National Academies Press: OpenBook

Guide for the Geometric Design of Driveways (2010)

Chapter: Chapter 4 - Driveway Location and Spacing

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Suggested Citation:"Chapter 4 - Driveway Location and Spacing." 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 4 - Driveway Location and Spacing." 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 19
Page 20
Suggested Citation:"Chapter 4 - Driveway Location and Spacing." 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.
×
Page 20
Page 21
Suggested Citation:"Chapter 4 - Driveway Location and Spacing." 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.
×
Page 21
Page 22
Suggested Citation:"Chapter 4 - Driveway Location and Spacing." 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.
×
Page 22
Page 23
Suggested Citation:"Chapter 4 - Driveway Location and Spacing." 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.
×
Page 23

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18 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- C H A P T E R 4 Driveway Location and Spacing

Driveway Location and Spacing 19 tional area extends both upstream and downstream from the physical intersection area and includes the longitudinal limits of auxiliary lanes. As a result, the functional area encom- passes the area where motorists are responding to the intersection, decelerating, and maneuvering into the appropriate lane to stop or complete a turn. The AASHTO Green Book also notes that a driveway influence area includes the following: • Impact length (the distance back from a driveway that cars begin to be affected by drive- way traffic), • Perception-reaction distance, and • Vehicle length. Additional guidance related to computing driveway influence areas is available in NCHRP Report 420: Impacts of Access Management Techniques (4-3, pp.48–62). Another general guideline that applies to driveway location is that sight distance must be sufficient. The AASHTO Green Book (4-5, pp.110–155 and 651–677) contains detailed 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. Lack of access control along arterial highways is the largest single factor resulting in functional obsolescence of highway facilities. Frequent driveways and curb cuts increase points of conflict and potential accident locations…. Few cities in the United States and Canada exercise effective access control along arterial streets … restrictions on driveway location and spacing are frequently minimal and the criteria are loose. Marks, H. Traffic Circulation Planning for Communities, Gruen Associates, Los Angeles, CA (1974) p. 232. Exhibit 4-1. Experts have long recognized the deficient state of the practice. (a) (b) Exhibit 4-2. Driveways too close to each other allow more conflicts to occur.

Exhibit 4-4. Driveways too close to exit ramp terminals allow more conflicts to occur. 20 Guide for the Geometric Design of Driveways Exhibit 4-3. Driveways too close to roadway intersections allow more conflicts to occur. • Where two lower-volume sites are adjacent, access to both can be provided by a single shared driveway. When access from the major roadway is required, sharing access with adjacent tracts reduces the overall number of connections to the major roadway. Shared access arrangement should be implemented by an appropriate joint easement. • For higher volume sites, additional access points may be needed. The assessment of this need must consider (1) whether or not good site planning principles have been applied and (2) the traffic safety and operational effects of the additional access. • Along major roadways, the left-turn exit movement from driveways should be kept to a min- imum. If a roadway is converted from undivided to divided, left-turn access may be closed in one or both directions. Where physically practical, direct left-turns can be replaced by right turns followed by u-turn movements. When access is not available from parallel or cross streets, or across adjacent tracts, it may be necessary to provide property access from the major roadway. This access often should be lim- ited to right turns only. However, in some situations, limiting access to only right turns will result in left-turning movements migrating to and overloading a nearby intersection—in such cases, it may be better to allow left-turn movements at the subject access point. An assessment may be needed as to which arrangement helps preserve the functionality of the roadway and the mobil- ity of the traffic. Driveway Location and Spacing Experience has shown that certain driveway locations tend to be problematic, and that for bet- ter safety and mobility, the frequency of driveways should be minimized. This section discusses the following four types of driveway spacing: • Spacing between unsignalized connections; • Spacing of driveways from signalized intersections (corner clearance); • Spacing for a signalized driveway; and • Spacing of a driveway from an interchange ramp.

Spacing Between Unsignalized Connections Spacing between unsignalized connections (whether between two driveways or a driveway and a roadway) should not interfere with safe and relatively unimpeded movement on the through roadway. Driveway spacing practices should provide reasonable access to abutting private property. General guidelines pertaining to unsignalized driveway spacing follow: • The needed distance between successive connections (both driveways and side streets) increases with higher operating speeds, higher access classifications for the public roadway, and higher driveway volumes. • A driveway should not be located within the functional area of an intersection or in the influ- ence area of the upstream and downstream driveways. • Left-turn lane storage requirements should be considered when determining the driveway influence area and can limit how closely driveways can be spaced. • On roadways that are undivided or have TWLTLs, the alignment of driveways on opposite sides of the road needs to be considered. Driveways on opposite sides of a lower-volume road- way may be aligned across from each other. Alternatively, they should be spaced so that those drivers desiring to travel between the driveways on opposing sides of the roadway need to make a distinct right turn followed by a left turn (or a left followed by a right). A much longer separation is needed on a higher-speed, higher-volume roadway (4-4). • On roadways with restrictive medians, the spacing between right-turn access points on oppo- site sides of the road can be treated separately. • Ideally, driveway access for a major development involving left-turn egress movements should be located where effective coordination of traffic signals would be achievable if there is a need to signalize the driveway. • Driveway connections to public roadways are subject to the same intersection control device analyses as are street intersections. If existing or future volumes warrant installing a traffic sig- nal, and signalized spacing requirements cannot be met, left-turn access should be subject to closure in one or both directions. Driveway spacing from roundabout considerations are similar to those of other types of inter- sections, but driveways may be closer to a roundabout because of shorter queuing. Driveways should not interfere with operation of the roundabout. General guidelines for unsignalized access spacing are contained in the Access Management Manual (4-1) and NCHRP Report 348 (4-2). Spacing of Driveways from Signalized Intersections The needed minimum separation distance (i.e., corner clearance) from a driveway to a signal- ized upstream or downstream location will depend on the function, operation, and design features of the roadway and the characteristics of the access connection. The basic principle of locating one connection outside of the functional area of another connection applies to driveways. For a driveway upstream of or approaching a signalized location on a major road, the func- tional area includes the perception-reaction time, maneuver distance, and storage length of the traffic on that approach. The spacing should provide separation between the conflicting move- ments occurring at the signal and the conflicting movements occurring at the driveway. In addi- tion, this spacing would enable the driveway to operate without being obstructed by the traffic backing up from the signal. Driveway Location and Spacing 21

Speed (mph) Cycle length 25 30 35 40 45 50 55 60 1100 1320 1540 1760 1980 2200 2420 70 1280 1540 1800 2050 2310 2570 2820 80 1470 1760 2050 2350 2640 2640 2640 90 1630 1980 2310 2640 2640 2640 2640 120 2200 2640 2640 2640 2640 2640 2640 NOTES: Spacing distances are in feet. Where the recommended spacing in the table exceeds ½ mile (2,640 ft), designers can limit the actual spacing to 2,640 ft. Exhibit 4-5. Signalized intersection spacing for various progression speeds and cycle lengths. 22 Guide for the Geometric Design of Driveways The spacing for a driveway downstream of the departure leg (i.e., far side) of a signalized location on a major road should be sufficient to minimize the adverse effects of the driveway operations on the intersection. According to Transportation and Land Development (4-4, p.6–28), the minimum downstream corner clearance should be no less than the stopping sight distance. Along the far side of an intersection of a crossroad with an arterial, the corner clearance distance to the first driveway varies. If the arterial does not have a channelized right-turn lane for traffic turning onto the crossroad, one source recommends that the driveway be spaced a minimum of 120 feet from the intersection. If the arterial has a channelized right-turn lane for traffic turning onto the crossroad, the clearance distance should reflect the inside corner radius. The clearance should be 200 feet for a 50-foot radius, 230 feet for a 75-foot radius, and 275 feet for a 100-foot radius (4-4, p. 6–35). The stopping sight distance principle also applies to driveways connecting to crossroads, along the far side of the intersections of cross- roads with major roads. For crossroads, the near side corner clearance should extend beyond the normal queuing dis- tance along the crossroad. Spacing for a Signalized Driveway Signal spacing is a function of travel speed and signal cycle length. The same criteria for sig- nal spacing apply to both a signalized driveway and a signalized public roadway intersection. If a driveway is going to be signalized, then it should be located to “fit” into the traffic signal progression along an arterial roadway and not interfere with the progression of traffic from one signalized intersection to the next. Desirable spacing is shown in Exhibit 4-5. When signalized driveways and intersections can be placed at these distances, there is no loss in green-band (through band) width. Small devia- tions (e.g., less than 10%) will have minimal negative effects on the progression. Further guidelines for green-band width are contained in NCHRP Report 348 (4-2, p.56–58) and in the Access Management Manual (4-1, p.140–149). Where the recommended spacing in the table exceeds 1⁄2 mile (2,640 feet), designers can limit the actual spacing to 2,640 feet. Spacing of a Driveway from an Interchange Ramp The needed driveway separation distance from an interchange area depends on the geomet- ric design of and methods of traffic control at the freeway ramp joining the roadway. It is also affected by the speed, volume, and number of lanes on the through roadway, the ramp volume

and speed, the number of vehicles turning into the driveway, the type of traffic control at the driveway, and whether the subject driveway is on the same or opposite side of the road from the entry ramp. Where the ramp entry is signalized, signal spacing criteria should govern where access con- nections are provided. A time-space analysis of the signals along the arterial, including any ramp signalization, can help in identifying the best locations for signalized access. Unsignalized ramp entry junctions can be either stop- or yield-controlled, with a geometry that is either free flowing or one that forces the ramp vehicle to come to a stop before entering the road- way. If a driveway is too close to an upstream ramp that is entering an arterial, this can cause con- gestion with spillback onto the ramp and additional conflict on the through roadway segment. This concern can be heightened where there is insufficient distance for the following sequence to occur: vehicles exit the ramp, merge into the outside lane of a multilane arterial, weave across through travel lanes, and finally enter an inside or left-turn lane to turn into a driveway on the opposite side of the roadway from the ramp. Vehicles making this maneuver have to wait for gaps in the through traffic lanes before weaving to the left. At locations with higher volumes, higher speeds, or free-flow movements from the ramp to the roadway, a longer distance is required to safely make this maneuver. NCHRP Report 420 is one source of spacing guidelines (4-3). References 4-1. TRB Committee on Access Management. Access Management Manual. Transportation Research Board, National Research Council, Washington, DC (2003) 388 pp. 4-2. Koepke, F. J., and Levinson, H. R. NCHRP Report 348: Access Management Guidelines for Activity Centers. Transportation Research Board, National Research Council, Washington, DC (1992) 111 pp. 4-3. Gluck, J. S., Levinson, H. R., and Stover, V. G. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, National Research Council, Washington, DC (1999) 157 pp. 4-4. Stover, V. G., and Koepke, F. J. Transportation and Land Development, 2nd edition. ITE (2002) 700 pp. 4-5. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) 896 pp. Driveway Location and Spacing 23

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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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