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33 Roadside Safety Analysis Program developers selected the Cooper encroachment data for use in their encroachment ratetraffic volume relationships, because the Cooper data are more recent, constitute a larger sample size, and are believed to be of better quality than the Hutchinson and Kennedy data. The developers then incorporated adjustment factors based on previous studies (Wright and Robertson 1976; Perchonok et al. 1978) that compared roadway characteristics with fatal single- vehicle run-off-road crashes, with the underlying assump- tion that differences in roadway characteristics between the fatal crash sites and the comparison sites are correlated with the occurrence of these fatal crashes. They cited studies that showed that crash rates on horizontal curves and vertical grades were significantly higher than those on tangent sections, and they assumed by extension that encroachment rates would also be similarly affected by horizontal curves and vertical grades. The developers also stated their belief that the adjust- FIGURE 8 Example of buffer between sidewalk and street ment factors overstated the effects of curvature on encroach- (Credit: Marcus Brewer, Texas Transportation Institute). ment rates, but represented the best information available at the time of the study. Safety Treatments NCHRP Project 16-04 (Dixon et al. 2008) was initiated to develop design guidelines for safe and aesthetically pleasing Volume 3 of NCHRP Report 500 (Neuman et al. 2003a) roadside treatments in urban areas and a toolbox of effective discusses modifying the clear zone in proximity to trees to roadside treatments to balance the safety and mobility needs reduce crashes. of pedestrians, bicyclists, and motorists, and accommodate community values. In fulfilling the first of those objectives, This strategy involves any change to the sideslope or roadside clear zone designed to reduce the likelihood of tree crashes by researchers recommended the following guidelines for road- increasing the chances that a [run-off-road] (ROR) vehicle can side treatments: successfully recover without striking a tree. While both tree removal and shielding strategies modify the roadside, this strat- Where possible at curb locations, provide a lateral off- egy may be implemented in a variety of ways, such as flattening or grading sideslopes, regrading ditch sections, adding shoulder set to rigid objects of at least 6 ft from the face of the improvements, or providing protective plantings on the roadside. curb and maintain a minimum lateral offset of 4 ft. [The authors state that] the cost to modify the roadside is often At lane merge locations, do not place rigid objects in considerably higher than tree removal and guardrail installation; an area that is 10 ft longitudinally from the taper point. however, applying this strategy on specific curves or short tangent This will result in a 20-ft object-free length at the taper sections of roadway may help manage the costs. point. The lateral offset for this 20-ft section should be consistent with the lane width, typically 12 ft. The authors of NCHRP Report 500 add that this strategy Although many auxiliary lanes, such as bus lanes or has been proven to reduce the severity of ROR crashes and bicycle lanes, have low volumes and may be included rollover crashes. Although they identified no specific studies as part of a clear zone in the urban environment, higher- that related to only trees, much work has been completed on the speed auxiliary lane locations, such as extended length benefits of improving the geometry of the roadside to allow right-turn lanes, are common locations for run-off-road vehicles to recover when they encroach on the roadside. crashes. A lateral offset of 6 ft from the curb face to rigid objects is preferred, and a 4-ft minimum lateral Summary of Key Findings offset should be maintained. At locations where a sidewalk buffer is present, such as This section summarizes key findings from the research in Figure 8, rigid objects are not to be located in a buffer noted in this chapter. This is an annotated summary; conclu- area with a width of 3 ft or less. For buffer widths greater sions and recommendations are those of the authors of the than 3 ft, lateral offsets from the curb face to rigid objects references cited. are to be maintained with a minimum offset of 4 ft. At these wider buffer locations, other frangible objects can be strategically located to help shield any rigid objects. Allocation of Traveled Way Width Rigid objects should not be located in the proximity of driveways, and care is to be taken to avoid placing rigid The benefits of 2+1 roads in Europe validated a recom- objects on the immediate far side of a driveway. In addi- mendation for their use in the United States to serve as tion, objects are not to be located within the required an intermediate treatment between an alignment with sight triangle for a driveway. periodic passing lanes and a full four-lane alignment.

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34 Such 2+1 roads are most suitable for level and rolling Shoulder Width terrain, with installations to be considered on roadways with traffic flow rates of no more than 1,200 veh/hr in a For horizontal curves on two-lane nonresidential facili- single direction. The use of a cable barrier as a separa- ties that have 3 degrees of curvature, the width of the tor is discouraged, and major intersections should be lane plus the paved shoulder should be at least 5.5 m located in the buffer or transition areas between oppos- (18 ft) throughout the length of the curve (Staplin ing passing lanes, with the center lane used as a turning et al. 2002). lane (Potts and Harwood 2003). Wider lane and shoulder widths are associated with a Passing activity on 2+1 roads was greatest at the beginning reduction in segment-related collisions on rural front- of the segments and the greatest benefits of decreased pla- age road segments (Lord and Bonneson 2007). tooning and increased safety occurred within the first 0.9 mi of a passing lane segment (Gattis et al. 2006). Rumble Strips Most passing on Super 2 passing lanes occurs within the first mile of a passing lane, so additional length may be Crashes at approximately 210 mi of undivided rural less useful than additional lanes in a Super 2 corridor, two-lane roads treated with CLRS were reduced by particularly at lower volumes. Designers should avoid 14% and injury crashes by an estimated 15%. All fron- intersections with state highways and high-volume tal and opposing-direction sideswipe crashes were county roads within passing lanes, consider terrain and reduced by an estimated 21%, and those crashes involv- right-of-way in determining alignment and placement ing injuries by an estimated 25%. All of the reductions of passing lanes, avoid the termination of passing lanes were determined to be statistically significant (Persaud on uphill grades, and discourage passing lane lengths et al. 2003). longer than 4 mi (Brewer et al. 2011). Crash data on roads treated with CLRS or shoulder TWLTLs could be used as a strategy to reduce head-on rumble strips revealed noticeable crash reductions on collisions on two-lane roads (Neuman et al. 2003b). all classes of roads (rural and urban two-lane roads and freeways). Shoulder rumble strips should be placed as Lane Width close to the edgeline as possible to maximize safety benefits. The safety benefits of CLRS for roadways on Researchers investigating the relationship between lane horizontal curves and on tangent sections are for practi- width and safety on urban and suburban arterials found cal purposes the same (Torbic et al. 2009). no general indication that the use of lanes narrower than 12 ft on urban and suburban arterials increased crash frequencies. They suggested that geometric design Shoulder Edge Treatments policies should provide substantial flexibility for use of lane widths narrower than 12 ft (Potts et al. 2007b). Plaxico et al. (2005) made the following recommenda- Lane widths of 11 or 12 ft provide optimal safety ben- tions on design guidelines for using curbs on roadways efit for common values of total paved width on rural with operating speeds greater than 60 km/h (37.3 mph): two-lane roads. Although 12-ft lanes appear to be the Any combination of a sloping-faced curb that is optimal design for 26- to 32-ft total paved widths, 11-ft 150 mm (6 in.) or shorter and a strong-post guardrail lanes perform equally well or better than 12-ft lanes for can be used where the curb is flush with the face of 34- to 36-ft total paved widths (Gross et al. 2009). the guardrail up to an operating speed of 85 km/h. Guardrails installed behind curbs are not to be located closer than 2.5 m (8.2 ft) for any operating Road Diet speed in excess of 60 km/h (37.3 mph). For roadways with operating speeds of 70 km/h Road diet crashes occurring during the period after instal- (43.5 mph) or less, guardrails may be used with lation were about 6% lower than that of matched compar- ison sites. However, controlling for possible differential sloping-face curbs no taller than 150 mm (6 in.) as changes in ADT, study period, and other factors indicated long as the face of the guardrail is located at least no significant effect of the treatment. Crash severity was 2.5 m (8.2 ft) behind the curb. virtually the same at road diets and comparison sites. Where guardrails are installed behind curbs on roads Conversion to a road diet should be made on a case-by- with operating speeds between 71 and 85 km/h (44.1 case basis in which traffic flow, vehicle capacity, and and 52.8 mph), a lateral distance of at least 4 m (13.1 ft) safety are all considered (Huang et al. 2002). is needed to allow the vehicle suspension to return to The effects of the road diet on crashes in Iowa, account- its pre-departure position. ing for monthly crash data and estimated volumes for At operating speeds greater than 85 km/h (52.8 mph), treatment and comparison sites, resulted in a 25.2% guardrails are only to be used with 100-mm (4-in.) or reduction in crash frequency per mile and an 18.8% shorter sloping-faced curbs, and be placed so that the reduction in crash rate (Pawlovich et al. 2006). curb is flush with the face of the guardrail. Operating

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35 speeds above 90 km/h (55.9 mph) require that the lateral offset for this 20-ft section is to be consistent sloping face of the curb must be 1:3 or flatter and with the lane width, typically 12 ft (Dixon et al. 2008). must be no more than 100 mm (4 in.) in height. A lateral offset of 6 ft from the curb face to rigid objects is The "Safety Edge" treatment produced small but posi- preferred for higher-speed auxiliary lane locations, such tive results in crash reduction at 56 of 81 treated sites. as extended length right-turn lanes, and a 4-ft minimum For all two-lane highway study sites in two states, the lateral offset is to be maintained (Dixon et al. 2008). best estimate of the treatment's effectiveness was a At locations where a sidewalk buffer is present, rigid reduction in total crashes of approximately 5.7%. The objects are not to be located in a buffer area with a results were not statistically significant, but they were width of 3 ft or less. For buffer widths greater than 3 ft, generally positive (Hallmark et al. 2006). lateral offsets from the curb face to rigid objects must be maintained with a minimum offset of 4 ft. At these Roadside wider buffer locations, other frangible objects can be strategically located to help shield any rigid objects Where possible at curb locations, provide a lateral (Dixon et al. 2008). offset to rigid objects of at least 6 ft from the face of Rigid objects are not to be located in the proximity of the curb and maintain a minimum lateral offset of 4 ft driveways, and care should be taken to avoid placing (Dixon et al. 2008). rigid objects on the immediate far side of a driveway. In At lane merge locations, do not place rigid objects in an addition, objects should not be located within the required area that is 10 ft longitudinally from the taper point. The sight triangle for a driveway (Dixon et al. 2008).