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8 Table 3. Fatal crash conditions on arterial, collector, and local roads (2005). Rural Urban Principal Minor Total Principal Minor Crash Condition Arterial Arterial Collector Local Rural Arterial Arterial Collector Local Total Urban Motor Vehicle Collision 2,256 1,912 2,211 891 7,270 2,262 1,442 447 877 5,028 Ped/Bike 269 221 377 322 1,189 1,353 865 257 748 3,223 Overturn (Rollover) 472 420 905 560 2,357 153 116 57 184 510 Fixed Object 821 1,088 2,740 1,811 6,460 805 851 442 1,058 3,156 Other Causes 119 154 271 270 814 145 148 66 266 625 Total 3,937 3,795 6,504 3,854 18,090 4,718 3,422 1,269 3,133 12,542 Source: Fatality Analysis Reporting System (118). classified as minor arterials. As depicted in Table 4, principal Because of such findings, the principal design strategy for arterials in urban areas have fatal crashes involving utility keeping vehicles on the roadway is to address horizontal shifts poles, light poles, and sign poles more often than their rural in the roadway. While eliminating the isolated sharp curve is counterparts. perhaps the most effective treatment, such applications are often prohibitively costly for mitigating safety problems on existing roads. As an alternative, designers have adopted a Preventing Vehicles from Leaving secondary strategy, which is to delineate potentially hazardous the Travelway environments, such as curves, using traffic control devices The logic behind keeping vehicles on the travelway is such as posted advisory speeds, chevrons, and other markings simple: if a vehicle does not leave the travelway, it cannot or strategies to more clearly indicate the edge of the travelway. be involved in a roadside crash. The difficulty posed by strate- Another secondary strategy that is commonly employed to gies aimed at keeping vehicles on the roadway is that, unlike alert drivers before their vehicle leaves the travelway is to providing appropriate clear zones or effective impact attenu- place rumble strips on the shoulder of the roadway. ators, these strategies are oriented toward the driver rather than the vehicle. Delineate Potentially Hazardous The literature on roadside safety shows that, of the strate- Roadside Environments gies identified in this report, knowledge on the design factors that may help prevent vehicles from leaving the roadway is the A common practice in minimizing run-off-road crashes is least developed. Typically, research has focused on the geo- to use signs to delineate potentially hazardous roadside con- metric characteristics of locations where vehicles leave the ditions. Signs and other indicators are used to increase the travelway and has found that a disproportionate share of these driver's awareness of changes to the operating characteristics crashes is associated with shifts in the horizontal curvature of of the roadway. Under conventional practice, the delineation the roadway, particularly an isolated, sharp, horizontal curve. of hazardous roadside conditions is limited almost exclusively Approximately 45 percent of all fixed-object crashes (5) and to the use of signs to denote shifts in the horizontal curvature up to 77 percent of tree-related crashes (6, 7) are due to vehicles of the roadway or other oncoming hazards. Other features, traveling off the roadway on the outside of a horizontal curve. such as the physical characteristics of the surrounding roadway, Table 4. Fatal fixed-object crashes on arterial, collector, and local roads (2005). Rural Urban Principal Minor Total Principal Minor Total Fixed Object Arterial Arterial Collector Local Rural Arterial Arterial Collector Local Urban Tree/Shrub 184 247 835 650 1,916 121 185 113 334 753 Utility Pole 39 73 181 138 431 103 116 59 119 397 Culvert/Ditch/Curb 143 237 583 401 1,364 240 239 111 242 832 Embankment 151 182 491 191 1,015 30 45 27 54 156 Guardrail 123 116 147 56 442 53 54 26 28 161 Building/Fence/Wall 30 50 160 110 350 40 46 26 82 194 Light/Sign Poles 86 84 139 95 404 108 81 36 72 297 Bridge 13 23 65 39 140 21 28 6 15 70 Other Fixed Object 52 76 139 131 398 89 57 38 112 296 Total 821 1,088 2,740 1,811 6,460 805 851 442 1,058 3,156 Source: Fatality Analysis Reporting System (118).
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9 also may give drivers cues as to safe operating behavior. Many In rural and select urban environments where roadways do governing jurisdictions consider the design of the street and not have curb lines, a common method for lane edge delin- the surrounding environment collectively, thus taking advan- eation is edge striping (using reflective paint for low-volume tage of environmental characteristics to alert the driver of safe roads and thermoplastic stripes for more densely traveled operating behavior. facilities). The use of edge striping in the urban environment varies. Many jurisdictions elect to use edge striping for major facilities only and allow the standard center striping combined Identifying Hazardous Conditions Using Signage with the curb line to delineate lane edges for lower speed local A common practice aimed at keeping vehicles on the road- roads. way is to post advisory speeds or other signage applications to A common strategy for enhancing roadway visibility on denote potentially hazardous conditions. While such a prac- urban streets is the use of street lighting. This not only illu- tice makes sense, the inconsistency of the practice of posting minates the travelway, but also provides safety and security advisory speeds (8) and the variability of posted speed limit for adjacent pedestrian facilities. Lighting is further discussed practices (9, 10, 11, 12) have led drivers to regularly disregard later in this chapter. speed signs. The use of reflective pavement markers (raised or snow- Further limiting the effectiveness of the signage practice is plowable) can also help delineate the vehicle travelway. These the fact that drivers are not merely disregarding signs; they are pavement markers often require regular maintenance (for lens failing to notice them. Studies have found that drivers typi- replacement or replacement of missing markers), so extensive cally comprehend only 56 percent of the signs posted along use of reflective pavement markers is generally reserved for the roadway (13). Further, even when drivers are conscientious high-volume locations or for locations that are perceived to in their attempts to adhere to factors such as posted speeds, be high risk. they naturally increase speeds toward the roadway's design Finally, many jurisdictions are experimenting with alter- speed when they begin to shift their concentration away from native pavement treatments. The use of skid-resistant pavement monitoring the speedometer (14). This has implications for surfaces has been a common recommendation for minimizing both geometric design and broader design practice, as it in- run-off-road crashes during inclement weather (1); however, dicates that violations of driver expectations may, to some an additional strategy is to change actual pavement color or extent, be directly associated with the design speed of the spe- pavement type at critical locations such as pedestrian cross- cific road. Overall, these findings suggest that signage and walks (for transverse delineation) and pavement edge (for lon- other similar applications may have only a moderate effect on gitudinal delineation). The City of Charleston, South Carolina, preventing run-off-road events. has maintained several cobblestone roads in their historic peninsula district. These roads serve to clearly identify the motor vehicle space from the adjacent pedestrian space and Enhancing Lane Delineation also provide the added benefit of dramatically slowing motor Run-off-road crashes often occur during reduced visibility vehicle operating speeds on these roads. The rough cobble- conditions (e.g., at dusk, dawn, and night, and during rain). stone pavement treatment is not conducive, however, to safe Thus, enhanced lane delineation may help keep an alert driver bicycle activity. In Denmark, the road surface treatments help from departing the road unexpectedly. There are several road users to clearly define who is to use a specific area of the methods for improving lane delineation in urban areas. These road system. These road surface treatments also help to de- may include curb lines, edge striping, street lighting, reflec- fine transitions from public to private space (15). Variations tive pavement markers, and pavement texture and/or color in road surface can be achieved by using patterns, textures, treatments. and similar treatments. The location of a concrete curb adjacent to an asphalt road is common for many urban regions. The contrasting light Taking Advantage of Characteristics color of the curb helps define the edge of the travelway. In re- of the Surrounding Environment gions where both roads and curbs are made of concrete, the face of the curb may sometimes be painted to create a con- While signage is most typically used to delineate hazardous trast. One disadvantage to using the curb line as the sole conditions, the FHWA scan of European practice suggests that method for lane edge delineation is that frequent curb cuts or signage is only one means of informing the driver of changes disruptions (at driveways or pedestrian crossing locations) in appropriate driving behavior (16). Drivers are monitoring may misdirect drivers who are fatigued or impaired. Infor- both traffic signs and the physical environment as part of the mation about the curb condition as a common urban road- driving task. While adequate signage is important for encour- side feature is included later in this chapter. aging safe driver behavior under changing environmental
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10 conditions, environmental hazards are signaled by more than and conducive to excessive speeds. The enclosed environment just the signs posted adjacent to the travelway. The geometric helps to mitigate speeding. In New Zealand, this enclosed design of the roadway and the characteristics of the surround- environment is captured using a vertical elements technique ing environment provide the driver with cues regarding safe in which the heights of vertical features are designed to be operating behavior. greater than the width of the street to provide the optical ap- One observation from the previously mentioned scanning pearance of a narrow street (22). These vertical elements can tour was that Europeans try to make the entire roadway send include trees, light poles, and other elements as long as the a clear and consistent message regarding safe operating be- human-made objects are frangible, and trees or shrubs have havior. Thus, design speeds are related to the physical envi- narrower trunks and do not interfere with sight lines. ronments in which the roadways are located, and the posted speed is meaningfully related to both. Typically, European Rumble Strips design guidance specifies tight design ranges for each road- way class, with a range of typically not more than 20 km/h Physical rumble strips are grooves placed into the roadway (approximately 12 mph) for any single road type in the urban or paved shoulder and are aimed at alerting the driver of environment. By narrowly specifying an appropriate design potentially hazardous conditions (see Figure 1). A similar speed range, designers are able to minimize the instances--such alert can also be achieved using thermoplastic rumble strips as an isolated sharp curve--that may be a potential hazard. which are generally placed on the surface of the road in con- An important aspect of this European practice is that it is junction with lane edge delineation. While transverse rumble adopted for the purposes of enhancing the safety of the road- strips are often used for purposes such as alerting the driver way. Agencies adopting such practices typically aim to achieve, of a downstream stop condition such as a toll booth or a stop- at a minimum, a 40-percent reduction in crashes over a 5-year controlled intersection, longitudinal rumble strips are also period, and, in many cases, agencies aim to have zero fatalities effective for alerting the driver that he or she is leaving the over a 10-year period (16). travelway. Although rumble strips do not have speed-reducing In a study of how people conceptualize urban environments, capabilities (23), they cause a vehicle to vibrate and make noise Kevin Lynch found that features such as architecturally unique when it crosses over them, thereby signaling to the driver that buildings, key viewsheds, and other environmental stimuli greater attention to the traveling environment is warranted. serve as central reference points by which individuals orient The sound made by a vehicle crossing rumble strips typically themselves and cognitively map their travel progress (17). The does not exceed that of the ambient sound experienced by the observation that such features figure prominently in the way driver (24); thus, the ability of rumble strips to alert drivers individuals visualize their travel activity suggests that environ- mental features provide drivers with important cues regard- ing appropriate driving behavior. The use of environmental factors to help inform drivers of safe operating conditions has received little attention in the literature (18), although the field of traffic psychology has begun to strongly encourage the use of environmental features as a key strategy for enhancing transportation system safety (19). Transit New Zealand's Guidelines for Highway Landscaping encourages agencies to use highway planting to help drivers understand the road ahead (20). Plantings are recommended to help with curve delineation, headlight glare reduction, visual containment, and speed awareness and stimulation. In 2001, the City of Las Vegas, Nevada, developed a guide for neighborhood traffic management. For this effort, they performed a community survey in which respondents rated pictures of various street cross sections (21). The most popular images were tree-lined streets in residential areas and com- mercial buildings placed close to the road in business districts. Both the trees and buildings provide a sense of enclosure that frames the street and narrows the driver's field of vision. The Photo reprinted from "New Focus for Highway Safety." (119) Las Vegas guide further suggests that when the buildings are set farther back from the street, the roadway appears to be wide Figure 1. Rumble strips.