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31 For roadways with operating speeds of 70 km/h (43.5 mph) or less, guardrails may be used with sloping- Asphalt Overlay face curbs no taller than 150 mm (6 in.) as long as the face of the guardrail is located at least 2.5 m (8.2 ft) behind the curb. In cases where guardrails are installed behind curbs on roads with operating speeds between 71 and 85 km/h (44.1 and 52.8 mph), a lateral distance of at least 4 m Existing Unpaved (13.1 ft) is needed to allow the vehicle suspension to Shoulder return to its pre-departure position. Once the suspen- sion and bumper have returned to their normal position, Existing Pavement For a 30 Safety Edge, = 30 impacts with the barrier would proceed successfully. At these speeds, guardrails may be used with sloping- FIGURE 5 Illustration of FHWA's Safety Edge Treatment (Hallmark et al. 2006). face curbs of 100 mm (4 in.) in height or less as long as the face of the guardrail is located at least 4 m (13.1 ft) behind the curb. asphalt material needs to be compacted to increase strength; At operating speeds greater than 85 km/h (52.8 mph), otherwise the material will break apart over time owing to guardrails are to only be used with 100-mm (4-in.) or forces of overrunning vehicles and runoff water. A specific shorter sloping-faced curbs, and they would be placed application of this treatment, called "Safety Edge" (shown so that the curb is flush with the face of the guardrail. in Figure 5), is being developed by FHWA, as discussed by Operating speeds above 90 km/h (55.9 mph) require Hallmark et al. (2006). An evaluation of this treatment by that the sloping face of the curb must be 1:3 or flatter Graham et al. (2010) indicated small but positive results in and must be no more than 100 mm (4 in.) in height. crash reduction at 56 of 81 treated sites. Their results indi- Curbs are to only be used on higher-speed roadways cated that for all two-lane highway study sites in two states, when concerns about drainage make them essential to the best estimate of the treatment's effectiveness was a reduc- the proper maintenance of the highway. tion in total crashes of approximately 5.7%. The results were not statistically significant, but they were generally positive. NCHRP Report 600C (Campbell et al. 2010) discusses the potential safety ramifications of shoulder edge drop-offs, which typically arise from tire rutting erosion, excessive wear, Work Zone Considerations or resurfacing. Guidelines for treating these locations are offered for purposes of design practices. The report authors There are a number of ways in which shoulders may be used cited a previous study by Graham and Glennon (1984), which under work zone traffic control conditions. The authors of stated that vertical or near-vertical shoulder drop-off heights in NCHRP Report 581 (Mahoney et al. 2007) discuss some con- work zones that exceeded the indicated values in Table 15 war- siderations for designers in the use of shoulders in work zones rant consideration for drop-off treatment or traffic control. The on high-speed roadways. They state that adoption of a work original source table also contained drop-off height thresholds zone design speed may be appropriate for the evaluation of that were greater than 3 in.; however, these were changed in superelevation and sight distance. Because the shoulders will the NCHRP report to reflect a more conservative assessment of be part of a permanent high-speed roadway, no horizontal or other related driver performance data on driver encounters with vertical alignment decisions are generally needed. Temporary drop-offs of various heights (Hallmark et al. 2006). work zone features can affect sight distance, and it was rec- ommended that the design be developed and evaluated from One potential treatment is a wedge-shaped application of that perspective. If the shoulder being used to carry traffic is asphalt; when placed between the roadway and the shoulder, on a horizontal curve, the magnitude and direction of its cross the material can help drivers recover from the shoulder to slope would be compared with the superelevation require- the driving surface. NCHRP Report 600C advises that the ment, and the agency's typical work zone policy for super- elevation would be applied. They added that the designer Table 15 is to also consider the adequacy of the shoulder in terms of Vertical Drop-Off Height Warranting Traffic structure (ability to carry the vehicle loads) and surface con- Control for Various Lane Widths ditions (friction and smoothness), with particular attention to Drop-Off Height (inches) the presence and placement of shoulder rumble strips. Speed for a Lane Width of (mph) 12 ft 11 ft 10 ft 9 ft 30 3 3 3 2 35 3 3 2 1 Medians 40 3 2 1 1 45 2 1 1 1 NCHRP Report 633 (Stamatiadis et al. 2009) presented rec- >50 1 1 1 1 ommendations for CMFs for shoulder and median width Adapted from Graham and Glennon (1984). for four-lane roads with 12-ft lanes. The authors made the