehicle Travel 1.8 (6) 1.8 (6) Garbage Pickup 1.8 (6) 1.8 (6) Mail and Other Deliveries 1.8 (6) 0.6 (2) Emergency Call Box Services 2.4 (8) 1.8 (6) Law Enforcement 2.4 (8) 1.8 (6) Parking, Residential 2.4 (8) 2.1 (7) Routine Maintenance 2.4 (8) 1.8 (6) Major Reconstruction and Maintenance 2.7 (9) 2.7 (9) Parking, Commercial 3.0 (10) 2.4 (8) Parking, Trucks 3.0 (10) N/A Slow-Moving Vehicles 3.0 (10) 2.7 (9) Turning and Passing at Intersections 3.0 (10) 2.7 (9) Sources: Adapted from A Guide for Achieving Flexibility in Highway Design (31) and NCHRP Report 254: Shoulder Geometrics and Use Guidelines (44).
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16 Wide shoulders may induce voluntary stopping and Strategy summary. Common shoulder treatment strate- therefore place a hazard immediately adjacent to the gies are as follows: travelway; Wide shoulders may entice drivers to use them as addi- Purpose Strategy tional lanes or for passing maneuvers on the right; and Discourage run-off-road Provide wider shoulders suitable Wider shoulders may encourage higher operating speeds. crashes for shoulder function (P) Evaluation of crash data without comprehensively consider- Provide traversable · Eliminate pavement drop-offs (P) ing these four contrasting tendencies may permit researchers transition for errant · Add a pavement safety edge (T) vehicles · Provide a paved or sealed to arrive at a variety of conclusions regarding shoulder shoulder (P) width safety. In general, on roads with wider shoulders, travel speeds are higher and crashes are more severe. How- ever, wider shoulders result in fewer run-off-road crashes, Channelization/Medians and therefore this benefit must be included. General information. The separation of traffic move- · Pavement edge treatments. A common problem with ments by the use of a raised median or turning island is often roadway shoulders is that they may not be flush either with referred to as channelization. For the purposes of this review, the travelway pavement surface (for the case of graded a flush or traversable median or island is considered part of shoulders) or with the adjacent roadside grading (for the the roadway, while a raised median and raised turn island are case of paved shoulders). There are many reasons that considered part of the roadside. pavement drop-offs may develop in the shoulder region. Channelized islands are generally used to reduce the area Erosion of the soil next to the pavement, rutting by frequent of pavement at an intersection while providing positive guid- tire wear, and pavement overlay maintenance are examples ance to turning vehicles. Channelized islands can be used for of how, over time, a pavement drop-off may develop. pedestrian refuge and traffic control device placement, and When a drop-off is encountered by an errant vehicle, the they can also be planted with landscaping treatments that vehicle's tires may have difficulty mounting the extra pave- contribute to an improved visual environment (15). For a ment lip, causing the vehicle to further lose control. raised island to be visible, it should have a minimum size of In the late 1970s and early 1980s, researchers at Texas 5 m2 (50 ft2) for urban conditions (37). The orientation of the A&M University performed a series of evaluations on pave- curb on a raised island should be slightly skewed to the adja- ment edge drop-offs (48, 49). They determined that ver- cent travel lane to give an illusion of directing vehicles into tical drop-offs as small as 7.6 cm (3 in.) could result in a the travel lane. Other cross-sectional characteristics of raised severe crash if encountered by an errant vehicle. The Texas islands are similar to those of raised medians. A&M researchers developed pavement edge shapes to pro- The raised median provides the primary function of sep- vide a more beveled edge and determined that for speeds arating opposing directions of vehicle travel. This physical up to 90 km/h (55 mph), a 45-deg angle could be applied separation has the added benefit of improving access man- to the drop-off. This sloped edge would then enable errant agement (restricting frequent left turns into driveways), pro- vehicles to regain access to the travelway safely. Currently, viding a location for pedestrian refuge (assuming the median the Federal Highway Administration promotes a pavement has adequate width), and providing road edge delineation edge treatment called the safety edge that uses a similar during inclement weather conditions (particularly snow). 45-deg angle with construction standards that permit com- A median may simply be raised using a vertical or sloped paction to provide pavement edge stability. curb. In urban regions, median width can vary dramatically de- · Safety of paved versus graded shoulder. The safety of pending on the proposed function of the median. As sug- paved versus graded shoulders is less controversial than the gested in the Maryland publication, When Main Street Is a pavement width consideration. Several studies have indi- State Highway: Blending Function, Beauty and Identity, the use cated that the addition of any paved shoulder will help with of a median can dramatically improve the visual quality of a crash reduction. Zegeer, Deen, and Mayes concluded that facility (51). increasing the width of a paved shoulder for rural roads by 0.3 m (1 ft) would reduce crashes by approximately 6 per- Safety research. Many of the recent research studies cent (50). They also concluded that paving at least 0.3 m about raised median safety focus on the influence of the (1 ft) of a shoulder would reduce crashes by 2 percent. median on access management and the resulting reduction of More recently, McLean found that for Australian roads the crashes due to restricted left-turn movements. Although this application of sealed shoulders with widths from 1.5 to 2 m crash reduction strategy falls outside the scope of this litera- (5 to 6.6 ft) would result in a decrease in crash rates and, ture review, it is worth noting that the median condition therefore, be a cost-effective treatment (124). has added safety benefits that should be considered in a
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17 comprehensive crash evaluation. In this review, however, the setbacks. They also found that with the increase in fixed- evaluation will be focused on median crashworthiness for the object collisions came a decrease in head-on and broadside purposes of roadside safety. collisions. Finally, the researchers found non-intersection In general, median research (excluding access management locations with median trees were positively associated with studies) has focused on the crash condition with specific atten- hit-pedestrian collision. tion to the following questions: Another common application for raised medians is as one element in a gateway treatment at transition locations · Do medians prevent pedestrian cross-median crashes? between rural and urban areas. The raised median studies What is the influence of a median barrier that completely for the purpose of use as gateway strategies are reviewed prohibits pedestrian crossing? later in this chapter in the section discussing traffic calm- · Do medians reduce the number of crashes and crash ing applications. severity? Research into appropriate median widths for safety pur- · Can landscaping and trees be safely located in medians? poses are focused primarily on the high-speed rural condi- · Should median barriers be used to improve median safety? tion. Similar median width studies for urban environments are not available. A median barrier review is included later in this chap- ter in the section on safety issues, so this section focuses on Strategy summary. Common channelized island and the influence of a median on crashes and landscaping treat- median strategies are as follows: ments. Several researchers have weighed the merits of a raised median (divided highway) versus no median or a Purpose Strategy flush median. Unfortunately, in most of the before-after re- Reduce likelihood of Widen median (T) search studies, a jurisdiction was implementing a median run-off-road collision improvement in conjunction with other improvements, Reduce crash severity · Place only frangible items in such as road widening, lane narrowing, and so forth. As a channelized island or median (P) result, the influence of divided versus undivided has re- · Shield rigid objects in median (P) sulted in a wide variety of crash observations. Harwood (52) studied several median conversion configurations from un- Roadside Grading divided to divided operations. After controlling for a vari- ety of variables, he concluded that the influence of the General information. The terrain adjacent to an urban median on safety was small. Many studies have resulted in road should be relatively flat and traversable. In general, the similar observations, that is, that raised medians have a neg- placement of common urban roadside features such as side- ligible effect on crash frequency. Crash severity varies de- walks and utilities tends to create a flatter urban roadside. The pending upon the median width (wider medians reduce the primary risk for irregular terrain adjacent to the travelway is chance for head-on collisions), the use of median barrier that an errant vehicle will either impact a rigid obstacle or that (to be discussed later), and the placement of rigid objects in the terrain will cause the vehicle to roll over. Rollovers were the median area. responsible for 20 percent of the fatal crashes in 2002, and the The issue of landscaping and the specific evaluation of largest number of rollovers occurred after a vehicle impacted tree placement are further discussed in the landscaping an embankment or a ditch (25, 54). The principal cause of section; however, a recent three-phase study performed at rollovers is a vehicle "tripping" on an element of the roadside California Polytechnic State University (53) specifically environment, such as a ditch or an embankment; neverthe- evaluated the placement of large trees in raised medians on less, sharp pavement drop-off on the shoulder may also lead urban and suburban highways. They evaluated sites with to vehicle tripping for roads without a curb. To prevent vehi- and without large trees and determined that at a 95-percent cle tripping, the grade of ditches, slopes, and embankments level of statistical confidence, an increased number of fatal should be minimized as much as possible, and pavement or injury crashes were associated with the presence of drop-offs must be kept to a minimum. median trees. The association between median tree crashes These strategies are potentially more relevant to rural and and left-side-only crashes, however, was only marginally suburban environments than to urban ones, however. In urban significant. The three-phase study also indicated that me- areas, the roadside is typically characterized not by shoulders dian trees on urban and suburban highways were associated and embankments, but by curb and gutter applications and by with an increase in collision frequency. Study researchers adjacent roadside development. This is evidenced when one were not able to identify any systematic relationships be- compares the absolute number of rollover crashes in urban tween the left-side crash rates and median widths or tree environments with the number of rollover crashes in rural
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18 environments. In 2002, there were roughly 1,800 rollover hazard associated with larger mail collection boxes, a common crashes in urban areas, compared with over 6,200 for rural re- feature in urban environments, as well as neighborhood de- gions. Accounting for exposure, roughly one rollover per billion livery units, which are associated with apartment complexes. miles of travel occurs in urban areas whereas almost six Crash tests of these features have shown them to fail safety rollovers per billion miles of travel occur in rural environments. requirements, and the Roadside Design Guide recommends While the conditions that lead to rollover crashes are not placing them outside of clear recovery areas. clear, crash data analyses indicate that these crash types are While making mailboxes crashworthy will satisfy safety generally associated with high-speed travel. Of the roughly associated with mailbox-related crashes, it is important to 8,000 rollovers that occurred in 2002, only about 600 occurred recognize that the placement of mailboxes may have an on roadways classified as urban minor arterials, collectors, important impact on the overall safety of the roadway. Mail- and locals. boxes should not obstruct intersection sight distance, nor The sideslope of an urban road should, in general, slope should they be located directly on higher-speed roadways, from the edge of the right-of-way toward the curb of the road. where stopping associated with mail delivery and collection This slope will prevent any road drainage from encroaching can lead to substantial speed differentials between vehicles on on adjacent property and enables the drainage to be contained the travelway, thereby increasing the possibility of a rear-end within a closed drainage system. As a result, the slope is often collision. Where such conditions exist, the Roadside Design quite flat (1V:6H typically) for curbed urban roads. For roads Guide recommends the use of a 2.4-m (8-ft) mailbox turnout without a curb, the design guidelines for rural roadside con- lane adjacent to the travelway to allow vehicles to leave the ditions should be applied. That is, the terrain, including travelway for mail collection and delivery purposes. This drainage channels, should be safely traversable by a motor turnout concept does not apply to urban curbed streets. At vehicle, and the placement of obstacles such as headwalls must curbed residential locations, the Roadside Design Guide rec- be flush with the ground surface and designed to be navigated ommends that the minimum distance from the roadside by an errant vehicle. face of the mailbox to the face of the curb should be 150 mm (6 in.), with a preferred offset ranging from 200 to 300 mm Safety research. The research team was not able to locate (8 to 12 in.). research specific to the urban roadside slope and safety A common issue regarding the placement of mailboxes in implications associated with this terrain. Most of the studies an urban environment is that the governing jurisdiction applicable to the urban condition focused on the presence of (often a city or county) may not adopt the guidelines com- roadside obstacles rather than the companion roadside slope. monly accepted by state departments of transportation. Many urban jurisdictions allow home owners to construct a mailbox Strategy summary. Common grading strategies are as of their choosing. In areas in which mailbox vandalism is follows: common, home owners have begun to erect increasingly rigid (less forgiving) mailbox units. A rigid brick mailbox is a com- Purpose Strategy mon site along many urban residential roadways. The problem Minimize crash likeli- Maintain traversable grades that are of rigid mailbox units is compounded by the general place- hood free of rigid obstacles (P) ment of such mailboxes adjacent to a driveway (to make it Minimize crash severity · Flatten grades to reduce chance of easy for the home owner to retrieve mail). Since the curb has vehicle rollover (P) · Create an object setback policy (T) a secondary function of delineating the edge of the roadway, a mailbox placed on the departure side of a driveway (where a curb cut interrupts the roadway delineation) is particularly Static Roadside Treatments vulnerable to errant vehicles that exit the road to the right. Mailboxes Safety research. NCHRP Report 350 provides recom- General information. The Roadside Design Guide (1) de- mended procedures to ensure roadside features such as mail- tails the preferred specifications for the design and installa- boxes are crashworthy (36). Since mailboxes are a common tion of mailboxes. In general, AASHTO recommends the use fixed object adjacent to urban streets (particularly residential), of a 100-mm by 100-mm (4-in. by 4-in.) wooden post or a they warrant particular attention when reviewing urban road- 38-mm (1.5-in.) light-gauge pipe for mounting mailboxes, side safety. Many urban jurisdictions do not require crash- with these posts embedded no deeper than 600 mm (24 in.) worthy mailboxes. There are several yielding mailbox designs in the ground. Mailboxes should further be mounted to their approved for the National Highway System (NHS) that could supports to prevent the mailbox from separating from the be incorporated in an urban setting. Chapter 11 of the Road- post during a crash event. Also of concern is the potential side Design Guide (1) provides a comprehensive summary of
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19 the safe placement of mailboxes. The use of yielding mail- surface and pedestrian facilities due to pavement heave and boxes is promoted in the Roadside Design Guide. This permits cracking. convenient mailbox placement adjacent to the road. Mailbox Placement criteria, in some cases, is based on the func- placement for urban commercial locations is not included in tional purpose or posted speed limits of adjacent roads. Com- the chapter and is a less common problem. In addition to mon landscape placement issues addressed in jurisdiction yielding mailbox support design, some jurisdictions promote plans include the following: the placement of reflective object markers on the mailbox or · Proximity to intersections, post to improve nighttime visibility (55). · Proximity to driveways, Strategy summary. Common mailbox safety strategies · Maintaining a clear vision space, are as follows: · Lateral offset placement of trees and landscaping, · Longitudinal placement of trees and landscaping, Purpose Strategy · Median planting strategies, and Minimize crash likeli- · Remove or relocate mailboxes to · Strategic placement strategies for visual perception. hood safe locations (P) · Add reflective object markers to These specific placement strategies are further described in improve nighttime visibility (T) the following: Minimize crash severity · Develop policies to require crashworthy mailboxes in urban · Proximity to intersections. Sight distance should be main- environments (P) · Shield rigid mailboxes where tained in the proximity of intersections. As a result, many practical (P) landscape guidelines restrict tree placement in the imme- diate vicinity of intersections. The North Carolina Tradi- tional Neighborhood Development (TND) Guidelines (56) Landscaping, Trees, and Shrubs and the City of Seattle Street Tree Planting Procedures (57), General information. Several types of roadside land- for example, recommend that trees should be located scaping are commonly employed to enhance the aesthetics no closer than 9 m (30 ft) from intersection corners. Land- of roadside environments. These treatments may include scape Design Guidelines for the City of Simi Valley (58) the placement of shrubs, street trees, or alternative treat- requires a clearance distance of 10.7 m (35 ft) from the ex- ments such as landscape berms. In addition to the concern tended curb at the near side of the cross street curb. of traversability in the event that an errant vehicle encoun- Another approach to intersection clearance is based on ters roadside landscaping, a common safety issue of adjacent street type and intersection configuration. For example, the landscape treatments is sight distance and the impact land- Montgomery, Alabama, Street Tree Master Plan (59) uses scape treatments may have for intersection, driveway, and street type and traffic control to determine tree offsets from stopping sight distance considerations. Regional jurisdic- intersections. Example minimum tree placement guidelines tions often have landscaping design guidelines, landscaping at intersections for Montgomery are depicted in Table 7. policies, and street tree master plans. These documents Figure 3 is based on an FHWA publication and demon- address a variety of landscaping issues including plant type, strates the sight triangle that is required to be free of trees maintenance, and plant placement. Since trees, in particu- at an example intersection (60). Higher-speed vehicles that lar, can vary from small, flexible species up to more rigid do not stop at the intersection require more sight distance varieties, the careful selection of tree species is critical. In than stopped vehicles, as shown. addition, different tree species can have substantially differ- Guidelines for Tree Planting and Maintenance on Urban ent root systems. Species selection should also focus on Roads, published by the Traffic Authority of New South the potential for the tree system to adversely impact road Wales, further recommends that skewed intersections, Table 7. Guidelines for minimum tree placement offsets at intersections for Montgomery, AL. Intersection Control Major Street Neighborhood Street Traffic Light 9.1 m (30 ft) -- Four-Way Stop 9.1 m (30 ft) 4.6 m (15 ft) Major Street Two-Way Stop 12.2 m (40 ft) -- Neighborhood Street Two- 9.1 m (30 ft) 4.6 m (15 ft)Stops Way Stop 9.1 m (30 ft)--Does Not Stop Source: Adapted from Montgomery Street Tree Master Plan (59).
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20 Graphic adapted from Trees in Hazardous Locations (60). Figure 3. Intersection sight triangles. locations with high turning speeds, or locations where fast- initial inspection; however, at this location the intersecting approaching vehicles veer into the left lane to avoid roadway is characterized by a horizontal curve. With the impacting right-turning vehicles are all locations where landscaping placement close to the driveway, the driver of cluster run-off-road crashes can be expected and where an exiting vehicle cannot detect approaching vehicles with- additional space free of roadside objects such as trees out edging into the active travel lane; therefore, these road- should be provided (61). side treatments encroach on the required sight distance. As · Proximity to driveways. The placement of trees near drive- can be viewed in the photo, the adjacent property owner ways poses similar sight distance issues as those identified also positioned large "ornamental" rocks at the corner, for intersections. As an example, the City of Simi Valley thereby adding a rigid obstacle in the immediate vicinity of guidelines requires a 1.5-m (5-ft) clearance between trees the roadway. and driveway edges (58). By contrast, the Montgomery · Maintaining a clear vision space. Traditional Neighbor- recommendations indicate that trees should not be placed hood Development (TND) Guidelines recommends that within 4.6 m (15 ft) of driveways (59). The City of Seattle, vertical space ranging from 0.6 to 2.1 m (2 to 7 ft) above in Street Tree Planting Procedures, requires maintaining a ground be maintained to preserve lines of sight (56). The minimum distance between trees and driveways of 2.3 m AASHTO publication Highway Safety Design and Opera- (7.5 ft) with a recommended distance of 3.0 m (10 ft) (57). tions Guide (30) recommends that the vertical "clear Many landscape policies do not directly stipulate tree vision space" range from 1 to 3 m (3.3 to 10 ft) to ensure placement near driveways, but use an approach similar to clear sight distance for drivers in low-riding sports cars as the New South Wales guidelines, which simply state that well as drivers in high trucks and buses. This vertical clear drivers exiting driveways should be able to see approaching space is common to many regional landscaping plans. The traffic and pedestrians (61). As an example, the attractive "clear vision space" is essentially the space above shrub landscaping depicted in Figure 4 seems reasonable upon growth and below tree overhang. A low tree overhang can also create an obstacle for pedestrian access, as shown in Figure 5. Figure 6 depicts another type of encroachment into the vertical clear vision space. Often landscape berms are used to screen adjacent parking from the roadway. The photo on the left shows a longitudinal landscape berm and the effect it has on horizontal sight distance. At the location shown, the road has a horizontal curve to the right (in the direction the vehicle shown is traveling) and has numer- ous driveways that are not easily visible due to the berm height. The photo on the right depicts the same location but more clearly shows that the height of the berm exceeds the height of a typical passenger car. This type of landscape treatment is a common roadside treatment in many urban areas. Photo by Karen Dixon. · Lateral offset placement of trees and landscaping. Tra- Figure 4. Landscaping in sight triangle for driveway. ditional Neighborhood Development (TND) Guidelines
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21 Graphic reprinted fromVegetation Control for Safety. A Guide for Street and Highway Maintenance Personnel, FHWA-RT-90-003 (120). Figure 5. Overhead object hazard. recommends that planting strips located between the curb equidistant between the pavement edge and the edge of the and sidewalk should be at least 1.8 m (6 ft) wide (56). This sidewalk (59). For major street locations, trees should not resource further suggests that for streets with design speeds be located closer to the edge of pavement than two-thirds at or below 32 km/h (20 mph) or for streets that permit of the distance from the pavement edge to the right-of-way on-street parking, small street trees can be planted within limits. The Georgia Department of Transportation Online 0.9 m (3 ft) of the back of curb or along the approximate Policy and Procedure System (62) recommends that in an centerline of the planting strip. The Seattle planting proce- urban environment, trees with diameters less than 100 dures permit tree planting 1.1 m (3.5 ft) from the face of mm (4 in.) should be laterally positioned 1.2 m (4 ft) for curb (57). The Montgomery, Alabama, plan recommends posted or design speeds of 56 km/h (35 mph) or less, 2.4 m that at neighborhood street locations trees should be in- (8 ft) for posted or design speeds of 64 to 72 km/h (40 to stalled at a point equidistant between the pavement edge 45 mph), and outside the clear zone for speeds greater than and the right-of-way limits or, for residential neighborhoods, 72 km/h (45 mph). For larger trees, the minimum lateral Photos by Karen Dixon. Figure 6. Landscape berm that blocks horizontal sight distance.
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25 Street Furniture Utility Poles, Luminaires, and Signposts/Hardware General information. In many urban areas, the use of General information. In both the national and interna- street furniture is a common approach to improving the aes- tional literature, the placement of utility poles, light poles, and thetic quality of a street. Street furniture includes items placed similar vertical roadside treatments and companion hard- adjacent to the road that are there to improve the adjacent ware is frequently cited as an urban roadside hazard. For land use or to improve the transportation operations. In some example, Haworth and Bowland have noted that while im- jurisdictions, street lights and signs are included in the cate- pacts with trees were more common outside the Melbourne, gory of street furniture; however, for the purposes of this Australia, metropolitan area, single-vehicle crashes with review, street furniture is considered to be supplemental poles or posts were more common within the metropolitan items such as benches, public art, trash receptacles, phone region (73). A 1998 study by the European Transport Safety booths, planters, bollards, fountains, kiosks, transit shelters, Council identified collisions with utility poles or posts as one bicycle stands, and so forth. Often the placement of these de- of the top two roadside hazards for Finland, Germany, Great vices can obscure sight distance, so their location should not Britain, and Sweden (74). occur in the sight triangles of intersections or driveways. Many street furniture items are placed along the right-of-way Safety research. This section discusses the research litera- by property owners, as in the case of the placement of a side- ture for utility poles, lighting supports, and signposts/roadside walk cafe in front of a restaurant, and are thus largely outside hardware. the engineer's control. Transit shelters are provided to protect · Utility poles. Utility poles are prevalent in urban environ- transit riders from inclement weather and must be located ments and can pose a substantial hazard to errant vehicles close to the curb to facilitate short bus dwell times. and motorists. A study of utility pole crashes, for example, One interesting way that some jurisdictions manage the found that crash frequency increases with daily traffic placement of street furniture by land owners is by a permit- volume and the number of poles adjacent to the travel- ting process that requires vendors or property owners to way (33). Utility poles are more prevalent adjacent to urban acquire liability insurance for street furniture located adja- roadways than rural highways, and demands for opera- cent to the road. Seattle is one city with this insurance tional improvements coupled with limited street right-of- requirement. Other jurisdictions restrict the placement of way often lead to placing these poles proximate to the private street furniture by a permitting process comple- roadway edge (see Figure 9). mented by a tax for each square meter or square foot of pub- The absolute number of fatalities related to utility lic right-of-way used. Dublin, Ireland, is one city that uses poles has remained around 1,200 since the mid 1980s this approach. (see Figure 10) (75). Utility poles are involved in the sec- ond highest number of fixed-object fatalities (trees are Safety research. The research team for this project was involved in the highest number of fixed-object fatalities). not able to locate any research evaluating the relative hazard In a statistical study performed by Washington State that may be posed by street furniture. Nevertheless, street Department of Transportation researchers, utility poles furniture can potentially create sight distance obstructions were identified as one of the roadside features that can when located near an intersection, particularly when large significantly affect the injury level or severity of run-off- numbers of people congregate as a result of the street furni- road crashes (68). ture. It is also important that the sight distance of pedestrians The literature regarding crashes related to utility poles be maintained when placing street furniture proximate to the has identified utility poles as being principally an urban roadway. hazard, with urban areas experiencing 36.9 pole crashes per 161 km (100 mi) of roadway, compared with 5.2 pole Strategy summary. Common street furniture safety strate- crashes per 161 km (100 mi) of roadway for rural areas gies are as follows: (76). Zegeer and Parker found that the variable that had Purpose Strategy the greatest ability to explain crashes related to utility poles was the average daily traffic (ADT) along the roadway (33). Minimize likelihood of · Locate street furniture as far crash from street as possible (P) The significance of ADT as the critical variable explains the · Restrict street furniture placement importance of vehicle exposure to understanding run-off- to avoid sight distance issues road crashes with utility poles. for road user (P) A common recommendation for addressing the utility Minimize crash severity Develop street furniture that meets pole safety issue is to place utilities underground and basic crashworthy standards (E) thereby remove the hazardous poles. The removal of all
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26 Photos by NCHRP Project 16-04 research team. Figure 9. Road widening resulting in utility pole hazards. poles in the urban roadside environment is not practical as or warning motorists of the obstacles (78). The report in- these poles often function as the supports for street lights cludes several initiatives in which pole relocation or re- and other shared utilities. There are, however, several moval is currently targeted as a safety strategy. known utility pole hazardous locations that should be Additional ways to minimize utility pole crashes are plac- avoided when feasible. In general, utility poles should be ing utilities underground (where feasible), using shared placed in the following locations: poles to reduce pole density, and relocating poles to less vul- As far as possible from the active travel lanes, nerable locations. The delineation of poles using reflective Away from access points where the pole may restrict tape or buttons may also help an alert driver identify a util- sight distance, ity pole and avoid it; however, this delineation treatment Inside a sharp horizontal curve (as errant vehicles tend may also act as an attractor for impaired drivers who are at- to continue straight toward the outside of curves), and tempting to guide their vehicles by road edge delineation. On only one side of the road (66, 77). The Land Transport Safety Authority in New Zealand rec- State of the Art Report 9: Utilities and Roadside Safety ommends that utility poles and large trees be highlighted summarizes categories for utility pole safety solutions as using a uniform method that cannot be removed, such as the following: changing the pole position; using safety de- reflectorized markers or paint markings (79). vices (crash cushions, safety poles, guardrail, and barriers); Increasing the lateral distance of utility poles from the travel lanes appears to be a promising improvement strategy. Many jurisdictions maintain an operational offset of 0.5 m (1.5 ft), but several agencies are seeking to increase the pole placement offset in urban regions. Haworth and colleagues (80) observed that in metropolitan Melbourne, Australia, poles involved in fatal crashes were most often less than 2 m (6.6 ft) from the edge of the road. The Clear Roadside Committee established by the Georgia Utilities Coordinat- ing Council suggests that for curbed sections, poles should be placed as far as is practical from the face of the outer curbs, with the following goals: Lateral clearance of 3.6 m (12 ft) from the face of the Graphic reprinted from State of the Art Report 9: Utilities and Roadside curb to the face of the pole. Safety (75). For speed limits greater than 56 km/h (35 mph) but Figure 10. Fatalities related to crashes with not exceeding 72 km/h (45 mph), a lateral clearance of utility poles (19802000). 2.4 m (8 ft).
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27 For roads with posted speed limits less than or equal to An important issue in addressing roadside safety is the 56 km/h (35 mph), a lateral clearance of 1.8 (6 ft) (81). role of lighting in making potentially hazardous roadside Similar to the Georgia policy, the Maine Utility Pole Lo- environments visible to the road users (motor vehicle driv- cation Policy suggests that offsets should be greater than ers, bicyclists, and pedestrians), particularly during night- 2.4 m (8 ft) for roadways with posted speed limits of 40 to time hours. Other issues with roadside lighting, as cited in 55 km/h (25 to 35 mph) and that offsets should be greater the literature, include frequency and spacing of lights (56) than 4.3 m (14 ft) on roadways with posted speed limits of and lighting color and associated visibility (85). These is- 65 to 70 km/h (40 to 45 mph) (82). sues are beyond the scope of this study. In Sweden, emphasis is placed on system level improve- · Signposts and roadside hardware. The design of signposts is ments. Based on the idea that the transportation system directed by NCHRP Report 350, and there has been substan- itself is unsafe, Sweden is redeveloping the system to re- tial research devoted to designing these features to be tra- duce user errors that lead to injury or death. One of the versable (36). Multiple designs for these features are included strategies to achieve this goal is to modify the system to in the current edition of the Roadside Design Guide (1), and ensure that users are not exposed to impact forces that can specifications for evaluating these features are contained in kill or severely injure the users (71). AASHTO's Standard Specifications for Structural Supports for Finally, some utility pole literature suggests the use of Highway Signs, Luminaires, and Traffic Signals (84). breakaway poles. In the event that an errant vehicle impacts Crash severity can be minimized through the use of tra- a breakaway pole, the pole will swing upward and then back versable hardware, such as break-away light posts, mail- down (thereby permitting the impacting vehicle to travel boxes, and utility poles. The current standard for break- safely under the pole). One concern with breakaway utility away hardware, as contained in the Roadside Design Guide poles has been whether they pose a threat to pedestrians and NCHRP Report 350, is that breakaway features func- when they swing back down after swinging up to avoid a tion omni-directionally to ensure that the features do not crash. A 1970s series of case studies performed in Australia constitute a hazard from any impact direction (1, 36). To by McLean, Offler, and Sandow evaluated crashes in the prevent vehicle snags, the stub height after breakaway proximity of Stobie poles (utility poles with two rolled-steel should not exceed 100 mm (4 in.) (see Figure 11). joists separated by concrete) (83). In evaluating the risk to While an unobstructed and traversable roadside is pre- pedestrians, the researchers determined that there were no ferred, it may be necessary at some locations to use break- cases in their studies where a pedestrian was in the imme- away features, which will minimize the severity of the initial diate vicinity of a collision between a car and a Stobie pole. impact by an errant vehicle. Breakaway poles and similar · Lighting and visibility. The design of luminaire posts is di- features must be designed to prevent intrusion on the pas- rected by NCHRP Report 350, and substantial research has senger compartment of the vehicle, either by minimizing been devoted to designing these light poles to be yielding the weight and load of such features, or by providing a sec- upon impact (using breakaway bolts) (36). Multiple designs ondary hinge, at least 2.1 m (7 ft) above the ground, that for these posts are included in the current edition of the permits the vehicle to pass safely beneath the post upon im- Roadside Design Guide (1), and specifications for evaluat- pact. The current edition of the Roadside Design Guide pro- ing these features are contained in AASHTO's Standard vides specifications for these devices and suggests that the Specifications for Structural Supports for Highway Signs, concern for pedestrians in urban areas has led to a trend of Luminaires, and Traffic Signals (84). using fixed supports for some urban locations (1). Graphic reprinted from Roadside Design Guide (1) with permission. Figure 11. Stub of a breakaway support.
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28 Strategy summary. Common strategies for utility poles, Also, in locations with bicycle activity, safety barriers located lighting supports, and signposts/roadside hardware are as immediately adjacent to the road may expose cyclists to un- follows: necessary risks because the barriers may give a sensation of "squashing" the cyclist between the barrier and an adjacent Purpose Strategy motor vehicle (79). Treat individual high · Remove or relocate poles (P) risk pole locations · Place poles on inside of horizontal Safety research. Considerable research has been per- curves and avoid placement on formed on a variety of traffic barriers. NCHRP Report 490: In- outside of roudabouts or too close Service Performance of Traffic Barriers includes an extensive to intersection corners (P) · Use breakaway or yielding literature review that discusses the evolution of traffic barrier poles (T) crashworthiness (87). The following summaries briefly re- · Shield poles (P) view the application of these tested barriers (barriers, bridge · Improve pole visibility (E) rails, and end treatments) in an urban environment. Treat multiple poles in · Establish urban clear zone offset high risk locations guidelines for pole setback · Barriers. Barriers can be categorized as flexible (cable barri- distances from curb (P) · Place utilities underground while ers and W-beam guardrail with weak post), semi-rigid (thrie maintaining appropriate beam and W-beam guardrail with strong post), and rigid nighttime visibility (P) (concrete barrier system such as the New Jersey barrier). · Combine utilities/signs onto Because guardrails are most typically associated with shared poles (reduce number rural and higher speed environments, the use of guardrails of poles) (P) in urban environments is often restricted to protection of · Replace poles with building- mounted suspended lighting bridge approaches and departures. In fact, the conventional (where suitable) (E) (86) use of guardrail is to shield roadside objects from impact Minimize level of Reduce travel speed on adjacent that pose a greater threat than impact to the guardrail it- severity road (P) self. Since the placement of guardrails at locations with frequent driveways is problematic due to the numerous breaks in the barrier treatment and the adverse effect of Safety Barriers the guardrail on driveway and intersection sight distance, Roadside barriers are subject to NCHRP Report 350 testing the use of conventional guardrail is minimal in urban criteria (36). There are several types of safety barriers that low-speed corridors. AASHTO indicates that for very low may be present in an urban environment. These include the traffic volume locations, traffic barriers are not generally following: cost-effective (32). This recommendation is confirmed by research performed by Stephens (88) and Wolford and · Barriers (flexible, semi-rigid, and rigid); Sicking (89). In the event that an engineer does endorse the · Bridge railings; and use of a barrier in an urban environment, factors in addi- · End treatments (crash cushions and end terminals). tion to the lateral offset, deflection distance, terrain effects, flare rate, and length of need (common to rural placement Generally, most of the research on safety barriers has been design) must be supplemented by consideration of corner oriented toward the design of barriers and their placement to sight distance, pedestrian activity (with particular atten- shield vehicles from hazardous roadside conditions. The tion to the needs of persons with disabilities), and bicycle Roadside Design Guide and NCHRP Report 350 provide con- activity (1). siderable information on placement and design of safe bar- When the use of a protective barrier is warranted in the rier systems (1, 36). FHWA maintains a roadside hardware urban environment, the application of aesthetic barrier treat- website that provides information about specific roadside ments may be considered. These treatments perform the hardware that has been tested (see http://safety.fhwa.dot.gov/ same general function as a guardrail (shield hazardous envi- roadway_dept/road_hardware/index.htm). ronments) while enhancing the aesthetics of a roadway. In the urban environment, many of the safety barriers The use of barriers in an urban environment can be to common to rural environments may not be suitable due to shield roadside obstacles (such as rigid utility poles), sepa- constraints regarding space available for flared end treatments, rate motorized and nonmotorized traffic, and provide a the constraining influence of safety barriers on pedestrian physical separation between the active travel lanes and pe- activity, and the potential obstruction of sight distance at the destrian activity. For barriers with a shielding objective, sev- many intersections and driveways in the urban environment. eral different barriers may be considered. For example, the
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29 California Department of Transportation published a report quacy). Transitions to guardrails must be located at both in 2002 with a focus on suitable aesthetic barriers (90). This the approach and departure end of all bridge rails. Bridge report includes the status of crash testing as well as the rails must be designed to retain a large passenger car at advantages and disadvantages of each treatment. Candidate the legal driving speed for local streets and roads (94). The barriers for urban environments include concrete barriers bridge rails must, therefore, be structurally designed and with textured and patterned surfaces, timber guardrail, pre- maintain their structural integrity after impact. cast concrete guardwall, and stone masonry guardwall. · End treatments. For locations where the end of the barri- The placement of barrier in the vicinity of a median is a ers cannot be adequately flared or protected, it is necessary common strategy in rural environments to help prevent to use an end treatment such as a barrier terminal or crash head-on collisions by errant vehicles. Washington State, for cushion. Aesthetic enhancements to end treatments have example, performed an evaluation of median treatments for not received much attention, so conventional treatments multi-lane, divided state highways with full access con- are necessary in the urban environment. These treatments trol (91). The researchers determined that the placement of should not allow a vehicle to penetrate, vault, or roll upon a barrier for medians up to a width of 15 m (50 ft) is cost- impact. They should have the strength and redirectional effective. In an urban environment, the median width is nar- qualities of a standard barrier. row and often serves the combined functions of separating opposing directions of travel and acting as pedestrian refuge Dynamic Roadside Conditions at certain locations. Currently, concrete Jersey barriers are Bicycle Facilities used most commonly for medians in urban locations. The placement of barriers adjacent to the road intro- General information. Bicycle facilities consist of road duces a new roadside hazard. For example, use of rigid bar- and roadside features intended for bicycle operation. These riers tends to result in a greater number of minor crashes, facilities may include standard lanes, wide outside lanes, but dramatically reduces the number of serious or fatal bicycle lanes, and off-road bicycle paths. Accompanying bicy- head-on and run-off-road crashes (92). Lee and Manner- cle facilities may be bicycle hardware located along the road- ing (68) determined that for urban environments, guard- side, such as bicycle racks. In general, the literature regarding rails are significantly associated with an increase in crash the relationship between bicycle facilities and roadside safety frequency, but the severity of these crashes is likely to re- is limited. Wide shoulders and bicycle lanes provide an addi- sult in possible injury only. tional "clear" area adjacent to the travelway, so these features In locations where aesthetics are important and a bar- could potentially provide a secondary safety benefit for rier is required, jurisdictions may develop crash-tested motorists, provided bicycle volumes are low. These bicycle options such as the Vermont-approved stone masonry facilities will also further separate the motor vehicle from any system shown in Figure 12 (in addition to the various aes- roadside obstructions and improve the resulting sight distance thetic barriers identified in the California report previ- for motor vehicle drivers at intersecting driveways and streets. ously indicated ). A second area of consideration is the placement of bicycle- · Bridge rails. In both urban and rural environments, bridges supportive hardware, such as bicycle racks, adjacent to the should be equipped with rails that do not permit vehicles travelway. Bicycle racks are commonly made of steel or other to penetrate the space beyond the rail (i.e., structural ade- metals and are typically bolted to the ground to secure locked bicycles from potential theft. These features are not designed to be yielding should a run-off-road event occur. To date, there has been little evaluation of the potential roadside hazard posed by such treatments, although they can clearly present a poten- tial fixed-object hazard. Making such features yielding would potentially minimize the core function of these features-- providing a secure location for locking up bicycles. Thus, a potentially more desirable alternative may be to encourage the placement of these features outside of the clear zone. Safety research. Most of the bicycle research focuses on specific bicycle safety issues such as safety helmets and train- ing. Several studies have developed and reviewed previous bicycle suitability or compatibility criteria (95, 96). The crite- Photo reprinted from Guardrail Study (93). ria for determining bicycle suitability include available lane Figure 12. Stone masonry barrier. width, traffic volume, and vehicle speeds. One best practices
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30 review has suggested that road safety for cyclists could be en- pedestrian, however, can be countered by the pedestrian who hanced by increased law enforcement to ensure that cyclists elects to cross the street mid-block in areas not designated for do not ride in the wrong direction (against traffic) or at night pedestrian crossing. The parked vehicles may act as a shield without adequate lighting (97). This best practices review also to prevent proper sight distance for the drivers of adjacent noted that the use of extruded curbs to separate a bike lane motor vehicles, often resulting in new conflicts between from traffic should be avoided. motor vehicles and pedestrians stepping between parked cars. Since the focus of this research effort is roadside safety for Similarly, there is an inherent conflict between the motor ve- the urban environment, it is helpful to understand the mag- hicle and drivers exiting or entering their parked vehicles on nitude of the safety risk to cyclists as they encounter roadside the traffic side of the roadway. environments. One FHWA report using hospital emergency An additional concern often cited regarding on-street department data noted that 70 percent of reported bicycle parking is the effect it may have on reducing emergency serv- injury events did not involve a motor vehicle and 31 percent ices' response rate. Often the narrowing effect of on-street occurred in non-roadway locations. For bicycle-only crashes, parking can be compounded by illegal parking too close to a total of 23.3 percent of the recorded crashes occurred at critical locations such as intersections. The Local Govern- sidewalk, driveway, yard, or parking lot locations (98). Stutts ment Commission released a publication called Emergency and Hunter (99) evaluated bicyclemotor vehicle crashes and Response Traffic Calming and Traditional Neighborhood determined that some factors associated with the crash were Streets (100) that suggests that the adverse effects of on-street variables such as age, gender, impairment, and time of day. parking can be mitigated by implementing the following Roadside variables (sidewalk, parking lot, and driveways) strategies: were not statistically significant in their model. · Placing a double set of driveways periodically (per fire Strategy summary. Common strategies to improve bi- department recommendation) to enable local access; cycle safety as well as bicyclemotor vehicle interactions are · Placing alleys on short blocks across from each other; as follows: · Placing mailbox clusters, curb extensions, or similar treat- ments where residents will find it inappropriate to park; and Purpose Strategy · Enforcing parking criteria to minimize illegal parking. Reduce likelihood · Use wider curb lanes (P) of crash · Increase bicycle enforcement (T) Finally, the severity of a roadside hazard constituted by a · Increase operational offsets (P) collision between a parked vehicle and a moving vehicle is Reduce severity of crash Locate bicycle racks as far away minimal. Since on-street parking is generally parallel to the from road as possible (T) moving vehicles, the impact by a moving vehicle is likely to be a sideswipe crash. This is one of the less severe crash types. For Parking locations with head-in or reverse-in parking, the crash severity likelihood is increased as the moving vehicle may impact a ve- General Information. In many urban environments, lim- hicle in reverse. Proper sight distance and separation of parked ited off-street parking often necessitates the use of on-street vehicles from the active travel lane (often by the use of a bulb- parking to address the needs of local businesses and stakehold- out at the intersection) will help minimize safety risks between ers. As noted in the Green Book (37), cars typically park 150 to moving and parked or parking vehicles. As indicated in the pre- 305 mm (6 to 12 in.) from the curb and have a normal width of vious section, curb extensions or bulb-outs can pose a hazard to roughly 2.1 m (7 ft). Thus, approximately 2.4 m (8 ft) are bicyclists by forcing them into the active travel lane. In the event needed to comfortably accommodate on-street parking. One that the parking lanes are not occupied, the extension could also common strategy in larger cities is to design wider outside park- create a hazard for drivers unfamiliar with it. On-street parking ing lanes, such as 3 m (10 ft), and convert them to travel lanes is generally not considered appropriate for higher speed roads during peak periods and anticipated high-volume conditions. such as suburban to urban transitional arterials. On-street parking can potentially have mixed results on a roadway's safety performance. On the one hand, these fea- Strategy summary. Common on-street parking strate- tures narrow the effective width of the roadway and may gies are as follows: result in speed reductions, thereby leading to a reduction in crash severity. Conversely, on-street parking may also lead to Purpose Strategy an increase in collisions associated with vehicles attempting Reduce likelihood of Restrict on-street parking to low- to pull in or out of an on-street parking space. crash speed roads (P) In addition to vehicle conflicts, on-street parking serves Reduce crash severity Where parking is appropriate, use as a physical buffer between the motor vehicle path and parallel parking rather than angular pedestrian facilities. The added safety buffer provided to the parking (P)
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31 Sidewalks and Pedestrian Facilities General information. Sidewalks and pedestrian facili- ties, in general, do not pose a particular hazard to motorists. The safety concern about locating these facilities adjacent to the road is the risk to the pedestrians using the facilities. Pro- viding safe facilities for pedestrians is an obvious strategy for increasing pedestrian safety. While shared streets may be appropriate if vehicle speeds and volumes are kept extremely low (see Figure 13), for most roads in urban areas, vehicle speeds warrant the use of sidewalks (21, 101, 102). The Green Book (37) recommends the use of sidewalks on urban streets, with sidewalk widths ranging between 1.2 and 2.4 m (4 and 8 ft) depending on the roadway classification and nearby land use characteristics (see Table 9). Photo reprinted from walkinginfo.org (121). Of the roughly 75,000 pedestrian-related crashes that occur each year, almost half occur while the pedestrian was at Figure 13. Low-speed shared street. a non-intersection, on-roadway location (see Table 10). The conventional approach to examining pedestrian and As shown in Table 11, approximately 13,600 crashes were bicycle safety is to examine crash records to determine the classified as "darting into road" crashes, which is where a non-motorized user's action prior to a crash event. Complete pedestrian rushes into the street and is struck by a motor and accurate motor vehicle crash data for pedestrian crashes vehicle. The majority of individuals involved in "darting- can be difficult to find. Stutts and Hunter evaluated police- into-road" crashes were children between the ages of 5 and 9, reported pedestrian crashes and compared these to hospital who may have been using the street for play activities (103). emergency room records (99). They found that crash events As Whyte has indicated, the street is often the preferred play that occurred in parking lots, driveways, and other off-road location for children, even when parks and other recreational locations were reported less frequently than those occurring amenities may be available (104). in the roadway. A 1999 FHWA study (98) also reviewed hos- An additional feature of the roadside environment is a pital emergency room records and determined that 11 percent pedestrian buffer area. The pedestrian buffer is a physical of the pedestrianmotor vehicle events recorded occurred at distance separating the sidewalk and the vehicle travelway. roadside locations (sidewalk, parking lot, and driveway). Buffer areas typically serve a host of secondary purposes as Of the data for which pre-crash action is known, improper well--they provide locations for on-street parking, transit crossings were the largest single crash category, accounting for stops, street lighting, and planting areas for landscape mate- 20 percent of the total crashes. The categories that have the rials, as well as a location for a host of street appurtenances, such most direct relationship to the roadside fell into the four cate- as seating and trash receptacles. Buffer strips may be either gories most strongly related to the design of roadsides: jogging, planted or paved. The Green Book supports the use of buffer walking with and against traffic, and miscellaneous activity. strips on urban arterials, collectors, and local streets (37). Table 9. Green Book sidewalk specifications (37). Road Class Side of Street Specification Urban Arterials Both Border area (buffer plus sidewalk) should be a minimum of 2.4 m (8 ft) and preferably 3.6 m (12 ft) or more. Collector Both sides of street for access to 1.2 m (4 ft) minimum in schools, parks, and shopping. residential areas. Both sides of streets desirable in 1.2 to 2.4 m (4 to 8 ft) in residential areas. commercial areas. Local Both sides of street for access to 1.2 m (4 ft) minimum in schools, parks, and shopping. residential areas. Both sides of streets desirable in 1.2 to 2.4 m (4 to 8 ft) in residential areas. commercial areas, although additional width may be desirable if roadside appurtenances are present. Source: Developed from A Policy on Geometric Design of Highways and Streets, 4th ed. (37 ).
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32 Table 10. Pedestrian location during a crash variety of options for improving pedestrian safety in the State (2002). of Utah (107). Many of his recommendations are similar to those already reviewed; however, he also included improving Pedestrian Location Number Percent Intersection - In Crosswalk 14,674 19.7 sidewalk security and visibility with street lights as an impor- Intersection - On Roadway 15,319 20.6 tant issue. Cottrell further noted that the failure to remove Intersection - Other 1,391 1.9 snow from sidewalks during winter conditions may result in Intersection - Unknown Location 810 1.1 Non-intersection - In Crosswalk 381 0.5 pedestrians entering the street either to cross it or to walk Non-intersection - On Roadway 35,785 48.0 along the cleared road. The document that to date most ex- Non-intersection - Other 3,518 4.7 Non-intersection - Unknown Location 173 0.2 haustively summarizes strategies for improving pedestrian In Crosswalk - Unknown if Intersection 16 0.0 safety is NCHRP Report 500: Guidance for Implementation of Other Location 1,830 2.5 the AASHTO Strategic Highway Safety Plan--Volume 10: Unknown Location 595 0.8 Total 74,492 100.0 A Guide for Reducing Collisions Involving Pedestrians (123). Source: General Estimates System (122). This document includes methods for enhancing pedestrian safety in the road as well as adjacent to the road. The recom- Safety research. Several studies have evaluated potential mendations in the document are consistent with those of the countermeasures to improve pedestrian safety. Generally, these research studies summarized in this review. studies were based on case studies, statistical models, or subjec- tive evaluation. Landis and colleagues (105) modeled several Strategy summary. Common strategies for eliminating roadside walking environment variables to evaluate the pedes- or minimizing motor vehiclepedestrian crashes at roadside trian's perception of risk versus actual risk. The researchers locations are as follows: expected that as the number of driveways increased they would Purpose Strategy observe a decrease in pedestrian safety, but this hypothesis was Reduce motor vehicle- · Provide continuous pedestrian determined not to be statistically significant. They did find that pedestrian crash facilities (P) motor vehicle volume and vehicle speeds were significant fac- likelihood at roadside · Install pedestrian refuge medians tors in pedestrian safety. Corben and Duarte evaluated the high locations or channelized islands (see number of pedestrian injuries along Melbourne's arterial roads previous section on medians and and recommended the adoption of three practices: islands) (P) · Offset pedestrian locations away · Reduce traffic volumes, from travelway with pedestrian · Reduce road widths, and buffers (P) · Reduce vehicle speeds (106). · Physically separate pedestrians from travelway at high-risk Corben and Duarte further suggested that strategies for locations (P) reducing the vehicle speed can include public awareness and · Improve sight distance by removing objects that obscure enforcement campaigns; gateway treatments (such as road driver or pedestrian visibility (T) narrowing, changing pavement texture, and implementing · Maintain pedestrian facilities free roundabouts); and streetscape improvements (106). Gateway of leaves, snow, or tree roots (T) treatments are addressed in the traffic calming section of this · Improve visibility by installing document, which follows this section. Cottrell reviewed a illumination for nighttime conditions (T) Table 11. Pedestrian crashes by type (2002). · Enforcement and public awareness campaigns (T) Pedestrian Crash Type No. of Crashes Percent Reduce severity of Reduce roadway design speed/ No Action 23,502 31.6 motor vehicle- operating speed in high pedestrian Darting into Road 13,594 18.3 pedestrian crashes at volume locations (T) Improper Crossing 15,344 20.6 roadside locations Inattentive 521 0.7 Jogging 211 0.3 Pushing Vehicle 103 0.1 Traffic Calming Applications--Gateway Walking with Traffic 1,746 2.3 Walking against Traffic 1,184 1.6 Treatments Playing, Working, etc. in Roadway 8,074 10.8 Other 5,964 8.0 Although traffic calming applications have been prevalent Unknown 4,249 5.7 throughout Europe for several decades, traffic calming is Total 74,492 100.0 relatively new in the United States, first emerging in the Source: General Estimates System. (122). late 1990s as a strategy for addressing community livability
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33 concerns associated with high vehicle traffic volumes and Safety research. Little information exists on the safety speeds. The practice of traffic calming applications has performance of gateway treatments. At present, such treat- sparked a substantial debate regarding the safety and appro- ments have not been subject to extensive crash testing, priateness of these applications, particularly when applied to undoubtedly because of the large degree of variation in roadways intended for higher volumes and/or higher operat- the design and materials used in the construction of such ing speeds, such as minor arterial roadways (23, 108, 109). features. Nevertheless, as noted in Skene, such features are Traffic calming applications come in a variety of forms, often used by Canadian and British transportation profes- including partial and full road closures and alterations in the de- sionals in speed transition zones to provide the driver sign of intersections and curb lines. This report is specifically fo- with visual cues of a forthcoming change in safe operating cused on traffic calming strategies deployed at arterial transitions conditions (108). In the United States, these gateway treat- from higher speed rural conditions to locations with lower speed ments are typically aimed at delineating the boundaries of arterial characteristics. This transitional traffic calming strategy specific communities. is known as a gateway and is reviewed in the following summary. Most of the research regarding gateway treatments focuses on their influence on operating speed or road users' percep- General information. The traffic calming strategy known tions of gateway treatments and their understanding that they as a gateway is defined by Burden as "a physical or geometric are, in fact, transitioning into a different and slower speed landmark on an arterial street which indicates a change in environment. environment from a major road to a lower speed residential In a 1997 study in the United Kingdom, researchers at the or commercial district (110)." Burden goes on to suggest that Transport Research Laboratory performed a before-after gateways can be a combination of street narrowings, medians, evaluation of a series of traffic calming strategies on major signs, arches, roundabouts, or other features. The objective of roads (112). The traffic volume on candidate roads was greater a gateway treatment is to make it clear to a motorist that he than 8,000 vehicles per day, and at least 10 percent of the traf- or she is entering a different road environment that requires fic was composed of heavy vehicles. Speeds at inbound gate- a reduction in speed. ways were reduced at eight out of nine locations tested. Mean Drivers need a certain transitional speed zone with the speed reductions ranged from 5 to 21 km/hr (3 to 13 mph). explicit guidance and roadway features to inform and encour- The study evaluated a variety of treatments, including speed age them to gradually slow down before they reach the urban reduction signage, narrowings, dragon teeth marking, speed residential area for a safe entry. A transitional speed zone can cushions, colored pavement, and advanced signing. The re- also help drivers to speed up within a certain timeframe when searchers determined that the signing provided a high visual leaving an urban area. This transition area is extremely im- impact and resulted in large speed reductions. Physical de- portant for drivers who are not familiar with the urban area. vices such as the speed cushions resulted in a greater level of They rely on the roadway features to indicate changes in sur- speed reduction than signing alone. The use of colored bands roundings that require an adjustment in their driving speed placed laterally across the road and placed in a series seemed and behavior. to result in some speed reduction, but did not result in large The gateway concept was presented in a 1998 paper by decreases in speed. The researchers did not test the speed re- Greg Pates in which he depicted the region between a rural duction signs and so could not comment on the effectiveness area and a fringe area (transitioning into urban use) as the of these devices. Dragon-teeth marking, identified as one of gateway location where travel speed should be reduced and the strategies, is depicted as a schematic and a photograph in motorists should become more alert (111). Figure 14. Photo and graphic reprinted from Traffic Calming on Major Roads (112) under the terms of the Click-Use License. Figure 14. Dragon-teeth marking.
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34 Berger and Linauer describe the operating speed influence medians of various dimensions to help slow traffic down. of five raised island configurations used as gateway treatments Based on a before-after site evaluation, the speed reduction at from high-speed rural locations in Austrian villages (113). control sections (where the strategies were not deployed) The five gateway island configurations are shown in Figure 15. ranged from 85 percent to 88 percent for initially observed The most dramatic influence on speeds occurred for Island speeds, while the speeding percentage reduction in test sections Number 5, where the path of the approach lane was shifted ranged from 47 percent to 67 percent of the original speeds. dramatically. Table 12 demonstrates the range of speed re- This 20-percent speeding reduction was determined to be ductions observed in this Austrian study, and Figure 16 shows statistically significant at a 99-percent confidence level. The the speed profile for Island Number 5. researchers were not able to compare corresponding crash A study performed for traffic calming strategies deployed data for the site. from 1993 to 1996 in Ireland evaluated the transition zone as Ewing reviewed a highway reconstruction project deployed the area between a high-speed and low-speed road (114). The in Saratoga Springs, New York (116). The case road transi- researchers evaluated the gateway transition from rural to tioned from a four-lane, semi-rural highway with a flush, urban environments in two phases: painted median and a speed limit of 88 km/hr (55 mph) to a three-lane urban road with a raised median and a posted · From the "Traffic Calming Ahead" sign to the "Do Not speed limit of 48 km/hr (30 mph). The length of road avail- Pass" sign and able for this transition was 550 m (1,800 ft). Because the road · From the "Do Not Pass" sign to the gateway treatments in passes the Saratoga Spa State Park, the Lincoln Baths, and the the form of raised islands. Museum of Dance, local representatives wanted a gateway for the transition of the road. The three photos shown in Fig- ure 17 depict the gateway transition ultimately constructed In the first phase, the researchers observed that the "Traf- for this facility. fic Calming Ahead" sign at the beginning of the transition Roundabouts are another commonly recommended gate- zone reduced inbound traffic speed. They determined this way treatment. Pates has discussed how Norwegian trial by comparing the speed reduction results in the transition projects using roundabouts experienced average speed re- zone with and without a "Traffic Calming Ahead" sign. ductions of 10 km/hr (6 mph) (111). Ewing (23) and Zein With the "Traffic Calming Ahead" sign present, the 85th and Montufar (92) identify roundabouts as safe traffic calm- percentile speeds ranged between 90 and 100 km/hr (56 and ing alternatives to conventional intersections that can serve 62 mph) at the start of the transition zone. The 85th percentile as both psychological and physical indicators of a transition speeds at the "Do Not Pass" signs were reduced by 6 to 8 km/hr from a rural high-speed environment to the lower speed (4 to 5 mph). At locations without the traffic calming signs, urban street. Ewing also indicates that the center islands of the 85th percentile speeds were observed to be reduced by the roundabouts can be landscaped and possibly include only 2 to 3 km/hr (1.2 to 1.9 mph) at the same approach sculptures or monuments. Although the research team was location. unable to locate published research regarding the use of In the second phase, the speed reduction analysis results street art in roundabout medians, they did speak with re- indicated that the gateway with raised traffic islands was an searchers from both the United Kingdom and Australia. effective traffic calming treatment. They found that speed Representatives from both countries suggested that the ap- reductions of approximately 14 km/hr (9 mph) relative to the plication of street art in roundabouts is generally hazardous speed recorded at "Do Not Pass" signs were achieved at gate- if these items are placed in the center of the first roundabout ways with raised islands compared with a reduction of only encountered by the driver on a rural road. The use of a series 10 km/hr (6 mph) at gateways without raised islands. of roundabouts as a transition, with the street art located A case study performed in Canada evaluated traffic calm- in the subsequent roundabouts, however, is a common prac- ing strategies on an arterial road connecting two residential tice and appears to be a safe strategy for these transitional areas (115). Mohawk Road is a two-lane road with a regions. 50-km/hr (31-mph) speed limit and was originally designed The evaluation of gateway treatments is a new area of re- to service rural conditions. Speeding is very common on this search for transportation and, as a result, very little is known road, and the test data showed that about 67 percent of the about crashworthiness issues. As reviewed in this summary, vehicles were exceeding the speed limit at the test location. the focus in using gateway treatments has been on the re- After evaluating several alternatives, the jurisdiction finally sulting speed reduction (thereby reducing ultimate crash elected to implement a series of landscaped speed control severity).
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35 Graphic reprinted from "Raised Traffic Islands at City Limits--Their Effect on Speed" (113). Figure 15. Gateway islands used in an Austrian study. Table 12. Gateway median speed results from Austrian study (113). Island Number Speed 1 2 3 4 5 Vmean Previous 54.0 58.0 60.0 65.0 65.0 (km/h) Subsequent 54.1 48.4 44.1 47.2 40.1 V85 Previous 62.0 67.0 70.0 76.0 77.0 (km/h) Subsequent 61.0 54.5 50.5 55.2 44.6 Vmax Previous 70.0 88.0 86.0 95.0 97.0 (km/h) Subsequent 76.2 59.3 56.1 65.8 46.9 Source: Adapted from "Raised Traffic Islands at City Limits--Their Effect on Speed" (113). Graphic reprinted from "Raised Traffic Islands at City Limits--Their Effect on Speed" (113). Figure 16. Island Number 5 speed profile.