National Academies Press: OpenBook

Guide for the Geometric Design of Driveways (2010)

Chapter: Chapter 2 - Terms and Definitions

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Suggested Citation:"Chapter 2 - Terms and Definitions." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Suggested Citation:"Chapter 2 - Terms and Definitions." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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Page 5
Page 6
Suggested Citation:"Chapter 2 - Terms and Definitions." National Academies of Sciences, Engineering, and Medicine. 2010. Guide for the Geometric Design of Driveways. Washington, DC: The National Academies Press. doi: 10.17226/14399.
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This chapter presents terms and definitions used in this report. Exhibit 2-1 illustrates the loca- tion of some of the named driveway design elements. AASHO – American Association of State Highway Officials AASHTO – American Association of State Highway and Transportation Officials ADA – Americans with Disabilities Act of 1990 Blended transition – A connection with a grade of 5% or less between the level of the pedestrian walkway and the level of the crosswalk (2-1). Breakover angle – The algebraic difference between two successive grades. CBD – Central business district: the established “downtown” core or center of a city that tradi- tionally included government, office, and retail activities. Commercial driveway – Driveways that serve uses such as offices, retail, or services. Connection – The junction of the subject roadway with a source of traffic from the side (e.g., a driveway, roadway, or ramp). Contrast – A marked difference between dark and light. With regard to ADA contrast for detectable warnings, the ADA Standards state the following in the advisory appendix section. A4.29.2 Detectable Warnings on Walking Surfaces. The material used to provide contrast should contrast by at least 70%. Contrast in percent is determined by: contrast = [(B1 – B2)/B1] × 100 where B1 = light reflectance value (LRV) of the lighter area and B2 = light reflectance value (LRV) of the darker area. Note that in any application both white and black are never absolute; thus, B1 never equals 100 and B2 is always greater than 0. Cross slope – The slope (or grade) perpendicular to the direction of travel. On a sidewalk or blended transition, it is measured perpendicular to the curb line or roadway edge. On a curb ramp, it is measured perpendicular to the longitudinal or running grade. Driveway triangular island (“pork chop”) – Roadway or driveway channelization in the form of a somewhat-triangular island. Dust pan –A driveway entry or exit shape with the plan view designed with a flared or tapered edge. With this design, the curb height along the roadway edge transitions from full height to no curb height. Thus, the design incorporates a taper in both the plan and in the front elevation views. Front overhang – The distance from the center of the front-most wheel to the front end of the vehicle. Functional area of intersection – The area that includes not only the physical area where roadways cross each other, but also the areas upstream and downstream of the physical intersection, where driver reaction, deceleration, queuing, and acceleration occur that are related to the operation of the intersection. Ground clearance – The distance from the bottom of a vehicle body to the ground. 4 C H A P T E R 2 Terms and Definitions

Terms and Definitions 5 Hang-up – When the underside of a vehicle comes into contact with the roadway surface, at grade breaks in the vertical profile, such that the vehicle is immobilized or stuck on the vertical geometry. Also referred to as lodged or high-centered. Interface – The broader area where a driveway joins the roadway, including the curved or flared turning areas. ITE – Institute of Transportation Engineers MUTCD – Manual on Uniform Traffic Control Devices. The federal MUTCD is incorporated by reference in 23 Code of Federal Regulations (CFR), Part 655, Subpart F. It is recognized as the national standard for all traffic control devices installed on any street, highway, or bicycle trail open to public travel in accordance with 23 U.S.C. 109(d) and 402(a). NCHRP – National Cooperative Highway Research Program Non-restrictive median – A median designed to be easily crossed by a motor vehicle, such as a two-way left-turn lane (TWLTL). Offset – The meaning of this term depends on the context. In the context of a driveway connec- tion transition, it can refer to the situation where due to the presence of on-street parking, a bicycle lane, a shoulder, or similar space generally parallel to and outside of the traveled way, the physical end of a driveway is some distance away from the edge of the traveled way. The effect of this is that part of the turning movement of those vehicles entering or exiting the driveway takes place in that area between the edge of the traveled way and the physical end of the driveway. Exhibit 2-1. Some driveway design elements.

PAR – Pedestrian Access Route Pedestrian Access Route – A continuous and unobstructed walkway within a pedestrian circu- lation path that provides accessibility. Ped – Pedestrian. From the 2003 MUTCD, Section 1A13.55: “a person afoot, in a wheelchair, on skates, or on a skateboard” (2-2). Pork chop (driveway triangular island) – Roadway or driveway channelization in the form of a somewhat-triangular island. P-vehicle – The passenger car design vehicle as defined by AASHTO. Also includes minivans, pick-up trucks, sport utility vehicles (SUVs), and standard size vans. Rear overhang – The distance from the centerline of the rearmost axle to the rear end of the vehicle. Restrictive median – A median, such as a raised or depressed median, designed not to be crossed by a motor vehicle except at selected locations. RV – Recreational vehicle (e.g., a motor home). Spillback – When a situation exists such that the traffic conditions at the subject driveway influence or affect the operation of vehicles in the outside through lane at or in advance of the driveway upstream of the subject driveway. TCD – Traffic control devices, including signs, pavement markings, and traffic signals Threshold – The edge, dividing line, or boundary where the driveway meets the public roadway. In many cases, this is a line along the curb edge. Throat length – The distance from the outer edge of the traveled way of the intersecting roadway to the first point along the driveway at which there are conflicting vehicular traffic movements. Also referred to as the driveway connection depth, driveway reservoir length, driveway stack- ing distance, driveway storage length. Traveled way – The portion of the roadway for movement of vehicles, exclusive of shoulders (2-3). TRB – Transportation Research Board Wheelbase – The distance between the centers of two axles or wheels. Sometimes shown as the length from the front axle to the rear axle. References 2-1. “R105.5 Defined Terms” in Revised Draft Guidelines for Accessible Public Rights-of-Way. http://www.access- board.gov/prowac/draft.htm#105 (as of Nov. 23, 2005). 2-2. FHWA. Manual on Uniform Traffic Control Devices. Washington, DC (2003) 760 pp. 2-3. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) p. 305. 6 Guide for the Geometric Design of Driveways

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 659: Guide for the Geometric Design of Driveways explores guidelines related to the geometric design of driveways. The report includes driveway-related terms and definitions, an examination of basic geometric controls, a summary of access spacing principles, and detailed discussions of various geometric design elements.

Material related to and supporting the contents of NCHRP Report 659, including an extensive review of literature, has been published as NCHRP Web-Only Document 151: Geometric Design of Driveways.

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