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76 Description The driveway throat width, effective curb return radius, and angle of intersection with the roadway act in combination to provide safe and efficient operations at the point where a drive- way intersects a roadway (1). Throat width is the normal driveway width, not including any widening associated with a radius or flare (2). Effective curb return radius considers both the physical construction of the curb and other roadway elements (e.g., a bicycle lane) that affect how vehicles make turning maneuvers (1). Figure 1 and Tables 55 through 57 follow. Quantitative Analysis Methods NCHRP Report 659 (2) provides guidance on throat widths, curb return radii, and driveway skew angles that, in combination, help provide safe and efficient driveway operations. Methods in the HCM6 (4) can be used to evaluate the capacity and queue storage benefits of adding one or more lanes to the driveway exit. A study in South Carolina (5) found that increasing the driveway width increases the crash rate at the driveway. The same study found that increasing the number of driveway entry lanes from 1 to 2 decreases the crash rate. An FHWA synthesis from the 1980s developed the following relationships between driveway width, curb return radius, and vehicle speed (1, 6), as shown in Figure 1. C H A P T E R 1 7 Driveway Width Source: Photograph provided by the authors.
Driveway Width 77 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Regulate maximum driveway width. Â â Â Â Â â â â Â Â Widen driveway to improve storage. â Â Â Â Â Â â â Â Â Â Install additional driveway exit lane. â Â Â Â Â Â â â Â Â Â Increase drivewayâs effective approach width. â Â Â Â â â â â Â â Install barrier to prevent uncontrolled access along property frontage. Â â Â Â Â â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Table 55. Multimodal operations and safety performance summary. Source: Access Management Manual, Second ed. (1), adapted from Hagenaur et al. (6). Figure 1. Relationships between driveway throat width, curb return radius, and average vehicle speed.
78 Guide for the Analysis of Multimodal Corridor Access Management Mode Operations Safety No documented effect. Vehicles enter and exit at random locations and are more likely to cross paths (2). Provides an opportunity to install a median in the driveway that can act as a pedestrian refuge (3), reducing pedestrian delay, and can help orient pedestrians with impaired vision (2). Providing a barrier between the sidewalk and roadway to control access would also improve pedestrian LOS (2). Increases the time and distance that pedestrians are exposed to conflicting traffic (2). May be more likely to seriously disorient a pedestrian with impaired vision (2). Increases average vehicle speeds entering the driveway (1). No documented effect. Increases the time and distance that bicyclists are exposed to conflicting traffic (2). Increases average vehicle speeds entering the driveway (1). No documented effect. No documented effect beyond that generally observed for motor vehicle traffic. No documented effect. No documented effect beyond that generally observed for motor vehicle traffic. Table 57. General trends associated with excessively wide driveways. Mode Operations Safety Vehicles may need to turn more slowly than normal into the driveway, which increases the potential for delaying following vehicles (2). At higher-volume driveways, the number of exiting lanes may provide insufficient capacity to serve demand (2). Increases the chances of vehicles encroaching on the curb, sidewalk, or exiting driveway lane (2). Larger entering vehicles may need to wait in the street until exiting vehicles have cleared (1). No documented effect. Increases the chances of vehicles encroaching on the curb or sidewalk (2). The existence of a bicycle lane provides a larger effective curb radius and may allow a smaller physical curb radius, although some agencies prefer not to do so in case the roadway cross- section is repurposed in the future, leaving an inadequate radius (2). Larger entering vehicles may need to wait in the street until exiting vehicles have cleared (1), which may force bicycles to go around them or wait. No documented effect. No documented effect beyond that generally observed for motor vehicle traffic. Trucks and other large vehicles (e.g., recreational vehicles) may not be able to enter until exiting vehicles have cleared the driveway (1). Trucks may need to encroach on the curb, sidewalk, or opposing driveway lane when entering or exiting (2). Larger entering vehicles may need to wait in the street until exiting vehicles have cleared (1). Table 56. General trends associated with inadequate throat widths or curb radii.
Driveway Width 79 Additional Information ⢠Chapters 11, 15, and 19 in this guide. ⢠Access Management Manual, Second ed.: Section 13.7. ⢠Access Management Application Guidelines: Chapter 10, Driveway Design and Geometrics. ⢠NCHRP Report 659: Guide for the Geometric Design of Driveways. References 1. 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. 2. 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. 3. 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. 4. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 5. Stokes, A., W. A. Sarasua, N. Huynh, K. Brown, J. H. Ogle, A. Mammadrahimli, W. J. Davis, and M. Chowdhury. Safety Analysis of Driveway Characteristics along Major Urban Arterial Corridors in South Carolina. Presented at 95th Annual Meeting of the Transportation Research Board, Washington, D.C., Jan. 2016. 6. Hagenaur, G. F., J. Upchurch, D. Warren, and M. J. Rosenbaum. Synthesis of Safety Research Related to Traffic Control and Roadway Elements. Volume 1, Intersections. Report No. FHWA-TS-82-232, Federal Highway Administration, Washington, D.C., 1982.
