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FHWA Home / Safety / Intersection / Intersection Safety Issue Briefs

Intersection Safety Issue Briefs

Issue Brief 13

Access Management

November 2009
FHWA-SA-10-005

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Access Management: A Key to Safety and Mobility

Access management refers to the design, implementation, and management of entry and exit points (e.g., driveways) between roadways and adjacent properties. The use of access management techniques is designed to increase roadway capacity, manage congestion, and reduce crashes while serving land uses appropriately.1

Through the years, extensive investment for public roadway infrastructure has been made. This has largely involved public funds, but private monies also have contributed to rebuilding and enhancing the street system. During the past 30 years or more, the ability to increase roadway capacity has been increasingly difficult due to both economic and environmental constraints. Areas that do not practice effective access management face the potential for more rapid deterioration of the quality of traffic flow than those areas with a well thought out access management policy in place.

The thoughtful application of access management can have a variety of positive consequences, including the following:

The general principles of access management can be applied to a variety of urban, suburban, and rural environments. Key principles include roadway hierarchy, the functional area of an intersection, conflict points, access point and signal density, and driveway design.

Roadway Hierarchy

A roadway hierarchy is based on the premise that different roads serve different functions within the transportation network. Freeways are at one end of the spectrum and are designed and constructed to accommodate large volumes of high-speed traffic with very little interference from traffic entering or leaving the roadway. At the other end of the spectrum, local/residential streets typically have very low traffic volumes and slow speeds while providing access to adjacent properties via separate driveways and/or on-street parking.

Within this hierarchy, public road intersections generally are of greater importance than individual driveways. Ideally, private driveways should connect to the lowest classification of roadway possible and with access limited to a local/residential street. Properties located at intersections with frontage to two roadways should access the one with lower travel speeds and traffic volumes. However, there are many cases where less than ideal conditions occur, and driveways connect to collectors and arterials.

Functional Area of an Intersection

Intersections require motorists to make several simultaneous decisions to determine a safe and prudent way to proceed. An intersection can be defined by both physical and functional areas, as illustrated in Figure 1. The physical area of an intersection is limited in size and typically represents the space confined within the corners of the intersection.

Figure 1: Physical and Functional Areas of an Intersection
Diagram depicting the elements of an intersection. The physical area of the intersection comprises the center of the crossroads, which includes all the central surface area between the crosswalks. The functional area of the intersection comprises the upstream and downstream roadways where vehicles approach and travel past the intersection.

The functional area of an intersection is that area beyond the physical intersection that comprises decision and maneuvering distance, plus any required vehicle storage length. The upstream area consists of distance for travel during a perception-reaction time, travel for maneuvering and deceleration, and queue storage. The downstream area includes the length of road downstream from the intersection needed to reduce conflicts between through traffic and vehicles entering and exiting a property.

Driveways located within the functional area may create too many conflict points within too small an area for motorists to safety negotiate. Limiting driveways within the functional area of an intersection helps reduce the number of decisions motorists face. The integrity of functional areas of intersections can be protected through corner clearance, driveway spacing, and intersection spacing requirements. Ideally, intersections should be spaced far enough apart so that functional areas do not overlap.

Driveway Location and Conflict Points

One of the key elements of access management is managing the potential conflict points that occur when streets and driveways intersect. These conflict points, particularly those involving left turns, manifest themselves as an increased risk for crashes.

Approximately 72% of the crashes at a driveway within the physical area of an intersection involve a left-turning vehicle.2 Of these left-turn crashes, approximately 39% (28% of all crashes) are attributed to the ingress movement, 47% (34% of all crashes) are attributed to the egress movement conflicting with the near-side through movement, and 14% (10% of all crashes) are attributed to the egress movement merging with the far-side through movement.3 This indicates that reducing or eliminating left turns to or from driveways where possible enhances safety.

Figure 2 identifies common locations of existing driveways serving the four corner properties at the intersection of major and minor roadways. Because the major roadway typically has higher traffic volumes than the minor road, property owners often prefer to have access to the major road. As shown in Figure 2, direct, full-movement access to a major roadway can result in a high number of conflict points, especially if the driveway is close to another driveway on the opposite side of the road. In many cases, a driveway may also be located within the functional area of an intersection along a major road, and the driveways on one side of the street frequently are located without regard to driveways on the opposite side (e.g., A versus B, or C versus D).

Figure 2: Potential Access Points to Serve Corner Properties at the Intersection of Two Public Roads
Diagram of the intersection of a major roadway on the x-axis crossing a minor roadway on the y-axis. Unrestricted access roads, also on the y-axis, intersect the major artery upstream and downstream of the intersection. The diagram illustrates the many points on the roadway both east and west of the intersection where vehicle conflicts can occur as drivers try to enter or leave the main roadway via the access roads.

Figure 3 illustrates how the application of various access management techniques could reduce the number of conflict points in this situation. The addition of a raised median limits the access to Driveways A, B, and D to right-in/right-out movements only. The relocation of Driveway C as a full-movement access point on the minor roadway reduces the conflicts on the major roadway. Even with the additional access points at E and F, the total number of conflicts is significantly reduced.

Figure 3: Potential Access Arrangement to Reduce the Number of Conflict Points for Properties Located at the Intersection of Two Public Roads
This diagram of the intersection of a major and minor roadway with restricted access roads shows how controlled access can dramatically reduce the points on the roadway where vehicle conflicts can occur.

Access Point Density

During the last 40 years, access point density (i.e., number of driveways per mile) has been studied on roadways that vary in geometry, operating speeds, and volumes. The results have consistently shown that "an increase in the number of access points translates into higher accident rates."4 Figure 4 shows this trend as identified under a variety of roadway conditions and environments across the United States and Canada. Research has shown that crash rates on roadways increase as the density of access points connecting to the roadway increase. Research in Iowa5 and Utah6 confirms these findings.

Figure 4: Effect of Access Point Density on Crash Rate
Chart shows that in 9 locations, as the number of acess points per mile increases, the number of crashes per million vehicle miles traveled increases as well.

Driveway Design

Driveway and site circulation must be adequately designed to ensure motorists are able to completely exit the roadway without being impeded by other vehicles in the inbound lane. Key elements include the following:

Additional Issues

Below is a partial list of additional issues to consider when evaluating the safety impacts of permitting driveways near intersections. Further discussion of these principles can be found in the TRB Access Management Manual and the AASHTO Policy on Geometric Design of Highways and Streets.

Access Management Tools and Techniques

There are a number of other tools and techniques available to consider for use as part of an access management plan. They include both physical design techniques as well as policy related addressing land development and roadway design standards. Some examples of common and highly effective techniques:

Resources

Transportation Research Board. Access Management Manual. National Academy of Sciences, Washington, DC, 2003.

Najm, W. G., J. D. Smith, and D. L. Smith. Analysis of Crossing Path Crashes. Report No. DOT-VNTSC-NHTSA-01-03. National Highway Traffic Safety Administration, U.S. Department of Transportation, Washington, DC, July 2001.

Iowa Department of Transportation and the Iowa Highway Research Board. Access Management Awareness Program, Phase II Report, the Iowa DOT Project TR-402 CTRE Management Project 97-1. Center for Transportation Research and Education, Ames, IA, December 1997.

Schultz, G. G., C. G. Allen, and D. L. Eggett. Crashes in the Vicinity of Major Crossroads. Report No. UT-08.25. Research and Development Division, Utah Department of Transportation, December 2008.

Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, National Academy of Sciences, Washington, DC, 1999.

American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets. Washington, DC: AASHTO, 2004.

Federal Highway Administration (FHWA). Benefits of Access Management. Document Number FHWA-OP-03-066. FHWA, Washington, DC, 2003.

Institute of Transportation Engineers, Traffic Engineering Handbook, 6th Edition. Washington, DC, 2009.

Traffic Safety Tool Box: A Primer on Traffic Safety. Washington, DC, 1999.

Urban Street Geometric Design Handbook. Washington, DC, 2008.

Parsonson, P. S., M. Walters, and J. S. Fincher. Effect on Safety of Replacing an Arterial Continuous Two-Way Left-Turn Lane with a Raised Median. National Conference on Access Management. TRB Access Management Conference, Vail, CO, 1993.

Parsonson, P.S., M. Walters, and J. S. Fincher. "Georgia Study Confirms the Continuing Safety Advantage of Raised Medians over Continuous Two-Way Left-Turn Lanes." National Conference on Access Management. Portland, OR, 2000.

Koepke, F. J., and H. S. Levinson. NCHRP Report 348: Access Management Guidelines for Activity Centers. Transportation Research Board, National Academy of Sciences, Washington, DC, 1992.

Driveway Handbook. Florida Department of Transportation, Tallahassee, FL, March 2005.

Bonneson, J., K. Zimmerman, and K. Fitzpatrick. Roadway Safety Design Synthesis. Texas Department of Transportation, 2005.


Footnotes:

1 Transportation Research Board. Access Management Manual. National Academy of Sciences, Washington, DC, 2003.

2 Najm, W. G., J. D. Smith, and D. L. Smith. Analysis of Crossing Path Crashes. Report No. DOT-VNTSC-NHTSA-01-03. National Highway Traffic Safety Administration, U.S. Department of Transportation, Washington, DC, July 2001.

3 Ibid.

4 Transportation Research Board. Access Management Manual. National Academy of Sciences, Washington, DC, 2003.

5 Iowa Department of Transportation and the Iowa Highway Research Board. Access Management Awareness Program, Phase II Report, the Iowa DOT Project TR-402 CTRE Management Project 97-1. Center for Transportation Research and Education (CTRE), Ames, IA, December 1997.

6 Schultz, G. G., C. G. Allen, and D. L. Eggett. Crashes in the Vicinity of Major Crossroads. Report No. UT-08.25. Research and Development Division, Utah Department of Transportation, December 2008.




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