Click for next page ( 19


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 18
CHAPTER 4 Driveway Location and Spacing For many decades, knowledgeable transportation professionals have recognized the need to manage access along roadways to preserve safety and mobility (see Exhibit 4-1 for an example). In practice, this includes regulating the number of, location of, spacing between, and geometric design of driveways. Several access management guidelines have been developed to assist agencies in balancing the competing needs for mobility along the roadways and access to abutting land developments. One of the most complete sources of information is the Access Management Manual (4-1). Other salient guidelines are contained in NCHRP Report 348: Access Management Guidelines for Activity Centers (4-2), NCHRP Report 420: Impacts of Access Management Techniques (4-3), Transportation and Land Development (4-4), and A Policy on Geometric Design of Highways and Streets, the AASHTO Green Book (4-5). Given that access management is addressed by other publications, this design guide will only briefly discuss the topic. For more information, refer to access management publications and websites. General Guidelines Although private property enjoys the right of access to the general system of public roadways, this is not an unlimited right. The right of access must be balanced with the needs of and poten- tial harm to the general traveling public. To preserve mobility and provide safety for the travel- ing public, many transportation agencies have established regulations and programs to manage access to their roadway network. The regulations are more restrictive for major arterials, the roadways intended to accommodate higher volumes and speeds; however, some objectives and practices apply to most driveways. Access management programs restrict the number of driveways allowed. These practices affect when and where direct driveway access will be allowed onto the roadway network, whether alter- native access should be provided, and the need for shared access. If direct access is allowed, the guidance includes the extent of that access (i.e., right-in and right-out versus full movement) and circumstances in which multiple driveways are allowed. In addition, agencies may require that steps be taken to mitigate projected traffic operations and/or safety impacts. An example of mit- igation would be providing an auxiliary lane to remove driveway turning traffic from the through traffic lanes on an arterial. As noted in the AASHTO Green Book (4-5, p.729), driveways should not be located within the functional area of an intersection or in the influence area of an adjacent driveway. The func- 18

OCR for page 18
Driveway Location and Spacing 19 tional area extends both upstream and downstream from the physical intersection area and Exhibit 4-1. Experts have includes the longitudinal limits of auxiliary lanes. As a result, the functional area encom- long recognized the deficient passes the area where motorists are responding to the intersection, decelerating, and state of the practice. maneuvering into the appropriate lane to stop or complete a turn. The AASHTO Green Lack of access control along arterial Book also notes that a driveway influence area includes the following: highways is the largest single factor resulting in functional obsolescence of Impact length (the distance back from a driveway that cars begin to be affected by drive- highway facilities. Frequent way traffic), driveways and curb cuts increase Perception-reaction distance, and points of conflict and potential accident locations.... Vehicle length. Few cities in the United States and Canada exercise effective access Additional guidance related to computing driveway influence areas is available in control along arterial streets ... NCHRP Report 420: Impacts of Access Management Techniques (4-3, pp.4862). restrictions on driveway location and spacing are frequently minimal and the Another general guideline that applies to driveway location is that sight distance must criteria are loose. Marks, H. Traffic Circulation Planning for be sufficient. The AASHTO Green Book (4-5, pp.110155 and 651677) contains detailed Communities, Gruen Associates, Los Angeles, CA (1974) p. 232. guidance on the purpose and computation of sight distance. In addition, driveways must be located so that they are conspicuous and clearly delineated for the various users. One major objective is to avoid driveway queuing that backs up into a public roadway. This is accomplished through design of the throat length, internal circulation, and traffic control within a site. Queuing of traffic exiting a site does not affect the operation of the public roadway, but could affect site circulation and parking lot operations. This internal queuing is affected by the throat length, number of egress lanes, and traffic control at the public roadway intersection. Exhibit 4-2 illustrates the confusion and potential for crashes when vehicles slow, change lanes, and try to enter or exit driveways that are too close to each other. Exhibit 4-3 clearly shows the increased potential for traffic conflicts when driveways are too close to the intersection of two public roadways. Exhibit 4-4 shows a vehicle conflict resulting from a driveway too close to the exit ramp off of a freeway. General guidelines often applied by agencies deciding whether to allow or deny access follow: Along the main roadways, limit the number of access points. Encourage property access from secondary roads and streets or "backage" roads. One carefully located and well-designed driveway per site is often adequate. Exhibit 4-2. Driveways too close to each other allow more conflicts to occur. (a) (b)