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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-
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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)
