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80 Description Vertical geometry elements of a driveway include the change in grade from the roadway cross slope to the slope of the driveway apron (grade breaks), the minimum and maximum grade of the driveway, vertical curve design criteria, and vertical clearance to overhead obstacles (e.g., utility lines) (1, 2). Tables 58 through 60 follow. Quantitative Analysis Methods Motor Vehicle Operations NCHRP Web-Only Document 151: Geometric Design of Driveways (3) documents a study comparing speeds of vehicles entering driveways with differing grades. The driveways studied had reasonably similar characteristics (e.g., throat width, curb radius, roadway speed, or right-hand travel lane width) other than the vertical grade. The driveways were divided into three groups by grade: flatter (1.5% to 5.0% grades), moderate (6.0% to 9.0% grades), and steeper (12.5% to 15.5% grades). Vehicle speeds observed prior to entering the driveway were similar for all three groups for right turns, but vehicle speeds were slightly lower at the driveway threshold and beyond the threshold for both right and left turns, as shown in Table 60. C H A P T E R 1 8 Driveway Vertical Geometry Source: Photograph provided by the authors.
Driveway Vertical Geometry 81 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Improve driveway vertical geometry. â Â â Â Â Â â Â â Â Â Table 58. Multimodal operations and safety performance summary. Mode Operations Safety Motor vehicles enter the driveway slightly faster (i.e., at a normal entering speed) (3). Reduces vehicle and roadway damage from scraping caused by too-severe changes in grade rate (2). Allows vehicles to enter and exit the roadway at the design speed produced by the horizontal geometry, thereby reducing the speed differential of vehicles on the roadway (4, 5). Improves sight distance between the egressing vehicle and the approaching vehicle. Where the sidewalk is adjacent to the curb, helps achieve conformance with Americans with Disabilities Act requirements for the maximum cross-slope of the pedestrian access route (5). Vehicle speeds increase slightly, but potential vehicleâ pedestrian conflicts still take place at low speeds (3). Improves sight distance between the egressing vehicle and the approaching pedestrian. No documented effect. Removes the potential for abrupt changes in pavement cross slope or elevation (e.g., bumps) that could cause a bicyclist to lose balance or control (3). Vehicle speeds increase slightly, but potential vehicleâbicycle conflicts still take place at low speeds. Improves sight distance between the egressing vehicle and the approaching cyclist. No documented effect beyond that generally observed for motor vehicle traffic. No documented effect beyond that generally observed for motor vehicle traffic. Improves sight distance between the egressing vehicle and the approaching bus. No documented effect beyond that generally observed for motor vehicle traffic. No documented effect beyond that generally observed for motor vehicle traffic. Improves sight distance between the egressing vehicle and the approaching truck. Table 59. General trends associated with improving driveway vertical geometry. Average Speed (mph) by Location and Type of Turn Driveway Grade Prior to Driveway Entering Driveway 15 Feet into Driveway Right Turn Left Turn Right Turn Left Turn Right Turn Left Turn Flatter 14.1 10.3 5.5 10.5 7.2 10.0 Moderate 14.7 10.0 5.8 10.2 7.2 9.5 Steeper 14.5 9.6 5.1 8.7 5.9 8.1 Note: Prior to driveway = 25 feet prior for right turns and 1 lane width prior for left turns; entering driveway = 2 feet beyond driveway threshold. Grade categories: flatter = 1.5%â5.0%, moderate = 6.0%â9.0%, and steeper = 12.5%â15.5%. Source: NCHRP Web-Only Document 151 (3). Table 60. Comparisons of speeds of vehicles entering driveways with differing grades.
82 Guide for the Analysis of Multimodal Corridor Access Management Motor Vehicle Safety NCHRP Report 659 (5) recommends a maximum grade break without a vertical curve of 10% for crests and 9% for sags and as low as 7% for driveways where vehicles towing trailers would be common, based on measurements of 31 driveways with scrape marks. When a driveway slopes downward from the roadway edge [e.g., a downward-sloping drive- way on the outside (or âhighâ side) of a superelevated roadway], sight distance for the motorist exiting the driveway may be restricted because the driveway is pointing upward and the vehicleâs structure restricts the motoristâs view of traffic (i.e., pedestrian, bicycle, transit, and truck) on the roadway (6). Reducing the driveway slope or providing a flatter landing area may improve sight distance. Additional Information â¢ Chapters 17 and 19 in this guide. â¢ Access Management Manual, Second ed.: Section 13.7.7. â¢ Access Management Application Guidelines: Chapter 10, Driveway Design and Geometrics. â¢ NCHRP Report 659: Guide for the Geometric Design of Driveways. References 1. Dixon, K. K., R. D. Layton, M. Butorac, P. Ryus, J. L. Gattis, L. Brown, and D. Huntington. Access Management Application Guidelines. Transportation Research Board, Washington, D.C., 2016. 2. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 3. Gattis, J. L., J. S. Gluck, J. M. Barlow, R. W. Eck, W. F. Hecker, and H. S. Levinson. NCHRP Web-Only Document 151: Geometric Design of Driveways. Transportation Research Board of the National Academies, Washington, D.C., July 2009. 4. Layton, R., G. Hodgson, and K. Hunter-Zaworski. Pedestrian and Bicyclist Impacts of Access Management. Proceedings of the Third National Access Management Conference, Fort Lauderdale, Fla., 1998. 5. Gattis, J. L., J. S. Gluck, J. M. Barlow, R. W. Eck, W. F. Hecker, and H. S. Levinson. NCHRP Report 659: Guide for the Geometric Design of Driveways. Transportation Research Board of the National Academies, Washington, D.C., 2010. 6. Florida Department of Transportation. Driveway Information Guide. Florida Department of Transportation, Tallahassee, Sept. 26, 2008.