The National Academies Press

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From page 20... ... Empirical Bayes analysis of crash data showed that there was a statistically significant crash reduction of 35% for segment-only (i.e., nonintersection) injury crashes on the study corridors, as compared with the expected number of crashes without passing lanes. Read the entire page →
From page 21... ... A chapter in their Managed Lanes Handbook contains recommendations and guidelines for design elements of freeway managed lanes. In general, they recommended that the features of the managed lane be commensurate with the design vehicle that is selected to be appropriate for the facility. Read the entire page →
From page 22... ... Minimum (%) 4.0 0.5 5.0 0.3 4.0 0.5 5.0 0.3 Clearance Vertical Lateral 16.5 ft 4 ft 14.5 ft 2 ft 5 m 1.2 m 4.4 m 0.6 m Lane Width Travel lanes 12 ft 11 ft 3.6 m 3.4 m Cross Slope Maximum Minimum 0.020 ft/ft 0.015 ft/ft 0.020 ft/ft 0.015 ft/ft 0.020 m/m 0.015 m/m 0.020 m/m 0.015 m/m Superelevation: Dependent on curve radii and design speed [0.10 ft/ft (0.10 m/m) Read the entire page →
From page 23... ... Individual state analyses did not indicate a clear preference for lane or shoulder width given a fixed paved width, but combined with findings from previous research, researchers described some potential trends: • For 26- to 32-ft total paved widths, 12-ft lanes provided the optimal safety benefit. The CMF ranged from 0.94 to 0.97, indicating a 3% to 6% crash reduction for 12-ft lanes compared with 10-ft lanes. Read the entire page →
From page 24... ... Crash severity was virtually the same at road diets and comparison sites, whereas there were some differences in crash type distributions between road diets and comparison sites, they found none between the "before" and "after" periods. They concluded that conversion to a road diet should be made on a case-by-case basis in which traffic flow, vehicle capacity, and safety are all considered. Read the entire page →
From page 25... ... Wider shoulders were found to increase roadside and TREATMENT: Convert Undivided Four-Lane Road to Three-Lane and TWLTL (Road Diet) CMF Level of Predictive Certainty: High METHODOLOGY: Empirical Bayes Before–After Crash Type Studied and Estimated Effect REFERENCE: NCHRP Project 17-25 research results State/Site Characteristics CrashType Number of Treated Sites CMF (std. Read the entire page →
From page 26... ... 2The average shoulder width for undivided highways is the average of the right shoulders; for divided, it is the average of left and right shoulder in the same direction. Read the entire page →
From page 27... ... Operational and safety treatments Rumble Strips Multiple studies have examined the effects of both shoulder and centerline rumble strips (CLRS) Read the entire page →
From page 28... ... . TABLE 13 RECOMMENDED CRASH MODIFICATION FACTOR FOR SHOULDER RUMBLE STRIPS TREATMENT: Add Centerline Rumble Strips CMF Level of Predictive Certainty: Medium-High METHODOLOGY: Empirical Bayes Before-After CRASH TYPE STUDIED AND ESTIMATED EFFECTS REFERENCE: Persaud et al. Read the entire page →
From page 29... ... They also stated that the safety benefits of CLRS for roadways on horizontal curves and on tangent sections are for practical purposes the same. With regard to rumble strip design, researchers concluded that shoulder rumble strip patterns for freeways and other roadways where bicyclists are not expected be designed to produce sound level differences between 10 to 15 dBA in the passenger compartment; for other roadways, the recommended sound level difference was 6 to 12 dBA. Read the entire page →
From page 30... ... They state that adoption of a work zone design speed may be appropriate for the evaluation of superelevation and sight distance. Because the shoulders will be part of a permanent high-speed roadway, no horizontal or vertical alignment decisions are generally needed. Read the entire page →
From page 31... ... They concluded that results were significantly different for the various classes of crash types, indicating that reducing the median width without adding barriers (even if the remaining median width is still reasonably wide) increases the severity of crashes, particularly opposite-direction crashes. Read the entire page →
From page 32... ... • Although many auxiliary lanes, such as bus lanes or bicycle lanes, have low volumes and may be included as part of a clear zone in the urban environment, higherspeed auxiliary lane locations, such as extended length right-turn lanes, are common locations for run-off-road crashes. A lateral offset of 6 ft from the curb face to rigid objects is preferred, and a 4-ft minimum lateral offset should be maintained. Read the entire page →
From page 33... ... Designers should avoid intersections with state highways and high-volume county roads within passing lanes, consider terrain and right-of-way in determining alignment and placement of passing lanes, avoid the termination of passing lanes on uphill grades, and discourage passing lane lengths longer than 4 mi (Brewer et al. Read the entire page →
From page 34... ... . roadside • Where possible at curb locations, provide a lateral offset to rigid objects of at least 6 ft from the face of the curb and maintain a minimum lateral offset of 4 ft (Dixon et al. Read the entire page →

From page 20...

... Empirical Bayes analysis of crash data showed that there was a statistically significant crash reduction of 35% for segment-only (i.e., nonintersection) injury crashes on the study corridors, as compared with the expected number of crashes without passing lanes.