Throughout the Guide links have been provided to resources that provide more detailed information. Additional information is available for unsignalized intersections for the following topics relevant to unsignalized intersection improvements:
- Access management.
- Alternative intersections.
- Crash data and analysis.
- FHWA Intersection Safety Implementation Plans (ISIPs).
- Human factors.
- Intersection case studies.
- Non-motorized users:
- Safe Routes to School.
- Road Safety Audits.
- Rural roads.
- Traffic calming.
- Types of drivers.
“The purpose of access management is to provide vehicular access to land development in a manner that preserves the safety and efficiency of the transportation system. The contemporary practice of access management extends the concept of access design and location control to all roadways—not just limited-access highways or freeways.” (Access Management Manual, TRB)
- The Access Management Manual prepared by the Transportation Research Board details access management practices including; effects, design, public involvement, legal considerations, and more. The manual is not available online, but a copy may be purchased from the Transportation Research Board website.
- Additional information on access management, including research papers on various aspects of the subject, can be found on the TRB Access Management (AHB70) website.
Also known as innovative, nontraditional, or unconventional intersections, alternative intersection designs offer opportunities to reduce congestion and improve safety at intersections by modifying traditional vehicular movements. This is often achieved by eliminating left turns and/or through movements that would normally occur against opposing traffic. While most alternative intersections operate under the control of a traffic signal, there are several unsignalized designs. The modern roundabout serves as the most familiar example, although, considering its prevalence in many pockets of the country, the roundabout has become increasingly mainstream in recent years. The restricted crossing U-turn (RCUT) intersection—also known as a J-turn—is a design that is quickly gaining traction across the U.S.
More information on the roundabout and J-turn is available in the resources below:
- In summer 2014, FHWA released a Restricted Crossing U-Turn Intersection Informational Guide.
- FHWA released a report in 2009 titled Alternative Intersections/Interchanges: Informational Report (AIIR) that describes four at-grade intersection designs, some with unsignalized applications.
- Resources for roundabouts include (but are not limited to) the following:
- Mini-Roundabouts: Technical Summary, FHWA, 2010.
- NCHRP Report 672—Roundabouts: An Informational Guide, Second Edition, Transportation Research Board, 2010.
Safety at an unsignalized intersection may be assessed through a review of police crash reports and an analysis of the crash data. Several manuals are available that define and detail crash types:
- American National Standard, Manual on Classification of Motor Vehicle Traffic Accidents, Seventh Edition, National Safety Council, 2007.
- Model Minimum Uniform Crash Criteria website.
The National Highway Traffic Safety Administration website contains further crash details in addition to national and state crash databases.
Methods for analyzing crashes are detailed in the Highway Safety Manual, AASHTO, 2010; copies may be purchased through the AASHTO Bookstore.
Crash Modification Factors: “A crash modification factor (CMF) is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site.” (Crash Modification Factors Clearinghouse, FHWA). Details about CMFs—including an extensive inventory list of various CMFs—can be found at the FHWA’s CMF Clearinghouse website.
Manuals on design of unsignalized intersections are available on the national and state levels. For example, AASHTO publishes a national guidance on geometric design (referred to as “the Green Book”):
- A Policy on the Geometric Design of Highways and Streets, 6th Edition, AASHTO, 2011; copies of this may be purchased through the AASHTO Bookstore.
A list of state and local DOT roadway design manuals can be found on the FHWA’s Office of Design web site.
The topic of intersection sight distance is detailed in the Handbook of Simplified Practice for Traffic Studies sponsored by Iowa DOT and the Iowa Highway Research Board.
The National Association of City Transportation Officials (NACTO) has published the Urban Street Design Guide that includes examples of design of unsignalized intersections in urban areas.
Intersection Safety Implementation Plans (ISIPs) are typically a key part of a state’s Strategic Highway Safety Plan (SHSP). In 2009, FHWA released a document describing the ISIP Process. Check with your state’s department of transportation to determine if it has developed an ISIP.
The characteristics and capabilities of humans as drivers, pedestrians, and bicyclists —human factors—is key to understanding their behavior and then to design and implement the various treatments based on their needs. This subject is comprehensively treated in NCHRP Report 600 Human Factors Guidelines for Road Systems and Chapter 10 is devoted to Non-signalized Intersections.
The FHWA Office of Safety has detailed nine successful case studies on safety measures that were applied to intersections and the resulting crash reductions.
Information on non-motorized users (i.e., pedestrians and bicyclists) are listed below. These include aspects of design, operations, planning, and safety, as well as advocacy.
- FHWA has produced online guides for pedestrians (PEDSAFE) and for bicyclists (BIKESAFE), which include engineering, education, and enforcement treatments.
- Pedestrian and Bicycle Information Center.
- Guide for the Planning, Design, and Operation of Pedestrian Facilities, 1st Edition, AASHTO, 2004; copies may be purchased through the AASHTO Bookstore.
For information on safely biking and walking to school, see the National Center for Safe Routes to School website.
Safety problems associated with pedestrian access to transit are discussed in the following publications:
- Pedestrian Safety Guide for Transit Agencies, FHWA, 2008.
- Accessing Transit: Design Handbook for Florida Bus Passenger Facilities, Florida DOT, 2008.
The Transportation Research Board (TRB) publishes the Highway Capacity Manual (HCM), which can be used to compute intersection operations such as capacity and level of service. The HCM also covers level of service for pedestrians and bicyclists at intersections.
- Highway Capacity Manual 2010, TRB, 2010; copies may be purchased through the Transportation Research Board website.
A Road Safety Audit (RSA) is a formal safety performance examination of an existing or future road or intersection by an independent, multidisciplinary team. It qualitatively estimates and reports on potential road safety issues and identifies opportunities for improvements in safety for all road users. More information about this procedure including can be found at the FHWA Office of Safety website.
The FHWA Office of Safety has prepared several manuals geared to local rural road owners on topics directly or indirectly related to the contents of this guide. Readers are encouraged to review the following guides that are available in PDF format.
- Non-Motorized User Safety: A Manual for Local Rural Road Owners
- Developing Safety Plans: A Manual for Local Rural Road Owners
- Roadway Safety Information Analysis: A Manual for Local Rural Road Owners
- Roadway Departure: A Manual for Local Rural Road Owners
- Intersection Safety: A Manual for Local Rural Road Owners
- Speed Management: A Manual for Local Rural Road Owners
The Institute of Transportation Engineers (ITE) has developed a Traffic Calming Library that “contains a searchable database of reports, articles and other documents related to traffic calming.”
A manual on traffic calming was developed through a joint effort by the American Planning Association (APA) and the American Society of Civil Engineers (ASCE):
- Ewing, Reid and Steven Brown, U.S. Traffic Calming Manual, American Planning Association (APA) and American Society of Civil Engineers (ASCE), 2009; copies may be purchased through the American Planning Association (APA) website or the American Society of Civil Engineers (ASCE) website.
The driving habits of one segment of the population may be quite different from those of another. For this reason, it may be relevant to consider whether certain demographic groups compose a disproportional segment of the driving population at a given intersection. Two groups of drivers that often receive special attention are young (or newly licensed) drivers and older drivers.
- Information related to young drivers is available on the National Highway Traffic Safety Administration website and the TRB Young Driver Subcommittee (ANB30) website.
- Information related to older drivers is available in FHWA’s Handbook for Designing Roadways for the Aging Population, the FHWA Office of Safety website, and the National Highway Traffic Safety Administration website.